Java Generics Example Tutorial – Generic Method, Class, Interface

Filed Under: Java
Generics In Java

Java Genrics is one of the most important features introduced in Java 5.

If you have been working on Java Collections and with version 5 or higher, I am sure that you have used it.

Generics in Java with collection classes is very easy but it provides a lot more features than just creating the type of collection.

We will try to learn the features of generics in this article. Understanding generics can become confusing sometimes if we go with jargon words, so I would try to keep it simple and easy to understand.

1. Generics in Java

Generics was added in Java 5 to provide compile-time type checking and removing risk of ClassCastException that was common while working with collection classes. The whole collection framework was re-written to use generics for type-safety. Let’s see how generics help us using collection classes safely.


List list = new ArrayList();
list.add("abc");
list.add(new Integer(5)); //OK

for(Object obj : list){
	//type casting leading to ClassCastException at runtime
    String str=(String) obj; 
}

Above code compiles fine but throws ClassCastException at runtime because we are trying to cast Object in the list to String whereas one of the element is of type Integer. After Java 5, we use collection classes like below.


List<String> list1 = new ArrayList<String>(); // java 7 ? List<String> list1 = new ArrayList<>(); 
list1.add("abc");
//list1.add(new Integer(5)); //compiler error

for(String str : list1){
     //no type casting needed, avoids ClassCastException
}

Notice that at the time of list creation, we have specified that the type of elements in the list will be String. So if we try to add any other type of object in the list, the program will throw compile-time error. Also notice that in for loop, we don’t need typecasting of the element in the list, hence removing the ClassCastException at runtime.

2. Java Generic Class

We can define our own classes with generics type. A generic type is a class or interface that is parameterized over types. We use angle brackets (<>) to specify the type parameter.

To understand the benefit, let’s say we have a simple class as:


package com.journaldev.generics;

public class GenericsTypeOld {

	private Object t;

	public Object get() {
		return t;
	}

	public void set(Object t) {
		this.t = t;
	}

        public static void main(String args[]){
		GenericsTypeOld type = new GenericsTypeOld();
		type.set("Pankaj"); 
		String str = (String) type.get(); //type casting, error prone and can cause ClassCastException
	}
}

Notice that while using this class, we have to use type casting and it can produce ClassCastException at runtime. Now we will use java generic class to rewrite the same class as shown below.


package com.journaldev.generics;

public class GenericsType<T> {

	private T t;
	
	public T get(){
		return this.t;
	}
	
	public void set(T t1){
		this.t=t1;
	}
	
	public static void main(String args[]){
		GenericsType<String> type = new GenericsType<>();
		type.set("Pankaj"); //valid
		
		GenericsType type1 = new GenericsType(); //raw type
		type1.set("Pankaj"); //valid
		type1.set(10); //valid and autoboxing support
	}
}

Notice the use of GenericsType class in the main method. We don’t need to do type-casting and we can remove ClassCastException at runtime. If we don’t provide the type at the time of creation, the compiler will produce a warning that “GenericsType is a raw type.

References to generic type GenericsType<T> should be parameterized”. When we don’t provide the type, the type becomes Object and hence it’s allowing both String and Integer objects. But, we should always try to avoid this because we will have to use type casting while working on raw type that can produce runtime errors.

Tip: We can use @SuppressWarnings("rawtypes") annotation to suppress the compiler warning, check out java annotations tutorial.

Also notice that it supports java autoboxing.

3. Java Generic Interface

Comparable interface is a great example of Generics in interfaces and it’s written as:


package java.lang;
import java.util.*;

public interface Comparable<T> {
    public int compareTo(T o);
}

In similar way, we can create generic interfaces in java. We can also have multiple type parameters as in Map interface. Again we can provide parameterized value to a parameterized type also, for example new HashMap<String, List<String>>(); is valid.

4. Java Generic Type

Java Generic Type Naming convention helps us understanding code easily and having a naming convention is one of the best practices of Java programming language. So generics also comes with its own naming conventions. Usually, type parameter names are single, uppercase letters to make it easily distinguishable from java variables. The most commonly used type parameter names are:

  • E – Element (used extensively by the Java Collections Framework, for example ArrayList, Set etc.)
  • K – Key (Used in Map)
  • N – Number
  • T – Type
  • V – Value (Used in Map)
  • S,U,V etc. – 2nd, 3rd, 4th types

5. Java Generic Method

Sometimes we don’t want the whole class to be parameterized, in that case, we can create java generics method. Since the constructor is a special kind of method, we can use generics type in constructors too.

