40 Java Collections Interview Questions and Answers

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Java Collections Interview Questions

Java Collections Framework is one of the core APIs of java programming language.

It’s one of the important topics for java interview questions. Here I am listing some important java collections interview questions and answers to help you in the interview. This is directly coming from my 14+ years of experience in Java programming.

Java Collections Interview Questions

  1. What are Collection related features in Java 8?
  2. What is Java Collections Framework? List out some benefits of Collections framework?
  3. What is the benefit of Generics in Collections Framework?
  4. What are the basic interfaces of Java Collections Framework?
  5. Why Collection doesn’t extend Cloneable and Serializable interfaces?
  6. Why Map interface doesn’t extend Collection interface?
  7. What is an Iterator?
  8. What is the difference between Enumeration and Iterator interface?
  9. Why there is not a method like Iterator.add() to add elements to the collection?
  10. Why Iterator don’t have a method to get next element directly without moving the cursor?
  11. What is different between Iterator and ListIterator?
  12. What are different ways to iterate over a list?
  13. What do you understand by iterator fail-fast property?
  14. What is difference between fail-fast and fail-safe?
  15. How to avoid ConcurrentModificationException while iterating a collection?
  16. Why there are no concrete implementations of Iterator interface?
  17. What is UnsupportedOperationException?
  18. How HashMap works in Java?
  19. What is the importance of hashCode() and equals() methods?
  20. Can we use any class as Map key?
  21. What are different Collection views provided by Map interface?
  22. What is difference between HashMap and Hashtable?
  23. How to decide between HashMap and TreeMap?
  24. What are similarities and difference between ArrayList and Vector?
  25. What is difference between Array and ArrayList? When will you use Array over ArrayList?
  26. What is difference between ArrayList and LinkedList?
  27. Which collection classes provide random access of its elements?
  28. What is EnumSet?
  29. Which collection classes are thread-safe?
  30. What are concurrent Collection Classes?
  31. What is BlockingQueue?
  32. What is Queue and Stack, list their differences?
  33. What is Collections Class?
  34. What is Comparable and Comparator interface?
  35. What is difference between Comparable and Comparator interface?
  36. How can we sort a list of Objects?
  37. While passing a Collection as argument to a function, how can we make sure the function will not be able to modify it?
  38. How can we create a synchronized collection from given collection?
  39. What are common algorithms implemented in Collections Framework?
  40. What is Big-O notation? Give some examples?
  41. What are best practices related to Java Collections Framework?
  42. What is Java Priority Queue?
  43. Why can’t we write code as List<Number> numbers = new ArrayList<Integer>();?
  44. Why can’t we create generic array? or write code as List<Integer>[] array = new ArrayList<Integer>[10];

Java Collections Interview Questions and Answers

  1. What are Collection related features in Java 8?

    Java 8 has brought major changes in the Collection API. Some of the changes are:

    1. Java Stream API for collection classes for supporting sequential as well as parallel processing
    2. Iterable interface is extended with forEach() default method that we can use to iterate over a collection. It is very helpful when used with lambda expressions because its argument Consumer is a function interface.
    3. Miscellaneous Collection API improvements such as forEachRemaining(Consumer action) method in Iterator interface, Map replaceAll(), compute(), merge() methods.
  2. What is Java Collections Framework? List out some benefits of Collections framework?

    Collections are used in every programming language and initial java release contained few classes for collections: Vector, Stack, Hashtable, Array. But looking at the larger scope and usage, Java 1.2 came up with Collections Framework that group all the collections interfaces, implementations and algorithms.
    Java Collections have come through a long way with the usage of Generics and Concurrent Collection classes for thread-safe operations. It also includes blocking interfaces and their implementations in java concurrent package.
    Some of the benefits of collections framework are;

    • Reduced development effort by using core collection classes rather than implementing our own collection classes.
    • Code quality is enhanced with the use of well tested collections framework classes.
    • Reduced effort for code maintenance by using collection classes shipped with JDK.
    • Reusability and Interoperability
  3. What is the benefit of Generics in Collections Framework?

    Java 1.5 came with Generics and all collection interfaces and implementations use it heavily. Generics allow us to provide the type of Object that a collection can contain, so if you try to add any element of other type it throws compile time error.
    This avoids ClassCastException at Runtime because you will get the error at compilation. Also Generics make code clean since we don’t need to use casting and instanceof operator. I would highly recommend to go through Java Generic Tutorial to understand generics in a better way.

  4. What are the basic interfaces of Java Collections Framework?

    Collection is the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Java platform doesn’t provide any direct implementations of this interface.

    Set is a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the deck of cards.

    List is an ordered collection and can contain duplicate elements. You can access any element from its index. The list is more like an array with dynamic length.

    A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.

    Some other interfaces are Queue, Dequeue, Iterator, SortedSet, SortedMap and ListIterator.

  5. Why Collection doesn’t extend Cloneable and Serializable interfaces?

    Collection interface specifies group of Objects known as elements. How the elements are maintained is left up to the concrete implementations of Collection. For example, some Collection implementations like List allow duplicate elements whereas other implementations like Set don’t.
    A lot of the Collection implementations have a public clone method. However, it doesn’t make sense to include it in all implementations of Collection. This is because Collection is an abstract representation. What matters is the implementation.
    The semantics and the implications of either cloning or serializing come into play when dealing with the actual implementation; so concrete implementation should decide how it should be cloned or serialized, or even if it can be cloned or serialized.
    So mandating cloning and serialization in all implementations is less flexible and more restrictive. The specific implementation should decide as to whether it can be cloned or serialized.

  6. Why Map interface doesn’t extend Collection interface?

    Although Map interface and its implementations are part of the Collections Framework, Map is not collections and collections are not Map. Hence it doesn’t make sense for Map to extend Collection or vice versa.
    If Map extends Collection interface, then where are the elements? The map contains key-value pairs and it provides methods to retrieve the list of Keys or values as Collection but it doesn’t fit into the “group of elements” paradigm.

