Inheritance in Java Example

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

Inheritance in Java is the method to create a hierarchy between classes by inheriting from other classes. Inheritance in java is transitive, so if Sedan extends class Car, then Sedan is also inherited from Vehicle class. 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.

In java, every class implicitly extends java.lang.Object class, so Object class is at the top level of inheritance hierarchy in java.

Let’s see how we implement inheritance in java with simple example:

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;
	}

}

Animal is the base class here, now lets create a class Cat which inherits from Animal class.

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;
	}

}

extends is the java keyword used to implement inheritance in java.

Now let’s write a simple test class to create Cat object and use some of it’s 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());
	}

}

Output of the above program is:

Cat is Vegetarian?false
Cat eats milk
Cat has 4 legs.
Cat color is black

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

Important Points for Inheritance in Java

  • Most important benefit of inheritance is code reuse because subclasses inherits the variables and methods of superclass.
  • 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.
  • Superclass members with default access is accessible to subclass ONLY if they are in same package.
  • Superclass constructors are not inherited by subclass.
  • 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.
  • Java doesn’t support multiple inheritance, a subclass can extends only one class. So here Animal is implicitly extending Object class and Cat is extending Animal class but due to java inheritance transitive nature, Cat class also extends Object class.
  • 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
    
  • 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
    
  • We can override the method of Superclass in the Subclass. However we should always annotate overridden method with @Override annotation so that Compiler knows that we are overriding a method and if something changes in the superclass method, we get to know at compile time rather than getting unwanted results at the runtime.
  • We can call the superclass methods and access superclass variables using super keyword. It comes handy when we have a 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.
  • We can use instanceof instruction to check the inheritance between objects, lets see this with below example.
    Cat c = new Cat();
    Dog d = new Dog();
    Animal a = c;
    
    boolean flag = c instanceof Cat; //normal case, returns true
    
    flag = c instanceof Animal; // returns true since c is-an Animal too
    
    flag = a instanceof Cat; //returns true because a is of type Cat at runtime
    
    flag = a instanceof Dog; //returns false for obvious reasons.
    
  • We can’t extend Final classes in java.
  • 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.

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


Leave a Reply

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