Here is a class showing an example of a java generic method.


package com.journaldev.generics;

public class GenericsMethods {

	//Java Generic Method
	public static <T> boolean isEqual(GenericsType<T> g1, GenericsType<T> g2){
		return g1.get().equals(g2.get());
	}
	
	public static void main(String args[]){
		GenericsType<String> g1 = new GenericsType<>();
		g1.set("Pankaj");
		
		GenericsType<String> g2 = new GenericsType<>();
		g2.set("Pankaj");
		
		boolean isEqual = GenericsMethods.<String>isEqual(g1, g2);
		//above statement can be written simply as
		isEqual = GenericsMethods.isEqual(g1, g2);
		//This feature, known as type inference, allows you to invoke a generic method as an ordinary method, without specifying a type between angle brackets.
		//Compiler will infer the type that is needed
	}
}

Notice the isEqual method signature showing syntax to use generics type in methods. Also, notice how to use these methods in our java program. We can specify type while calling these methods or we can invoke them like a normal method. Java compiler is smart enough to determine the type of variable to be used, this facility is called type inference.

6. Java Generics Bounded Type Parameters

Suppose we want to restrict the type of objects that can be used in the parameterized type, for example in a method that compares two objects and we want to make sure that the accepted objects are Comparables. To declare a bounded type parameter, list the type parameter’s name, followed by the extends keyword, followed by its upper bound, similar like below method.


public static <T extends Comparable<T>> int compare(T t1, T t2){
		return t1.compareTo(t2);
	}

The invocation of these methods is similar to unbounded method except that if we will try to use any class that is not Comparable, it will throw compile-time error.

Bounded type parameters can be used with methods as well as classes and interfaces.

Java Generics supports multiple bounds also, i.e <T extends A & B & C>. In this case, A can be an interface or class. If A is class then B and C should be an interface. We can’t have more than one class in multiple bounds.

7. Java Generics and Inheritance

We know that Java inheritance allows us to assign a variable A to another variable B if A is subclass of B. So we might think that any generic type of A can be assigned to generic type of B, but it’s not the case. Let’s see this with a simple program.


package com.journaldev.generics;

public class GenericsInheritance {

	public static void main(String[] args) {
		String str = "abc";
		Object obj = new Object();
		obj=str; // works because String is-a Object, inheritance in java
		
		MyClass<String> myClass1 = new MyClass<String>();
		MyClass<Object> myClass2 = new MyClass<Object>();
		//myClass2=myClass1; // compilation error since MyClass<String> is not a MyClass<Object>
		obj = myClass1; // MyClass<T> parent is Object
	}
	
	public static class MyClass<T>{}

}

We are not allowed to assign MyClass<String> variable to MyClass<Object> variable because they are not related, in fact MyClass<T> parent is Object.

8. Java Generic Classes and Subtyping

We can subtype a generic class or interface by extending or implementing it. The relationship between the type parameters of one class or interface and the type parameters of another are determined by the extends and implements clauses.

For example, ArrayList<E> implements List<E> that extends Collection<E>, so ArrayList<String> is a subtype of List<String> and List<String> is subtype of Collection<String>.

The subtyping relationship is preserved as long as we don’t change the type argument, below shows an example of multiple type parameters.


interface MyList<E,T> extends List<E>{
}

The subtypes of List<String> can be MyList<String,Object>, MyList<String,Integer> and so on.

9. Java Generics Wildcards

Question mark (?) is the wildcard in generics and represent an unknown type. The wildcard can be used as the type of a parameter, field, or local variable and sometimes as a return type. We can’t use wildcards while invoking a generic method or instantiating a generic class. In the following sections, we will learn about upper bounded wildcards, lower bounded wildcards, and wildcard capture.