  7. What is an Iterator?

    The Iterator interface provides methods to iterate over any Collection. We can get iterator instance from a Collection using iterator() method. Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection during the iteration. Java Collection iterator provides a generic way for traversal through the elements of a collection and implements Iterator Design Pattern.

  8. What is difference between Enumeration and Iterator interface?

    Enumeration is twice as fast as Iterator and uses very little memory. Enumeration is very basic and fits basic needs. But the Iterator is much safer as compared to Enumeration because it always denies other threads to modify the collection object which is being iterated by it.
    Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection that is not possible with Enumeration. Iterator method names have been improved to make its functionality clear.

  9. Why there is not method like Iterator.add() to add elements to the collection?

    The semantics are unclear, given that the contract for Iterator makes no guarantees about the order of iteration. Note, however, that ListIterator does provide an add operation, as it does guarantee the order of the iteration.

  10. Why Iterator don’t have a method to get next element directly without moving the cursor?

    It can be implemented on top of current Iterator interface but since its use will be rare, it doesn’t make sense to include it in the interface that everyone has to implement.

  11. What is different between Iterator and ListIterator?

    • We can use Iterator to traverse Set and List collections whereas ListIterator can be used with Lists only.
    • Iterator can traverse in forward direction only whereas ListIterator can be used to traverse in both the directions.
    • ListIterator inherits from Iterator interface and comes with extra functionalities like adding an element, replacing an element, getting index position for previous and next elements.
  12. What are different ways to iterate over a list?

    We can iterate over a list in two different ways – using iterator and using for-each loop.

    
    List<String> strList = new ArrayList<>();
    
    //using for-each loop
    for(String obj : strList){
        System.out.println(obj);
    }
    
    //using iterator
    Iterator<String> it = strList.iterator();
    while(it.hasNext()){
        String obj = it.next();
        System.out.println(obj);
    }
    

    Using iterator is more thread-safe because it makes sure that if underlying list elements are modified, it will throw ConcurrentModificationException.

  13. What do you understand by iterator fail-fast property?

    Iterator fail-fast property checks for any modification in the structure of the underlying collection everytime we try to get the next element. If there are any modifications found, it throws ConcurrentModificationException. All the implementations of Iterator in Collection classes are fail-fast by design except the concurrent collection classes like ConcurrentHashMap and CopyOnWriteArrayList.

  14. What is difference between fail-fast and fail-safe?

    Iterator fail-safe property work with the clone of underlying collection, hence it’s not affected by any modification in the collection. By design, all the collection classes in java.util package are fail-fast whereas collection classes in java.util.concurrent are fail-safe.
    Fail-fast iterators throw ConcurrentModificationException whereas fail-safe iterator never throws ConcurrentModificationException.
    Check this post for CopyOnWriteArrayList Example.

  15. How to avoid ConcurrentModificationException while iterating a collection?

    We can use concurrent collection classes to avoid ConcurrentModificationException while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList.
    Check this post for ConcurrentHashMap Example.

  16. Why there are no concrete implementations of Iterator interface?

    Iterator interface declare methods for iterating a collection but its implementation is responsibility of the Collection implementation classes. Every collection class that returns an iterator for traversing has its own Iterator implementation nested class.
    This allows collection classes to chose whether iterator is fail-fast or fail-safe. For example ArrayList iterator is fail-fast whereas CopyOnWriteArrayList iterator is fail-safe.

  17. What is UnsupportedOperationException?

    UnsupportedOperationException is the exception used to indicate that the operation is not supported. It’s used extensively in JDK classes, in collections framework java.util.Collections.UnmodifiableCollection throws this exception for all add and remove operations.

  18. How HashMap works in Java?

    HashMap stores key-value pair in Map.Entry static nested class implementation. HashMap works on hashing algorithm and uses hashCode() and equals() method in put and get methods.

    When we call put method by passing key-value pair, HashMap uses Key hashCode() with hashing to find out the index to store the key-value pair. The Entry is stored in the LinkedList, so if there is an already existing entry, it uses equals() method to check if the passed key already exists, if yes it overwrites the value else it creates a new entry and stores this key-value Entry.

    When we call get method by passing Key, again it uses the hashCode() to find the index in the array and then use equals() method to find the correct Entry and return its value. The below image will explain these detail clearly.

    java-hashmap-entry-impl

    The other important things to know about HashMap are capacity, load factor, threshold resizing. HashMap initial default capacity is 16 and load factor is 0.75. The threshold is capacity multiplied by load factor and whenever we try to add an entry if map size is greater than the threshold, HashMap rehashes the contents of the map into a new array with a larger capacity. The capacity is always the power of 2, so if you know that you need to store a large number of key-value pairs, for example in caching data from the database, it’s a good idea to initialize the HashMap with correct capacity and load factor.

  19. What is the importance of hashCode() and equals() methods?

    HashMap uses the Key object hashCode() and equals() method to determine the index to put the key-value pair. These methods are also used when we try to get value from HashMap. If these methods are not implemented correctly, two different Key’s might produce the same hashCode() and equals() output and in that case, rather than storing it at a different location, HashMap will consider the same and overwrite them.

    Similarly all the collection classes that doesn’t store duplicate data use hashCode() and equals() to find duplicates, so it’s very important to implement them correctly. The implementation of equals() and hashCode() should follow these rules.

    • If o1.equals(o2), then o1.hashCode() == o2.hashCode()should always be true.
    • If o1.hashCode() == o2.hashCode is true, it doesn’t mean that o1.equals(o2) will be true.
  20. Can we use any class as Map key?

    We can use any class as Map Key, however following points should be considered before using them.

    • If the class overrides equals() method, it should also override hashCode() method.
    • The class should follow the rules associated with equals() and hashCode() for all instances. Please refer earlier question for these rules.
    • If a class field is not used in equals(), you should not use it in hashCode() method.
    • Best practice for user defined key class is to make it immutable, so that hashCode() value can be cached for fast performance. Also immutable classes make sure that hashCode() and equals() will not change in future that will solve any issue with mutability.
      For example, let’s say I have a class MyKey that I am using for the HashMap key.

