Inheritance in Java Example

Filed Under: Java

Inheritance in java is one of the core concepts of Object Oriented Programming. Java Inheritance is used when we have is-a relationship between objects. Inheritance in Java is implemented using extends keyword.

Inheritance in Java

Inheritance in Java is the method to create a hierarchy between classes by inheriting from other classes.

Java Inheritance is transitive – so if Sedan extends Car and Car extends Vehicle, then Sedan is also inherited from Vehicle class. The Vehicle becomes the superclass of both Car and Sedan.

Inheritance is widely used in java applications, for example extending Exception class to create an application specific Exception class that contains more information like error codes. For example NullPointerException.

Java Inheritance Example

Every class in java implicitly extends java.lang.Object class. So Object class is at the top level of inheritance hierarchy in java.

Let’s see how to implement inheritance in java with a simple example.

Superclass: Animal


package com.journaldev.inheritance;

public class Animal {

	private boolean vegetarian;

	private String eats;

	private int noOfLegs;

	public Animal(){}

	public Animal(boolean veg, String food, int legs){
		this.vegetarian = veg;
		this.eats = food;
		this.noOfLegs = legs;
	}

	public boolean isVegetarian() {
		return vegetarian;
	}

	public void setVegetarian(boolean vegetarian) {
		this.vegetarian = vegetarian;
	}

	public String getEats() {
		return eats;
	}

	public void setEats(String eats) {
		this.eats = eats;
	}

	public int getNoOfLegs() {
		return noOfLegs;
	}

	public void setNoOfLegs(int noOfLegs) {
		this.noOfLegs = noOfLegs;
	}

}

The Animal is the base class here. Let’s create a Cat class that inherits from Animal class.

Subclass: Cat


package com.journaldev.inheritance;

public class Cat extends Animal{

	private String color;

	public Cat(boolean veg, String food, int legs) {
		super(veg, food, legs);
		this.color="White";
	}

	public Cat(boolean veg, String food, int legs, String color){
		super(veg, food, legs);
		this.color=color;
	}

	public String getColor() {
		return color;
	}

	public void setColor(String color) {
		this.color = color;
	}

}

Notice that we are using extends keyword to implement inheritance in java.

Java Inheritance Test Program

Let’s write a simple test class to create Cat object and use some of its methods.


package com.journaldev.inheritance;

public class AnimalInheritanceTest {

	public static void main(String[] args) {
		Cat cat = new Cat(false, "milk", 4, "black");

		System.out.println("Cat is Vegetarian?" + cat.isVegetarian());
		System.out.println("Cat eats " + cat.getEats());
		System.out.println("Cat has " + cat.getNoOfLegs() + " legs.");
		System.out.println("Cat color is " + cat.getColor());
	}

}

Java Inheritance Program Output

Output of the above program is shown in below image.

inheritance in java, java inheritance example

Cat class doesn’t have getEats() method but still it works because it’s inherited from Animal class.

Java Inheritance Important Points

  1. Code reuse is the most important benefit of inheritance because subclasses inherits the variables and methods of superclass.
  2. Private members of superclass are not directly accessible to subclass. As in this example, Animal variable noOfLegs is not accessible to Cat class but it can be indirectly accessible via getter and setter methods.
  3. Superclass members with default access is accessible to subclass ONLY if they are in same package.
  4. Superclass constructors are not inherited by subclass.
  5. If superclass doesn’t have default constructor, then subclass also needs to have an explicit constructor defined. Else it will throw compile time exception. In the subclass constructor, call to superclass constructor is mandatory in this case and it should be the first statement in the subclass constructor.
  6. Java doesn’t support multiple inheritance, a subclass can extends only one class. Animal class is implicitly extending Object class and Cat is extending Animal class but due to java inheritance transitive nature, Cat class also extends Object class.
  7. We can create an instance of subclass and then assign it to superclass variable, this is called upcasting. Below is a simple example of upcasting:
    
    Cat c = new Cat(); //subclass instance
    Animal a = c; //upcasting, it's fine since Cat is also an Animal
    
  8. When an instance of Superclass is assigned to a Subclass variable, then it’s called downcasting. We need to explicitly cast this to Subclass. For example;
    
    Cat c = new Cat();
    Animal a = c;
    Cat c1 = (Cat) a; //explicit casting, works fine because "c" is actually of type Cat
    

    Note that Compiler won’t complain even if we are doing it wrong, because of explicit casting. Below are some of the cases where it will throw ClassCastException at runtime.