9.1) Java Generics Upper Bounded Wildcard

Upper bounded wildcards are used to relax the restriction on the type of variable in a method. Suppose we want to write a method that will return the sum of numbers in the list, so our implementation will be something like this.


public static double sum(List<Number> list){
		double sum = 0;
		for(Number n : list){
			sum += n.doubleValue();
		}
		return sum;
	}

Now the problem with above implementation is that it won’t work with List of Integers or Doubles because we know that List<Integer> and List<Double> are not related, this is when an upper bounded wildcard is helpful. We use generics wildcard with extends keyword and the upper bound class or interface that will allow us to pass argument of upper bound or it’s subclasses types.

The above implementation can be modified like the below program.


package com.journaldev.generics;

import java.util.ArrayList;
import java.util.List;

public class GenericsWildcards {

	public static void main(String[] args) {
		List<Integer> ints = new ArrayList<>();
		ints.add(3); ints.add(5); ints.add(10);
		double sum = sum(ints);
		System.out.println("Sum of ints="+sum);
	}

	public static double sum(List<? extends Number> list){
		double sum = 0;
		for(Number n : list){
			sum += n.doubleValue();
		}
		return sum;
	}
}

It’s similar like writing our code in terms of interface, in the above method we can use all the methods of upper bound class Number. Note that with upper bounded list, we are not allowed to add any object to the list except null. If we will try to add an element to the list inside the sum method, the program won’t compile.

9.2) Java Generics Unbounded Wildcard

Sometimes we have a situation where we want our generic method to be working with all types, in this case, an unbounded wildcard can be used. Its same as using <? extends Object>.


public static void printData(List<?> list){
		for(Object obj : list){
			System.out.print(obj + "::");
		}
	}

We can provide List<String> or List<Integer> or any other type of Object list argument to the printData method. Similar to upper bound list, we are not allowed to add anything to the list.

9.3) Java Generics Lower bounded Wildcard

Suppose we want to add Integers to a list of integers in a method, we can keep the argument type as List<Integer> but it will be tied up with Integers whereas List<Number> and List<Object> can also hold integers, so we can use a lower bound wildcard to achieve this. We use generics wildcard (?) with super keyword and lower bound class to achieve this.

We can pass lower bound or any supertype of lower bound as an argument, in this case, java compiler allows to add lower bound object types to the list.


public static void addIntegers(List<? super Integer> list){
		list.add(new Integer(50));
	}

10. Subtyping using Generics Wildcard


List<? extends Integer> intList = new ArrayList<>();
List<? extends Number>  numList = intList;  // OK. List<? extends Integer> is a subtype of List<? extends Number>

11. Java Generics Type Erasure

Generics in Java was added to provide type-checking at compile time and it has no use at run time, so java compiler uses type erasure feature to remove all the generics type checking code in byte code and insert type-casting if necessary. Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead.

For example, if we have a generic class like below;


public class Test<T extends Comparable<T>> {

    private T data;
    private Test<T> next;

    public Test(T d, Test<T> n) {
        this.data = d;
        this.next = n;
    }

    public T getData() { return this.data; }
}

The Java compiler replaces the bounded type parameter T with the first bound interface, Comparable, as below code:


public class Test {

    private Comparable data;
    private Test next;

    public Node(Comparable d, Test n) {
        this.data = d;
        this.next = n;
    }

    public Comparable getData() { return data; }
}

12. Generics FAQs

12.1) Why do we use Generics in Java?

Generics provide strong compile-time type checking and reduces risk of ClassCastException and explicit casting of objects.

12.2) What is T in Generics?

We use <T> to create a generic class, interface, and method. The T is replaced with the actual type when we use it.

12.3) How does Generics work in Java?

Generic code ensures type safety. The compiler uses type-erasure to remove all type parameters at the compile time to reduce the overload at runtime.

13. Generics in Java – Further Readings

That’s all for generics in java, java generics is a really vast topic and requires a lot of time to understand and use it effectively. This post here is an attempt to provide basic details of generics and how can we use it to extend our program with type-safety.

Comments

  1. Roberto says:

    Lost from “Java Generic Method”…..