      
      //MyKey name argument passed is used for equals() and hashCode()
      MyKey key = new MyKey("Pankaj"); //assume hashCode=1234
      myHashMap.put(key, "Value");
      
      // Below code will change the key hashCode() and equals()
      // but its location is not changed.
      key.setName("Amit"); //assume new hashCode=7890
      
      //below will return null because HashMap will try to look for key
      //in the same index as it was stored but since the key is mutated, 
      //there will be no match and it will return null.
      myHashMap.get(new MyKey("Pankaj")); 
      

      This is the reason why String and Integer are mostly used as HashMap keys.

  21. What are different Collection views provided by Map interface?

    Map interface provides three collection views:

    1. Set<K> keySet(): Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
    2. Collection<V> values(): Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator’s remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
    3. Set<Map.Entry<K, V>> entrySet(): Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s remove operation, or the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
  22. What is difference between HashMap and Hashtable?

    HashMap and Hashtable both implements Map interface and looks similar, however, there is the following difference between HashMap and Hashtable.

    1. HashMap allows null key and values whereas Hashtable doesn’t allow null key and values.
    2. Hashtable is synchronized but HashMap is not synchronized. So HashMap is better for single threaded environment, Hashtable is suitable for multi-threaded environment.
    3. LinkedHashMap was introduced in Java 1.4 as a subclass of HashMap, so incase you want iteration order, you can easily switch from HashMap to LinkedHashMap but that is not the case with Hashtable whose iteration order is unpredictable.
    4. HashMap provides Set of keys to iterate and hence it’s fail-fast but Hashtable provides Enumeration of keys that doesn’t support this feature.
    5. Hashtable is considered to be legacy class and if you are looking for modifications of Map while iterating, you should use ConcurrentHashMap.
  23. How to decide between HashMap and TreeMap?

    For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

  24. What are similarities and difference between ArrayList and Vector?

    ArrayList and Vector are similar classes in many ways.

    1. Both are index based and backed up by an array internally.
    2. Both maintains the order of insertion and we can get the elements in the order of insertion.
    3. The iterator implementations of ArrayList and Vector both are fail-fast by design.
    4. ArrayList and Vector both allows null values and random access to element using index number.

    These are the differences between ArrayList and Vector.

    1. Vector is synchronized whereas ArrayList is not synchronized. However if you are looking for modification of list while iterating, you should use CopyOnWriteArrayList.
    2. ArrayList is faster than Vector because it doesn’t have any overhead because of synchronization.
    3. ArrayList is more versatile because we can get synchronized list or read-only list from it easily using Collections utility class.
  25. What is difference between Array and ArrayList? When will you use Array over ArrayList?

    Arrays can contain primitive or Objects whereas ArrayList can contain only Objects.
    Arrays are fixed-size whereas ArrayList size is dynamic.
    Arrays don’t provide a lot of features like ArrayList, such as addAll, removeAll, iterator, etc.

    Although ArrayList is the obvious choice when we work on the list, there are a few times when an array is good to use.

    • If the size of list is fixed and mostly used to store and traverse them.
    • For list of primitive data types, although Collections use autoboxing to reduce the coding effort but still it makes them slow when working on fixed size primitive data types.
    • If you are working on fixed multi-dimensional situation, using [][] is far more easier than List<List<>>
  26. What is difference between ArrayList and LinkedList?

    ArrayList and LinkedList both implement List interface but there are some differences between them.

    1. ArrayList is an index based data structure backed by Array, so it provides random access to its elements with performance as O(1) but LinkedList stores data as list of nodes where every node is linked to its previous and next node. So even though there is a method to get the element using index, internally it traverse from start to reach at the index node and then return the element, so performance is O(n) that is slower than ArrayList.
    2. Insertion, addition or removal of an element is faster in LinkedList compared to ArrayList because there is no concept of resizing array or updating index when element is added in middle.
    3. LinkedList consumes more memory than ArrayList because every node in LinkedList stores reference of previous and next elements.
  27. Which collection classes provide random access of its elements?

    ArrayList, HashMap, TreeMap, Hashtable, and Vector classes provide random access to its elements. Download java collections pdf for more information.

  28. What is EnumSet?

    java.util.EnumSet is Set implementation to use with enum types. All of the elements in an enum set must come from a single enum type that is specified, explicitly or implicitly, when the set is created. EnumSet is not synchronized and null elements are not allowed. It also provides some useful methods like copyOf(Collection c), of(E first, E… rest) and complementOf(EnumSet s).

    Check this post for java enum tutorial.

  29. Which collection classes are thread-safe?

    Vector, Hashtable, Properties and Stack are synchronized classes, so they are thread-safe and can be used in multi-threaded environment. Java 1.5 Concurrent API included some collection classes that allows modification of collection while iteration because they work on the clone of the collection, so they are safe to use in multi-threaded environment.

  30. What are concurrent Collection Classes?

    Java 1.5 Concurrent package (java.util.concurrent) contains thread-safe collection classes that allow collections to be modified while iterating. By design Iterator implementation in java.util packages are fail-fast and throws ConcurrentModificationException. But Iterator implementation in java.util.concurrent packages are fail-safe and we can modify the collection while iterating. Some of these classes are CopyOnWriteArrayList, ConcurrentHashMap, CopyOnWriteArraySet.

    Read these posts to learn about them in more detail.

  31. What is BlockingQueue?

    java.util.concurrent.BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

    BlockingQueue interface is part of the java collections framework and it’s primarily used for implementing the producer-consumer problem. We don’t need to worry about waiting for the space to be available for producer or object to be available for consumers in BlockingQueue as it’s handled by implementation classes of BlockingQueue.

    Java provides several BlockingQueue implementations such as ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue, etc.
    Check this post for use of BlockingQueue for producer-consumer problem.