    
    Dog d = new Dog();
    Animal a = d;
    Cat c1 = (Cat) a; //ClassCastException at runtime
    
    Animal a1 = new Animal();
    Cat c2 = (Cat) a1; //ClassCastException because a1 is actually of type Animal at runtime
    
  9. We can override the method of Superclass in the Subclass. However we should always annotate overridden method with @Override annotation. The compiler will know that we are overriding a method and if something changes in the superclass method, we will get a compile-time error rather than getting unwanted results at the runtime.
  10. We can call the superclass methods and access superclass variables using super keyword. It comes handy when we have the same name variable/method in the subclass but we want to access the superclass variable/method. This is also used when Constructors are defined in the superclass and subclass and we have to explicitly call the superclass constructor.
  11. We can use instanceof instruction to check the inheritance between objects, let’s see this with below example.
    
    Cat c = new Cat();
    Dog d = new Dog();
    Animal an = c;
    
    boolean flag = c instanceof Cat; //normal case, returns true
    
    flag = c instanceof Animal; // returns true since c is-an Animal too
    
    flag = an instanceof Cat; //returns true because a is of type Cat at runtime
    
    flag = an instanceof Dog; //returns false for obvious reasons.
    
  12. We can’t extend Final classes in java.
  13. If you are not going to use Superclass in the code i.e your Superclass is just a base to keep reusable code then you can keep them as Abstract class to avoid unnecessary instantiation by client classes. It will also restrict the instance creation of base class.

Java Inheritance Videos

I have recently published two videos on YouTube explaining Inheritance in detail with sample programs, you should watch them below.

You can checkout more inheritance examples from our GitHub Repository.

Reference: Oracle Documentation

Comments

  1. Pragun says:

    Just a correction in the wording:
    ////When an instance of Superclass is assigned to a Subclass variable, then it’s called downcasting. We need to explicitly cast this to Subclass.///

    The instance is of sub class which is referred by Superclass variable. I mean the object instance is of Cat type, just that it is referened by superclass variable and we want to downcast it to subclass variable.

  2. Carlos says:

    There’s a typo error at the example in point 11.

    Please, add examples at some points (let’s say: points 9 and 10, and others).

    Thanks for your help.

    1. Pankaj says:

      Thanks for noticing the typo, I have fixed it.

  3. prashant pardhi says:

    Hi Pankaj, your article are too good man, i like the detail explanation about the topic which is given by you, i also like the interview question section provided by you, thank, really appreciate your work.

  4. CHINTUL says:

    why we don’t write getter and setter method for private int noofleg… plz describe

  5. Kishan says:

    Hi Pankaj,

    Would you please brief me the below point

    “If superclass doesn’t have default constructor, then subclass also needs to have an explicit constructor defined. Else it will throw compile time exception. In the subclass constructor, call to superclass constructor is mandatory in this case and it should be the first statement in the subclass constructor.”

    As per my understanding ,Java compiler inserts a default constructor into the code during compilation and exists in .class file . Could you please explain me how can we create a super class without a default constructor.

    1. Pankaj says:

      You can always create a class without default constructor and then extend it. In that scenario you will have to explicitly write a constructor in child class. This is common if you are extending a class from third party jars and you have no control on them.

      1. Kishan says:

        Thank you Pankaj. got it
        So if a class contain No Arg and Parameterised Constructor then Default constructor is removed. In this case there is no default constructor.In that case we have to explicitly write a constructor in Child class.

  6. Evgeniy says:

    Hi, Pankaj!

    I have found a small typo in the list of important points:

    “Subclass members with default access is accessible to subclass ONLY if they are in same package.”

    Seems it has to start with “Superclass members…”

    Anyway great tutorial, thank you!

    1. Pankaj says:

      Thanks for noticing the typo error, I have corrected it.

  7. Anonymous says:

    Interesting articles on information like this is a great find. It’s like finding a treasure. I appreciate how you express your many points and share in your views. Thank you.

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