  2. ranjit kumar yadav says:

    public class GenericsTypeOld {

    private Object t;

    public Object get() {
    return t;
    }

    public void set(Object t) {
    this.t = t;
    }

    public static void main(String args[]){
    GenericsTypeOld type = new GenericsTypeOld();
    type.set(“Pankaj”);
    112 //String str = (String) type.get(); //type casting, error prone and can cause ClassCastException
    }

    I think line number 112 is not giving erroe

    1. Venkata Rao says:

      Your right it does not give error because type.set(“Pankaj”) pankaj name is type String, It is type casting to String. ;

    2. Balaji says:

      It won’t throw error

  3. Gaurav says:

    Hi Pankaj,

    quite amid follower of your tutorials. Short but well explained topics. However, felt some topics are missing in this article. Like PECS of generics, how shall we use them. or maybe it’s me only who is finding it difficult.

  4. priya says:

    Generics forces the java programmer to store specific type of objects. The language is very easy to understand. Thanks for sharing.

  5. Yash Rathore says:

    sir plz muje bato ki generics methode me hum 2 object ko jo generics h ko “+” operant se add kyo nahi kr pa rahe.

  6. Raj Gopal Bhallamudi says:

    interface Parent {
    }

    interface Child extends Parent {

    }

    public class Subtyping implements Child {

    T tobj;
    U uobj;

    public Subtyping(T t, U u) {
    tobj = t;
    uobj = u;
    }

    public static void main(String[] args) {

    Parent obj = new Subtyping(4, “raj”);
    Parent obj1 = new Subtyping(4, 40.0);

    /*
    * The subtypes of Parent can be
    * Subtyping , Subtyping and so on.
    * but not Subtyping
    *
    * this statement will give error
    * Parent obj2 = new Subtyping( “raj”,4);
    */

    System.out.println(obj);
    System.out.println(obj1);

    }

    @Override
    public String toString() {
    return ” t= ” + tobj + ” ” + uobj;
    }

    }

  7. Raj Gopal Bhallamudi says:

    An example for Java Generic Classes and Subtyping .

    interface Parent {
    }

    interface Child extends Parent {

    }

    public class Subtyping implements Child {

    T tobj;
    U uobj;

    public Subtyping(T t, U u) {
    tobj = t;
    uobj = u;
    }

    public static void main(String[] args) {

    Parent obj = new Subtyping(4, “raj”);
    Parent obj1 = new Subtyping(4, 40.0);

    /*
    * The subtypes of Parent can be
    * Subtyping , Subtyping and so on.
    * but not Subtyping
    *
    * this statement will give error
    * Parent obj2 = new Subtyping( “raj”,4);
    */

    System.out.println(obj);
    System.out.println(obj1);

    }

    @Override
    public String toString() {
    return ” t= ” + tobj + ” ” + uobj;
    }

    }

  8. swathi says:

    public class GenTest
    {
    T t;
    public void setType(T t)
    {
    this.t=t;
    }
    public T getType()
    {
    return t;
    }
    public static void main(String[] arg)
    {
    GenTest gt=new GenTest();
    Kamala k=new Kamala();
    gt.setType(k);
    //Object o=gt.getType();
    System.out.println(gt.getType().aboutu());
    }
    }

    //kamala class
    public class Kamala
    {
    public void aboutu()
    {
    System.out.println(“I am kamala ! i am helping class to check generic concept”);
    }
    }

    Sir above program i am getting error: ‘ void’ type not allowed here

    1. shubham says:

      1. First of all while using T t in GenTest you need to declare T as type parameter as class GenTest.
      2. While instantiating the generic class you must have used GenTest gt = new GenTest();
      3. calling aboutu in sysout – no need to use System.out.println(gt.getType().aboutu());
      use gt.getType().aboutu() as method return type is void.

      Hope this solves your problem and I recommend you to study the generics post thoroughly.

  9. Shafali says:

    Thanks for the nice article.

    I couldnt understand Raw & Unbounded generics.
    What is the difference in below?

    Set setOfRawType = new HashSet();
    setOfRawType = new HashSet();
    Set setOfUnknownType = new HashSet();
    setOfUnknownType = new HashSet();

    Please explain its very urgent.

    TIA!

    1. GOPINATH M B says:

      1. a raw type (Set) treats the type as if it had no generic type information at all. Note the subtle effect that not only will the type argument T be ignored, but also all other type arguments that methods of that type might have. You can add any value to it and it will always return Object.
      2. Set is a Set that accepts all Object objects (i.e. all objects) and will return objects of type Object.
      3. Set is a Set that accepts all objects of some specific, but unknown type and will return objects of that type. Since nothing is known about this type, you can’t add anything to that set (except for null) and the only thing that you know about the values it returns is that they are some sub-type of Object.

      http://stackoverflow.com/a/7360664

  10. Shashikumar says:

    Hi..
    Have a doubt. Could you please help?