  32. What is Queue and Stack, list their differences?

    Both Queue and Stack are used to store data before processing them. java.util.Queue is an interface whose implementation classes are present in java concurrent package. Queue allows retrieval of element in First-In-First-Out (FIFO) order but it’s not always the case. There is also Deque interface that allows elements to be retrieved from both end of the queue.
    The stack is similar to queue except that it allows elements to be retrieved in Last-In-First-Out (LIFO) order.
    Stack is a class that extends Vector whereas Queue is an interface.

  33. What is Collections Class?

    java.util.Collections is a utility class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, “wrappers”, which return a new collection backed by a specified collection, and a few other odds and ends.

    This class contains methods for collection framework algorithms, such as binary search, sorting, shuffling, reverse, etc.

  34. What is Comparable and Comparator interface?

    Java provides a Comparable interface which should be implemented by any custom class if we want to use Arrays or Collections sorting methods. The comparable interface has a compareTo(T obj) method which is used by sorting methods. We should override this method in such a way that it returns a negative integer, zero, or a positive integer if “this” object is less than, equal to, or greater than the object passed as an argument.

    But, in most real-life scenarios, we want sorting based on different parameters. For example, as a CEO, I would like to sort the employees based on Salary, an HR would like to sort them based on age. This is the situation where we need to use Comparator interface because Comparable.compareTo(Object o) method implementation can sort based on one field only and we can’t choose the field on which we want to sort the Object.

    Comparator interface compare(Object o1, Object o2) method need to be implemented that takes two Object argument, it should be implemented in such a way that it returns negative int if the first argument is less than the second one and returns zero if they are equal and positive int if the first argument is greater than the second one.

    Check this post for use of Comparable and Comparator interface to sort objects.

  35. What is difference between Comparable and Comparator interface?

    Comparable and Comparator interfaces are used to sort collection or array of objects.

    Comparable interface is used to provide the natural sorting of objects and we can use it to provide sorting based on single logic.
    Comparator interface is used to provide different algorithms for sorting and we can choose the comparator we want to use to sort the given collection of objects.

  36. How can we sort a list of Objects?

    If we need to sort an array of Objects, we can use Arrays.sort(). If we need to sort a list of objects, we can use Collections.sort(). Both these classes have overloaded sort() methods for natural sorting (using Comparable) or sorting based on criteria (using Comparator).
    Collections internally uses Arrays sorting method, so both of them have same performance except that Collections take sometime to convert list to array.

  37. While passing a Collection as argument to a function, how can we make sure the function will not be able to modify it?

    We can create a read-only collection using Collections.unmodifiableCollection(Collection c) method before passing it as argument, this will make sure that any operation to change the collection will throw UnsupportedOperationException.

  38. How can we create a synchronized collection from given collection?

    We can use Collections.synchronizedCollection(Collection c) to get a synchronized (thread-safe) collection backed by the specified collection.

  39. What are common algorithms implemented in Collections Framework?

    Java Collections Framework provides algorithm implementations that are commonly used such as sorting and searching. Collections class contain these method implementations. Most of these algorithms work on List but some of them are applicable for all kinds of collections.
    Some of them are sorting, searching, shuffling, min-max values.

  40. What is Big-O notation? Give some examples?

    The Big-O notation describes the performance of an algorithm in terms of the number of elements in a data structure. Since Collection classes are data structures, we usually tend to use Big-O notation to chose the collection implementation to use based on time, memory and performance.

    Example 1: ArrayList get(index i) is a constant-time operation and doesn’t depend on the number of elements in the list. So its performance in Big-O notation is O(1).
    Example 2: A linear search on array or list performance is O(n) because we need to search through entire list of elements to find the element.

  41. What are best practices related to Java Collections Framework?

    • Chosing the right type of collection based on the need, for example if size is fixed, we might want to use Array over ArrayList. If we have to iterate over the Map in order of insertion, we need to use LinkedHashMap. If we don’t want duplicates, we should use Set.
    • Some collection classes allows to specify the initial capacity, so if we have an estimate of number of elements we will store, we can use it to avoid rehashing or resizing.
    • Write program in terms of interfaces not implementations, it allows us to change the implementation easily at later point of time.
    • Always use Generics for type-safety and avoid ClassCastException at runtime.
    • Use immutable classes provided by JDK as key in Map to avoid implementation of hashCode() and equals() for our custom class.
    • Use Collections utility class as much as possible for algorithms or to get read-only, synchronized or empty collections rather than writing own implementation. It will enhance code-reuse with greater stability and low maintainability.
  42. What is Java Priority Queue?

    PriorityQueue is an unbounded queue based on a priority heap and the elements are ordered in their natural order or we can provide Comparator for ordering at the time of creation. PriorityQueue doesn’t allow null values and we can’t add any object that doesn’t provide natural ordering or we don’t have any comparator for them for ordering. Java PriorityQueue is not thread-safe and provided O(log(n)) time for enqueing and dequeing operations. Check this post for java priority queue example.

  43. Why can’t we write code as List<Number> numbers = new ArrayList<Integer>();?

    Generics doesn’t support sub-typing because it will cause issues in achieving type safety. That’s why List<T> is not considered as a subtype of List<S> where S is the super-type of T. To understanding why it’s not allowed, let’s see what could have happened if it has been supported.

    
    List<Long> listLong = new ArrayList<Long>();
    listLong.add(Long.valueOf(10));
    List<Number> listNumbers = listLong; // compiler error
    listNumbers.add(Double.valueOf(1.23));
    

    As you can see from above code that IF generics would have been supporting sub-typing, we could have easily add a Double to the list of Long that would have caused ClassCastException at runtime while traversing the list of Long.

  44. Why can’t we create generic array? or write code as List<Integer>[] array = new ArrayList<Integer>[10];

    We are not allowed to create generic arrays because array carry type information of its elements at runtime. This information is used at runtime to throw ArrayStoreException if elements type doesn’t match to the defined type. Since generics type information gets erased at compile time by Type Erasure, the array store check would have been passed where it should have failed. Let’s understand this with a simple example code.

