What does it mean when a method is overridden?

In the previous chapter, we talked about superclasses and subclasses. If a class inherits a method from its superclass, then there is a chance to override the method provided that it is not marked final.

The benefit of overriding is: ability to define a behavior that's specific to the subclass type, which means a subclass can implement a parent class method based on its requirement.

In object-oriented terms, overriding means to override the functionality of an existing method.

Example

Let us look at an example.

class Animal {
   public void move() {
      System.out.println("Animals can move");
   }
}

class Dog extends Animal {
   public void move() {
      System.out.println("Dogs can walk and run");
   }
}

public class TestDog {

   public static void main(String args[]) {
      Animal a = new Animal();   // Animal reference and object
      Animal b = new Dog();   // Animal reference but Dog object

      a.move();   // runs the method in Animal class
      b.move();   // runs the method in Dog class
   }
}

This will produce the following result −

Output

Animals can move
Dogs can walk and run

In the above example, you can see that even though b is a type of Animal it runs the move method in the Dog class. The reason for this is: In compile time, the check is made on the reference type. However, in the runtime, JVM figures out the object type and would run the method that belongs to that particular object.

Therefore, in the above example, the program will compile properly since Animal class has the method move. Then, at the runtime, it runs the method specific for that object.

Consider the following example −

Example

class Animal {
   public void move() {
      System.out.println("Animals can move");
   }
}

class Dog extends Animal {
   public void move() {
      System.out.println("Dogs can walk and run");
   }
   public void bark() {
      System.out.println("Dogs can bark");
   }
}

public class TestDog {

   public static void main(String args[]) {
      Animal a = new Animal();   // Animal reference and object
      Animal b = new Dog();   // Animal reference but Dog object

      a.move();   // runs the method in Animal class
      b.move();   // runs the method in Dog class
      b.bark();
   }
}

This will produce the following result −

Output

TestDog.java:26: error: cannot find symbol
      b.bark();
       ^
  symbol:   method bark()
  location: variable b of type Animal
1 error

This program will throw a compile time error since b's reference type Animal doesn't have a method by the name of bark.

Rules for Method Overriding

• The argument list should be exactly the same as that of the overridden method.

• The return type should be the same or a subtype of the return type declared in the original overridden method in the superclass.

• The access level cannot be more restrictive than the overridden method's access level. For example: If the superclass method is declared public then the overridding method in the sub class cannot be either private or protected.

• Instance methods can be overridden only if they are inherited by the subclass.

• A method declared final cannot be overridden.

• A method declared static cannot be overridden but can be re-declared.

• If a method cannot be inherited, then it cannot be overridden.

• A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.

• A subclass in a different package can only override the non-final methods declared public or protected.

• An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However, the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.

• Constructors cannot be overridden.

Using the super Keyword

When invoking a superclass version of an overridden method the super keyword is used.

Example

class Animal {
   public void move() {
      System.out.println("Animals can move");
   }
}

class Dog extends Animal {
   public void move() {
      super.move();   // invokes the super class method
      System.out.println("Dogs can walk and run");
   }
}

public class TestDog {

   public static void main(String args[]) {
      Animal b = new Dog();   // Animal reference but Dog object
      b.move();   // runs the method in Dog class
   }
}

This will produce the following result −

Output

Animals can move
Dogs can walk and run

Introduction

Overriding and overloading are the core concepts in Java programming. They are the ways to implement polymorphism in our Java programs. Polymorphism is one of the OOPS Concepts.

When the method signature (name and parameters) are the same in the superclass and the child class, it’s called overriding. When two or more methods in the same class have the same name but different parameters, it’s called overloading.

Comparing overriding and overloading

Overriding and overloading example

Here is an example of overloading and overriding in a Java program:

package com.journaldev.examples;

import java.util.Arrays;

public class Processor {

	public void process(int i, int j) {
		System.out.printf("Processing two integers:%d, %d", i, j);
	}

	public void process(int[] ints) {
		System.out.println("Adding integer array:" + Arrays.toString(ints));
	}

	public void process(Object[] objs) {
		System.out.println("Adding integer array:" + Arrays.toString(objs));
	}
}

class MathProcessor extends Processor {

	@Override
	public void process(int i, int j) {
		System.out.println("Sum of integers is " + (i + j));
	}

	@Override
	public void process(int[] ints) {
		int sum = 0;
		for (int i : ints) {
			sum += i;
		}
		System.out.println("Sum of integer array elements is " + sum);
	}

}

Overriding

The process() method and int i, int j parameters in Processor are overridden in the child class MathProcessor. Line 7 and line 23:

public class Processor {

    public void process(int i, int j) { /* ... */ }

}

/* ... */

class MathProcessor extends Processor {
 
    @Override
    public void process(int i, int j) {  /* ... */ }

}

And process() method and int[] ints in Processor are also overridden in the child class. Line 11 and line 28:

public class Processor {

    public void process(int[] ints) { /* ... */ }

}

/* ... */

class MathProcessor extends Processor {

    @Override
    public void process(Object[] objs) { /* ... */ }

}

Overloading

The process() method is overloaded in the Processor class. Lines 7, 11, and 15:

public class Processor {

    public void process(int i, int j) { /* ... */ }

    public void process(int[] ints) { /* ... */ }

    public void process(Object[] objs) { /* ... */ }

}

Conclusion

In this article, we covered overriding and overloading in Java. Overriding occurs when the method signature is the same in the superclass and the child class. Overloading occurs when two or more methods in the same class have the same name but different parameters.

What happens when you override a method?

Why should a method be overridden?

Which methods are overridden?

What is method overriding give an example?