    Three classes are there. A,B & C. Both B & C extends A.
    So in generics how can I make the class C not to be included. It should allow only B

    Thanks,
    Shashi

  11. Rashmi says:

    Hi,

    I have a question what it exactly means?

  12. sairam says:

    really helpfull thanks

  13. Abhinav Joshi says:

    Sir as I was going through your tutorial. I got stuck at a point

    public Object get() {
    return t;
    }

    public void set(Object t) {
    this.t = t;

    As I have seen in getter(), setter() methord, we write in such a way…

    public Object getT() {
    return t;
    }

    public void setT(Object t) {
    this.t = t;

    What you have done is OK for 1 variable???

    1. Corey says:

      public Object getT() {
      return t;
      }

      public void setT(Object t) {
      this.t = t;

      Should be …

      public T getT() {
      return t;
      }

      public void setT(T t) {
      this.t = t;

      This allows any type. If you do it how you had it originally, you’re just making 2 Object type getters/setters with different names.

      1. rohit bhardwaj says:

        this program gets error bro

  14. Satpal Singh says:

    How converting int to String is autoboxing in the Class generic example given above.
    type1.set(10); //valid and autoboxing support

  15. anonymous says:

    good explanation

  16. Very good explanation… Thanks

  17. sony says:

    great explanation!!!

  18. Abbas says:

    I was looking for this:
    Thanks 🙂

  19. Manish Verma says:

    Excellent article. I am wondering for simple cases, is there a reason to use Wildcard instead of generics?
    e.g. The printData method in the “Generics Unbounded Wildcard” could have been written like that?

    public static void printData(List list) {
    for(T obj : list) {
    System.out.print(obj + “::”);
    }
    }

    And we don’t have the limitations of wildcard either (add method not allowed). So, is there any benefit of using wildcard in such cases?

  20. SUDIP GHOSH says:

    Hi Pankaj,

    Your explanations and sample programs are very easy to understand.

    I have a query regarding generics.

    A sample progam:

    package main.test;

    import java.util.ArrayList;
    import java.util.List;

    public class MainClass {

    /**
    * @param args
    */
    public static void main(String[] args) {

    List list = new ArrayList();
    list.add(8);
    list.add(7);
    printList1(list);
    printList2(list);
    printList3(list);
    printList4(list);

    }

    /**
    * Using Generics Bounded Type Parameters
    * @param list
    */
    public static void printList1(List list){
    System.out.println(list);
    }

    /**
    * Using Generics Upper Bounded Wildcard
    * @param list
    */
    public static void printList2(List list){
    System.out.println(list);
    }

    /**
    * Using Simple Generics Method
    * @param list
    */
    public static void printList3(List list){
    System.out.println(list);
    }

    /**
    * Using Generics Unbounded Wildcard
    * @param list
    */
    public static void printList4(List list){
    System.out.println(list);
    }

    }

    There are 4 methods printing a list:
    printList1: Using Generics Bounded Type Parameters
    printList2: Using Generics Upper Bounded Wildcard
    printList3: Using Simple Generics Method
    printList4: Using Generics Unbounded Wildcard

    I have two questions:
    1) When to use Generics Upper Bounded Wildcard? Because the same thing can be done using Generics Bounded Type Parameters?(methods: printList1 and printList2)
    I mean how to decide which one to use between these two?

    2) Similar Question regarding method: printList3 and printList4.When to use the Generics Unbounded Wildcard?Because the same thing can be done using Using Simple Generics Method.

    1. SUDIP GHOSH says:

      The generics syntax(angel bracket) are not coming while I’m pasting it. 🙁

    2. Pankaj says:

      Generic Upper/Lower Bounded Wildcard and Bounded Type parameters are similar, however bounded type parameters help us in writing generic algorithms, for example in above code, we are using bounded type parameters with Comparable.

  21. Amishi Shah says:

    Could you please explain what is that which can be done in a generic class but not in a normal java class without generics?