    
    List<Integer>[] intList = new List<Integer>[5]; // compile error
    Object[] objArray = intList;
    List<Double> doubleList = new ArrayList<Double>();
    doubleList.add(Double.valueOf(1.23));
    objArray[0] = doubleList; // this should fail but it would pass because at runtime intList and doubleList both are just List
    

    Arrays are covariant by nature i.e S[] is a subtype of T[] whenever S is a subtype of T but generics doesn’t support covariance or sub-typing as we saw in the last question. So if we would have been allowed to create generic arrays, because of type erasure we would not get an array store exception even though both types are not related.

    To know more about Generics, read Java Generics Tutorial.

I will keep on adding more questions on java collections framework as and when I found them, if you found it useful please share it with others too, it motivates me in writing more like these. 🙂

Please let me know if I have missed any important question, I will include that to list.

Comments

  1. saksham says:

    Why HashMap allows null key and values whereas Hashtable doesn’t allow null key and values ?

  2. Ashutosh Ghimire says:

    41
    Chosing the right type of collection based on the need, for example if size is fixed, we might want to use Array over ArrayList. If we have to iterate over the Map in order of insertion, we need to use TreeMap. If we don’t want duplicates, we should use Set.
    Shouldn’t Treemap be replaced with LinkedHashMap ,How would you know the order of insertion in a TreeMap unless you are inserting a sorted entry ?

    1. AS says:

      Yes..agree with your comment..TreeMap maintains the natural ordering or ordered according to provided comparator !

      1. Pankaj says:

        Yes, you guys are right. I have corrected the error. I appreciate the comment.

  3. Meenal Jain says:

    Hello Pankaj,

    There is a question in above list:

    Which collection classes provide random access of it’s elements?

    And vector is not there in answer, although Vector implements random access.

    Can you tell me does vector and stack support random access or not?

    1. Pankaj says:

      Vector implements random access, Stack doesn’t.

  4. PREM SINHA says:

    Very helpful for interview and good collections.
    One thing i would like to mention for the difference between Comparable and Comparator is can’t we use comparable to sort using many fields?
    As per the documentation we can have multiple sorting sequence using comparator that’s not possible using Comparable. I meant to say we can have multiple classes for Comparator like SortUsingAge, SortUsingName, SortUsingAgeAndName. But using Comparable we can have only one class and we need to construct CompareTo method whithin the same Object which you want to sort but we have options in CompareTo method to sort using many fields.

    Please update If I am wrong because it’s nice tutorial and very helpful for interview and I am expecting this to be updated If I am correct.

  5. Gaurav Sahu says:

    Excellent work..Very useful material..Thanks

  6. vicky kumar jaiswal says:

    Map.Entry static nested class implementation..

    It is nested interface or implementation class.

  7. Meenakshi says:

    Very good source of info. Great job indeed. Thanks Pankaj !!

  8. Vaibhav says:

    Thanks for such good Questions

  9. Vivek Kurmi says:

    Very Nice Interview Questions..

  10. Java says:

    Please make correct question 12. For-each loop under the hood use iterator to go through elements of collection and It throw ConcurrentModificationException if something has changed. The explanation to this question sounds like “Iterator is better than Iterator”.

  11. Tsetso says:

    In the “What are the basic interfaces of Java Collections Framework?” is missed Queue Collection Interface which is one of the four based Java Collection interfaces

  12. Venus says:

    Can someone pls tell difference between for loop and iterator .. even in Iterator we can not modify collection as it throws Concurrent ModificationException.

    1. Tsetso says:

      If I remember correctly iterator is released in java 1.2
      Advantage of iterator is that you can remove element during the iteration. Also you can you iterate backward.
      Hope this help you

      1. Raghib says:

        Iterator doesn’t move backward it is a One Directional Cursor Whereas ListIterator is a Bidirectional cursor.
        In Enumeration:- Read Only
        In Iterator:- Read/Remove
        In ListIterator:- Read/Remove/Add/replace.

  13. Nand says:

    Hi Pankaj.

    Please expalin once again below line .

    Hashtable is synchronized but HashMap is not synchronized. So HashMap is better for single threaded environment, Hashtable is suitable for multi-threaded environment.

    1. csg says:

      HashMap is better for multi-threaded environment because it is not synchronized and multiple threads can access the hashmap at the same time,but it is not the same with hashtable.multiple threads cannot access the hashtable at the same time.

  14. Sanjay Roy says:

    Hi,
    I do follow your posts for various topics in Java. As I am not very old as a programmer , I was going through the fail-fast property of Iterator by writing small programs to just get accustomed with the exception it throws when any modification is done to the underlying Collection being iterated by it,
    For me the concept goes well with List but when I was implementing the fail-fast property of Iterator with Map, I was not getting any Exception even after modification to the Collection. I am attaching a snippet of the code below. Please do check and let me know why I wasn’t getting any Exception even after I add Entries in the Map.

    Map test = new HashMap();
    test.put(1, “abc”);
    test.put(2, “cde”);
    test.put(3, “efg”);
    test.put(4, “ghi”);
    test.put(5, “ijk”);
    test.put(6, “klm”);

    Iterator iterator = test.keySet().iterator();
    while(iterator.hasNext()) {
    if(it.next().equals(6)) {
    test.put(6, “yui”);
    test.put(7, “lll”);
    test.put(8, “kkk”);
    test.put(9, “sss”);
    }
    }

    for(Map.Entry m: test.entrySet()) {
    System.out.println(m.getKey() + ” ” + m.getValue());
    }

    Do bare me with all the reference names been used in the program. It was only for test purpose

    1. Pankaj says:

      The problem is with your if condition, you are doing the modification on the last item. Change it to if(it.next().equals(4)) and you will get the exception.

      1. Sanjay Roy says:

        Thanks for the response. It does work but can you tell me if possible why the exception is not getting thrown if the condition is made on the last entry?. I am modifying something there also.
        Thanks in advance

        1. arvind says:

          Because it is the next() method that is throwing the ConcurrentModificationException and if it is not called then no exception.
          And this is what happening in your code.