    1. Finishing School says:

      Firstly u need to understand that GENERIC in java represents a General Model( be it a method or a class).

      Secondly, by creating generic class u r creating a General Class which can operate on any type of Object.

      Thirdly, without using generics u need to create separate class for supporting various types of objects.

      thus use of generic class helps u in reusability of code i.e less LOC & Compile time Type Checking.

      thanks
      Finishing School

      1. Amishi Shah says:

        Thanks a lot.

  22. viswam says:

    Nice explanation, easy to understand

    Thanks for the post.

  23. laukendra says:

    Awesome man 🙂

  24. Anup says:

    Awesome work!
    Really informative

    1. Pablo says:

      Thank you very much, really well explained.

  25. Mayur Bote says:

    How can I use binary operators in generics ???

  26. smokes says:

    Tnx man.
    really great explanation for beginners.
    Much appreciated.

  27. Ashish says:

    Dear sir i have a query regarding generics plz solve this…
    I have a generic super class Employee and this super class have two sub class Admin and Finance…so tell me can i use Admin data in Finance sub class and Finance data in Admin sub class.

    1. Pankaj says:

      This is not generics, its inheritance. Think of it like Integer and String class, both have super class as Object. Can you assign Integer to String and vice versa, NO.

  28. Dev says:

    Hi
    Respected Sir

    My expectation is growing high after getting answer from you and solving my previous problem.

    I face another problem while using java generic in my program.

    The problem is i have to GROUP BY a ganeric list based on one or two field.

    Suppose i have a POJO class named StationInformation with CODE,STATION,NAME,ADDRESS
    i have created a list ArrayList StationInformationList= new ArrayList

    and populate this list with station data.

    After some operation with this list object i have to GROUP BY this list based on CODE field and after that CODE and STATION field. This GROUP BY should be same as GROUP BY in database.
    There is no way to go database from here because data is coming from api.

    This is very essential to me and i have to resolve it as soon as possible.

    I hope i’ll get another feedback from you with solution my problem.
    This GROUP BY is vary urgent and please give me a solution.

    I am waiting for your answer/reply.

    Please please sir give me a solution.

    If possible please give me code example.

  29. Siddu says:

    What are the drawbacks of Generics?

  30. Dev says:

    Thanks a lot. I made this change in my code and this is working correctly.

  31. Mohsen says:

    Thanks for this great article … it was very helpful for me

  32. Dev says:

    I want to create xlsx file from a ArrayList.
    TableBean means a class with some fields and its getter and setter.

    I have three (3) list
    1. ArrayList CompanyBeanList (CompanyBean has 4 fileds with getter – setter)
    2.ArrayList EmployeeBeanList (EmployeeBean has 10 fileds with getter – setter)
    3.ArrayList ClientBeanList (ClientBeanBean has 8 fileds with getter – setter)

    I want to call exportToExcel method as exportToExcel(“D:\Back Up\PROJECT\”,CompanyBeanList ,”CompanyBean”)
    and exportToExcel(“D:\Back Up\PROJECT\”,EmployeeBeanList ,”EmployeeBean”)
    and exportToExcel(“D:\Back Up\PROJECT\”,ClientBeanList ,”ClientBean”)

    exportToExcel is a method which create an .xlsx file in the specified path with data from the beanList (2 nd parameter)

    But problem is i can not pass these 3 different type of list in exportToExcel.
    Error says The method exportToExcel(String, ArrayList, String) in the type Utility is not applicable for the arguments (String, List, String) (same for other two class)

    I do not want to write BeanClass name in parameter. it should be parametrized.

    public static void exportToExcel(String outputPath , ArrayList beanClassName,String sheetName)
    {
    try{

    Map data = new TreeMap();
    XSSFWorkbook workbook = new XSSFWorkbook();
    XSSFSheet sheet = workbook.createSheet(sheetName);