  15. Rajesh says:

    Very good collection…

  16. Sanju says:

    Question#44 : “Since generics type information gets erased at runtime ”
    but according to https://docs.oracle.com/javase/tutorial/java/generics/erasure.html information erased at compile time. Is there any misunderstand from my side?

    1. Pankaj says:

      You are absolutely right, it was a typo error. I have fixed the post. Thanks for the comment, I appreciate it.

  17. Anurag Singh says:

    this post is awesome for every one who wanted to crack the java interview.
    thank you so much.

    1. Veera Kanisetty says:

      Really superb , looking for these kind of stuff in all Java frameworks as well. Thanks

  18. liverm0r says:

    4 point is wrong.
    Map is not under the Collections interface.

    1. Vineet Kapoor says:

      Yes True.

    2. Vikas Palakurthi says:

      The question is “What are the basic interfaces of Java Collections Framework”. It specifically says “Collections Framework”.
      Map is an interface in Collections framework, but is not a sub-interface of Collection interface.

  19. Avisha Jindal says:

    I was once asked in an interview. What is the meaning of making an array list ‘final’? Does it mean that we won’t be able to add elements to it? Till date whomever i’ve asked this question to , has not been able to answer it correctly. Please add more such type of tricky interview questions.

    1. Tarun says:

      @avisha if you use final with any attribute or any other thing you can’t instantiate again eg . final int i =6;
      you not change value of i again same as
      final List list = new ArrayList (); you cant instantiate it again like list = new LinkedList(); or somthing
      but you add the vlaue inside the list .

    2. Vineet Kapoor says:

      This means that you cannot rebind the variable to point to a different collection instance:

      final List list = new ArrayList();
      list = new ArrayList(); // Since `list’ is final, this won’t compile

      1. zurreyab says:

        compile

    3. Rupa says:

      It simply means that you cant change or modify its reference.
      final List al = new ArrayList();
      The al reference is pointing to the object Arraylist() cannot be modified

  20. Dabbiru Gautam says:

    HI,
    Great work done for collating all questions at one place.
    For 36. The object needs to Implement Comparable or Comparator first and should have the implementation for compareTo() or compare(). Then we can use Colletions.sort().

    Regards,
    Gautam

    1. Kannan says:

      When you use collection to hold any wrapper class objects, then there is no need of implementing Comparable or Comparator interface.

      Ex: List list = new ArrayList();
      Collections.sort(list); //will sort based on lexicographical.

      When you use any custom objects (says Person, Employee), then you have implement any of interface and tell based which fields collections has to be sorted.

      Say ArrayList holds list of Employee object, and you want to sort by employee salary or name then you have to go with any above interface.

      Ex. List empList = new ArrayList();
      Collections.sort(empList); //compile time error if you don’t implement Comparable or Comparator interface

      1. Vineet Kapoor says:

        Yes true

  21. vedant maheshwari says:

    hi Pankaj ,
    Can you give more explanation on Q44.
    As it says that Arrays doesn’t allow to be generic. so what happen in case of Q44.
    Will it give error or simply store the list of elements of integer and double?

    1. Pankaj says:

      It will give compile error, the code is just to show what will happen if we assume that code will not give compile error.

  22. GOPINATH M B says:

    Good collection bro…thank you:)

  23. Dhananjay says:

    Good work, very clear explanation.

  24. Shreya says:

    Thank you so much for providing the explanations.

  25. ramana reddy says:

    Amazing work, very detailed. Thank you so much.

  26. Raju says:

    @Pankaj how are writing this much long tutorial. You didn’t get married ?? 🙂
    But any how i love your work and effort its very useful. thanks

    1. Pankaj says:

      This is funny and nice. Actually I am married and my wife provide a lot of support in this and help me motivated.

      1. Inder says:

        Hi Pankaj ,

        Can you please revisit the answer to the question 22. I guess in HashMap only null keys are supported and not null values..

        1. Pankaj says:

          For these, best way to confirm is write a simple program. See below program compiles and runs fine.

          import java.util.HashMap;
          
          public class Test {
          
          public static void main(String args[]){
          
          HashMap<String,String> m = new HashMap<String,String>();
          
          m.put(null,null);
          m.put("1","1");
          
          System.out.println(m);
          }
          
          }
          
        2. Kannan says:

          HashMap will allow only one null but n number of null values.

  27. Bharath says:

    Very Nice collection Interview questions.
    Thanks Pankaj.

  28. Pratap Shinde says:

    Great work, Pankaj

  29. vivek says:

    Please do a program by adding scanning without using add method.

  30. Bhaumik says:

    Hello sir i cant see questions answers after the 21 no questions

  31. Viswa says:

    Excellent material..A big thanks for your efforts !!

  32. Ashish Mane says:

    Grate efforts….keep it up.

  33. Arun says:

    Dear Sir;
    Greetings!!!
    I have some doubt in Collection Frame Work-
    1-Why we use interface why we did not use directly class.
    like list interface accept duplicacy and arraylist iimplement it
    then why we did not implement all the list feature in array list directly
    and what is advantage of list iterface

    1. Quang says:

      The interface useful as you want to change the implementation classes.
      Maybe this time, you’re using ArrayList and in five years I assume that your business will be upgraded so you have to replace ArrayList by LinkedList. If you use the implementation classes in function arguments , you have to replace all of them but if you use the interface, you dont need to update.
      Let see example below:

      void getInfo(List b) -> you can pass any implementation classes
      void getInfo(ArrayList b) -> you can only pass ArrayList class.

  34. RAHUL SAXENA says:

    Can you please explain in concurrent collection why iterator does not throw concurrent modification exception.You have written that it works on the clone of that object.

    So please explain it deeply like where this clone is maintained and what operations are performed in the memory??

    1. Ankita says:

      In concurrent collection like CopyOnWriteArrayList and CopyOnWriteArrayset, for every update operation a cloned copy of original object is created and the update operation is performed on that cloned copy rather than original copy.
      Later on this cloned copy and original copy object are synched by JVM internally to perform the update.Since the update is performed on cloned copy rather than original copy so we don’t get concurrentModifationException.
      Also if we have many write operations it is not recommended to use this concurrent class becoz for every update one cloned copy is created and performance degrades.

  35. Vishwadeep Aggarwal says:

    Could you please explain Que 29. and if Collection Classes are thread safe then please explain how??

  36. Paresh Bhavsar says:

    very good Collections of questions and answers … thanx.

  37. Vishy says:

    Fine list of questions & answers..

  38. Eswaran says:

    please tell about equals and hashcode method deeply

  39. Vinod Karathiya says:

    Really Nice work, As experienced person I don’t have these much knowledge. Again Thanks a lot.

  40. Sharmi says:

    Please use the language short and precise. I read a question “Why Collection doesn’t extend Cloneable and Serializable interfaces?” it took me 5 minutes to understand what exactly are you trying to say.

  41. dakshina gandikota says:

    In Question#20

    Line 12: myHashMap.get(new MyKey(“Pankaj”));

    If Line 12: were to be

    myHashMap.get(key);

    would the “theory” that mutable class as key is a bad idea hold?

    By the way, the definition of mutable is up in the air. Is it a final class with final methods and fields?

  42. dakshina gandikota says:

    Questions 14 and 30 contradict each other wrt ConcurrentHashMap.

    1. Pankaj says:

      No it was not contradicting, however answer in Q30 was confusing. I have modified it and it should be clear now.

      1. dakshina gandikota says:

        Thanks. Why not Vector for random access of its contents, along side ArrayList, HashMap, TreeMap and Hashtable?

  43. Anoop says:

    Correction: “If we have to iterate over the Map in order of insertion, we need to use TreeMap”. Its LinkedHashMap.

    1. Vu Le says:

      If we have to iterate over the Map in order of insertion, we need to use LinkedHashMap. With TreeMap if you need to iterate by sorted order

  44. kiran says:

    Great Job sir….. Thank u

  45. Pabel Lopez says:

    It will be very helpful to have an image of the main collections (collection, set, list, Queue and map) sometimes is easier to watch the image as a tree than reading the description.

    Thank you.

  46. sampath kumar madala says:

    Q: 30– ” By design iterator is fail fast and throws ConcurrentModificationException” Is this statement conflicts with explanation in Q:14 !!!

    1. Pankaj says:

      Don’t get confused, both are right explanation. Iterator is just an interface and their corresponding implementation are either fail-fast or fail-safe.

      All the iterator implementation in java.util package collection classes are fail-fast whereas collection classes in java.util.concurrent implement fail-safe iteraator.

  47. Prakash says:

    What is ThreadLocal? Pleas explain with some realtime example?

  48. Jagadeesh says:

    HI Pankaj,

    I great to see this site, It is very useful for Interviews, But here I found only 21 questions and answers,Can u post all the answers?

    Thanks a lot

  49. Dharmendra says:

    Very good collection of Collection questions 🙂

  50. Danny says:

    each and every question is very nicely explained, good job!

  51. Aditya Peshave says:

    Hi,

    really nice tutorial.

    Just a correction:

    the default capacity of HashMap is 16.

    reference:
    http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/HashMap.java#HashMap.%3Cinit%3E%28%29

  52. Karl says:

    Very good collection of Collections interview questions.

  53. Karl says:

    Very good Collections interview questions!

  54. Amit says:

    wow! Really very nice articles

  55. vinita says:

    good collection of question….thanx

  56. pk says:

    gud collection of questions

  57. sudheer says:

    awesome posts by you pankaj…thanks a lot.

  58. Arti says:

    I am not able to view all the questions though i am following you on twitter. I was able to view all the 40 questions for some days but not now. Please check.

  59. chandan says:

    Thanks Pankaj…Excellent post. Please also add answers for questions from 22 onwards.

  60. Abhay says:

    Hi Pankaj,

    You have not added answers to all questions listed above ? Please provide answers also if possible.
    Your answers are easy to understand and very clear. Thanks in advance.

    1. Abhay says:

      Please ignore:)

  61. Maruthi says:

    Good collection of collections interview questions with nice explanation

  62. jkbkrk says:

    Great site! One question about no. 40:

    “If we have to iterate over the Map in order of insertion, we need to use TreeMap.”

    Shouldn’t it be:

    “If we have to iterate over the Map in order of insertion, we need to use LinkedHashMap.” or
    “If we have to iterate over the Map in natural order, we need to use TreeMap.”

    Thanks.

  63. Sateesh Kumar U says:

    Great work. Thanks alot Pankaj..!!

  64. Sushil says:

    Thank you very much.

    I deeply appreciate your efforts.

  65. nikhil sethiya says:

    great work sir…..!!

  66. nikhil sethiya says:

    Excellent Work……..!!

  67. jameer says:

    Hello, Pankaj.

    Awesome Explanation.

  68. vidhya says:

    Amazing work, thanks a lot.

  69. Narayanarao Menda says:

    It’s amazing tutorial…thank u very much..

  70. Shelly says:

    Hi Pankaj,

    Its a great article just one thing that i question 40 you mention If we have to iterate over the Map in order of insertion, we need to use TreeMap it shouldn’t be linkedhashmap instead of treemap.

    Thanks

    1. Pankaj says:

      We can use both TreeMap or LinkedHashMap.

      1. Brijeshwar says:

        Hi Pankaj,

        Insertion order is maintained only in LinkedHashMap whereas TreeMap maintains key based SortedOrder, Please correct me if I am wrong.

    2. Vu Le says:

      It should be LinkedHashMap not TreeMap, TreeMap with natural order

  71. Divya says:

    Hello Pankaj,

    Can you please explain the the question no 43 “Why can’t we create generic array” in more detail and in easy way. It’s very difficult to understand it.
    Please help me out.