    Object[] colName =new Object[20];
    if(beanClassName.size()>0)
    {
    Field[] field=beanClassName.get(0).getClass().getDeclaredFields();
    for(int i=0;i<field.length;i++)
    {
    colName[i]=field[i].getName().toUpperCase();
    }
    data.put("1", colName);

    for(int i=0;i<beanClassName.size();i++)
    {
    Object[] colValue =new Object[20];
    Field[] field1=beanClassName.get(0).getClass().getDeclaredFields();
    for(int f=0;i<field1.length;i++)
    {
    Method method=null;
    try {
    method=beanClassName.get(i).getClass().getMethod("get"+field1[f].getName().toUpperCase(), null);
    }catch (IllegalArgumentException e) {
    log.error("User: "+getUserName()+" : "+"exportToExcel..1.."+e.getMessage());
    e.printStackTrace();
    } catch (Exception e) {
    log.error("User: "+getUserName()+" : "+"exportToExcel..2…."+e.getMessage());
    e.printStackTrace();
    }
    colValue[f]=method.invoke(beanClassName.get(i), null);
    }
    data.put((i+2)+"", colValue);

    }
    Set keyset = data.keySet();
    int rownum = 0;
    for (String key : keyset)
    {
    Row row = sheet.createRow(rownum++);
    Object [] objArr = data.get(key);
    int cellnum = 0;
    for (Object obj : objArr)
    {
    Cell cell = row.createCell(cellnum++);
    if(obj instanceof String)
    cell.setCellValue((String)obj);
    else if(obj instanceof Integer)
    cell.setCellValue((Integer)obj);
    else if(obj instanceof Double)
    cell.setCellValue((Double)obj);
    else if(obj instanceof Float)
    cell.setCellValue((Float)obj);
    else if(obj instanceof Number)
    cell.setCellValue((Double)obj);
    }
    }
    try
    {
    //Write the workbook in file system
    FileOutputStream out = new FileOutputStream(new File(outputPath+File.separator+”OrderCounts.xlsx”));
    workbook.write(out);
    out.close();

    }
    catch (Exception e)
    {
    e.printStackTrace();
    }

    }
    else
    {
    return;
    }

    }catch (Exception e) {
    log.error(“User: “+getUserName()+” : “+”exportToExcel……”+e.getMessage());
    e.printStackTrace();
    }
    }

    I only want to call
    exportToExcel(“D:\Back Up\PROJECT\”,CompanyBeanList ,”CompanyBean”)
    and exportToExcel(“D:\Back Up\PROJECT\”,EmployeeBeanList ,”EmployeeBean”)
    and exportToExcel(“D:\Back Up\PROJECT\”,ClientBeanList ,”ClientBean”)

    and it will create .xlsx file in the path. That is my main aim.

    Is it possible.??
    Which modification is necessary in exportToExcel method. ???
    If solution is known to you please write / modify exportToExcel method as you wish and reply me over mail or post in this site.

    It is very urgent to me. I am waiting for your reply.

    I hope i’ll get a suitable solution from you.I believe on you.

    1. Pankaj says:

      its simple, the error is coming because your method argument is expecting ArrayList but you might be creating at client side as List myList = new ArrayList();.

      All you need to do is change the method argument type to List and you should be good to go.

  33. RANJITH KUMAR says:

    i want usecase digram for generics vs collection, its for my job,please give quickly

  34. varun says:

    List str=null;
    str.add(“varun”);
    str.add(new Integer(10), str);

    im getting compile time error..plz give me solution .how to add string and integer both in arrylist ???

    1. Pankaj says:

      Create list of Object and then you can add any type of Object. In above code snippet str is not initialized.

  35. sriram says:

    hi, I want to define my arraylist which allows to insert only class A,B,C .How to achieve in genrics

    1. Pankaj says:

      The classes should have something in common, if nothing specified then Object is the common parent class that we can use to define the type of list.

  36. Sarah says:

    hi! i want to devlop my future in java ,i want to learn it i am beginner no from whrere do i start please help me!!!

    1. Pankaj says:

      Start with core java and then move on to J2EE technologies.

  37. Avinash says:

    Hi

    Thanks for the very informative article..

    However could you please explain the difference between:

    public static void printData(List list){
    for(Object obj : list){
    System.out.print(obj + “::”);
    }
    }

    And

    public static void printData(List list){
    for(Object obj : list){
    System.out.print(obj + “::”);
    }
    }

    1. Pankaj says:

      I think both the methods are same, guessing some typo.

Leave a Reply

Your email address will not be published. Required fields are marked *

close
Generic selectors
Exact matches only
Search in title
Search in content
Search in posts
Search in pages