    Thanks,
    Divya 🙂

    1. Pankaj says:

      Let’s get through each line in the sample code and try to understand what is happening.

      1. List<Integer>[] intList = new List<Integer>[5];

      This line will throw compile time exception now because we can’t have generic array. Just for the sake of argument, let’s assume that it’s allowed and compiler don’t throw any error.

      Since generics type information gets erased at runtime by Type Erasure, it will be like an array of List at runtime like List[] intList = new List[5];

      2. Object[] objArray = intList;

      Perfectly valid statement, creating an object array.

      3. List<Double> doubleList = new ArrayList<Double>();

      At runtime List doubleList = new ArrayList();

      4. doubleList.add(Double.valueOf(1.23));

      Valid statement, adding Double to the list.

      5. objArray[0] = doubleList;

      You see the problem here, since the objArray is referring to intList, we are basically setting doubleList to one of the elements of the array that is supposed to contain only Integer List. And it will work fine because for Array both intList and doubleList are just list, since there is no type information available.

      That’s why Generic Arrays are not supported in Java.

  72. Harsha says:

    Good one !!! Keep it up , your explanation is good and practical .

  73. Rajan says:

    Hi Pankaj,

    Thanks for such a nice detail on collections.

  74. Akhil says:

    Hi Pankaj,
    In question no 19, below line will always return null.
    myHashMap.get(new MyKey(‘Pankaj’));

    since its creating new key everytime and there is no value available for this key. Please update it,if i’m correct and one more please replace all the strings enclosed with double quotes.

    Thanks,
    Akhil

    1. Pankaj says:

      Not that is not true. As stated HashMap uses equals() and hashcode() functions to find the value in the map. So if MyKey override these methods and provide correct implementation, then the hashcode will be same as any other value stored with same hashcode and we will get it. The code given in the question is to prove that immutability is required to avoid any data integrity issues in the Map, because if we change the data hashcode is changed and we will never be able to get the associated value.

      And thanks for pointing out the quotation marks, I will change it.

  75. sam says:

    Hi Pankaj ,

    I was looking for some good interview question and found this . Its really very helpful and descriptive.
    You deserve praise for it . Excellent work Sir and keep it up !

  76. Malli says:

    Hi Pankaj,
    Thanks for given a good kind of materiel. very useful to us. I request you, please provide JSF, EJB and JPA.

  77. krishna says:

    Thanks a TON 🙂

  78. Snehal Masne says:

    Thanks a lot for this blog. I bookmarked it 🙂
    Thanks a lot buddy!!

    1. test says:

      wow gr8 that u bukmrkd it….

  79. Ganesh says:

    Good Knowledge sharing and valuable informations not only from interview aspective but also to grow technically strong.

    You are doing a fantastic job, Pls continue.

  80. Arun says:

    good work Pankaj. its a wonderful collection..

  81. siri says:

    Collections is a class it will be helpful to collection framework by providing methods.

  82. Rameshwar Singh says:

    Hi,
    Kindly help to remove extra characters from the code like “"”.

    1. siri says:

      It is collection framework not collections framework……

  83. Abhishek Singh says:

    Wow, beautyfull artical in simple language………Thanks.

  84. Naresh says:

    Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?

    if No, How?

  85. Sandeep says:

    Really helpful. keep it up 🙂

  86. Yogesh says:

    Thanks.It was really nice. Keep such good work up!

  87. G.Palanikumar says:

    Respected Sir,

    Really i proud of you. you were posted many questions always i am welcome. but company never discuss to candidate in first time they give some programs for interview time and write the output. i am expecting these kind of sample question with answer. thank you sir. if you find any mistake my words please forgive me sir. that is my suggestion thats all

    1. Pankaj says:

      It depends on the interview type, if its telephonic round then expect more questions on core java related technologies. For programming questions, until unless you have deep knowledge of core technologies, you won’t be able to answer them with confidence and answer the followup questions.

  88. Manish Dass says:

    Good collection of questions from Collections.
    Fantastic and very helpful.

  89. Amit says:

    This is really one of the best !!. Thanks a lot. Please keep sharing more.

  90. vivek says:

    Hi,
    This is regarding question 17, there you mention “if map size is greater thn threshold , hashmap rehashes the content s of hashmap into new ‘ARRAY’ with larger capacity”
    Don’t we use linkedlist for for storing hasmap, I am beginner.
    Can you please explain ?
    Regards
    Vivek

    1. Pankaj says:

      Here I am referring to the array that keeps indexes to the linked list, check the image in question 17 for better understanding.

  91. Leo says:

    Excellent post…. Really learnt lot of things in collections with full clarity….

    thanks alot..

  92. Rahul Sharma says:

    Very well written and covering almost all the interviews related questions….great job indeed!!!

  93. Tarun says:

    nice collection of quesction thank for sharing.

  94. Brijs Modi_1 says:

    Good one

  95. shashi says:

    good work very useful

  96. Rameshwar Singh says:

    Thanks a lot!!!
    This covers everything what an individual expects 🙂
    Really helpful.. keep up the good work..

    1. Brijs says:

      Good one

  97. praveen says:

    great work pankaj…. this helped me a lot

  98. Praveen Kumar says:

    Thanks a lot for this blog!! I bookmarked it..Thanks a lot buddy!!

  99. abc says:

    Very Nice …

  100. Anupam says:

    itz realy so good

  101. Genia says:

    After looking at a number of the blog posts on your blog, I seriously appreciate
    your way of blogging. I book marked it to my bookmark site
    list and will be checking back soon. Take a look at my web site as well and
    tell me your opinion.

  102. I M Futane says:

    Good Question Collect From Util Package

  103. Adinath Bhawani says:

    a good collection of Java

  104. Sushil says:

    Its a very good collection of Collection IQ.

    Keep the work going and Thank you very much for sharing

    1. Ambuj Tailang says:

      Awesome collection

  105. GNS says:

    Very good collection of Collections interview questions. …

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