Overriding is a core concept in Object Oriented Programming as well as in Java programming language. Understanding what it is, how it works and what the rules are is very important for every Java programmer, especially those who are preparing for the OCPJP exam (exam topic: Java Class Design > Override methods).Therefore, this tutorial compiles a comprehensive summary of concepts and rules regarding overriding in the Java language, with easy-to-understand code examples.

 

1. What is Overriding in Java?

Overriding refers to the ability of a subclass to re-implement an instance method inherited from a superclass. Let’s take a look at the following class diagram:

Here, Animal is the superclass and Dog is the subclass, thus Dog inherits the move() method from Animal. However, Dog re-implements the move() method for some behaviors which are specific to only dogs (like walk and run). In this respect:

• The Dog’s move() method is called the overriding method.
• The Animal’s move() method is called the overridden method.

Basically, the overriding method must have same name and same arguments list as the overridden one. It’s the way by which a subtype extends or re-defines behaviors of its supertype.

 

2. What methods can be overridden?

Rule #1:Only inherited methods can be overridden.

Because overriding happens when a subclass re-implements a method inherited from a superclass, so only inherited methods can be overridden, that’s straightforward. That means only methods declared with the following access modifiers: public, protected and default (in the same package) can be overridden. That also means private methods cannot be overridden. Let’s see some examples:

• The Dog class overrides both the move() (public) and eat() (protected) methods from the Animal class (regardless of packages where the both classes are declared):

public class Animal {

	public void move() {
		// animal moving code...
	}

	protected void eat() {
		// animal eating code...
	}
}

public class Dog extends Animal {

	public void move() {
		// dog moving code...
	}

	protected void eat() {
		// dog eating code...
	}
}

 

• In the following example, the Dog class perfectly overrides the move() method which is declared with default access modifier in the Animal class, as long as both the classes are in the same package:

package net.codejava.core;

public class Animal {

	void move() {
		// Animal moving code...
	}
}

package net.codejava.core;

public class Dog extends Animal {

	void move() {
		// Dog moving code...
	}
}

 

• In the following example, the Dog and Animal classes are in different packages. Thus it isn’t considered an overriding because the Dog class does not inherit the Animal’s move() method:

package net.codejava.animal;

public class Animal {

	void move() {
		// Animal moving code...
	}
}

package net.codejava.dog;

import net.codejava.core.animal.Animal;

public class Dog extends Animal {

	void move() {
		// Dog moving code...
	}
}

 Here, the Dog’s move() method is just a new method, not an overriding one.

 

• In the following example, the Animal’s move() method is private, so the Dog’s move() method is just a new method, not an overriding one: 

public class Animal {

	private void move() {
		// Animal moving code...
	}
}

public class Dog extends Animal {

	public void move() {
		// Dog moving code...
	}
}

 

3. What methods that cannot be overridden?

Rule #2:Final and static methods cannot be overridden.

A final method means that it cannot be re-implemented by a subclass, thus it cannot be overridden. Consider the following example:

public class Animal {

	final void sleep() {
		// animal sleeping code...
	}
}

public class Dog extends Animal {

	public void sleep() {
		// Dog sleeping code...
	}
}

Here, the Animal’s sleep() method is marked as final, therefore the Dog class won’t compile. The compiler will complain:

error: sleep() in Dog cannot override sleep() in Animal
	public void sleep() {
	            ^
  overridden method is final
1 error

In case of static method, because a static method is available to all instances of the superclass and its subclasses, so it’s not permissible to re-implement the static method in a particular subclass. Consider the following example:

public class Animal {

	static void sleep() {
		// animal sleeping code...
	}
}

public class Dog extends Animal {

	public void sleep() {
		// Dog sleeping code...
	}
}

The compiler will issue the following complaint when trying to compile the Dog class:

error: sleep() in Dog cannot override sleep() in Animal
	public void sleep() {
	            ^
  overridden method is static
1 error

 

4. Requirements for the overriding method

With respect to the overridden method, the overriding method must obey the following rules:

Rule #3: The overriding method must have same argument list.

Let’s see the following example:

public class Animal {

	protected void eat(String food) {
		// animal eating code...
	}
}

public class Dog extends Animal {

	protected void eat(String food) {
		// dog eating code...
	}
}

The eat() method of the Dog class is a legal overriding, as it keeps the same argument (String food) as the superclass’ version. If we add a new argument to the method like this:

protected void eat(String food, int amount) {
	// dog eating code...
}

Then this method is not an overriding, it is an overload instead.

 

Rule #4: The overriding method must have same return type (or subtype).

Suppose that a Food class has a subclass called DogFood, the following example shows a correct overriding:

public class Animal {

	protected Food seekFood() {

		// animal seeking for food code...

		return new Food();
	}
}

public class Dog extends Animal {

	protected Food seekFood() {

		// dog seeking for food code...

		return new DogFood();
	}
}

It’s possible to modify the return type of the Dog’s seekFood() method to DogFood - a subclass of Food, as shown below:

protected DogFood seekFood() {

	// dog seeking for food code...

	return new DogFood();
}

That’s perfectly a legal overriding, and the return type of Dog’s seekFood() method is known as covariant return type.

The Dog class won’t compile if we change the seekFood() method’s return type to another, as shown below:

protected String seekFood() {

	// dog seeking for food code...

	return new String();
}

As the complier issues this error:

error: seekFood() in Dog cannot override seekFood() in Animal
	protected String seekFood() {
	                 ^
  return type String is not compatible with Food
1 error

 

Rule #5: The overriding method must not have more restrictive access modifier.

This rule can be understood as follows:

• If the overridden method is has default access, then the overriding one must be default, protected or public.
• If the overridden method is protected, then the overriding one must be protected or public.
• If the overridden method is public, then the overriding one must be only public.

In other words, the overriding method may have less restrictive (more relaxed) access modifier. The following example shows a legal overriding:

public class Animal {

	protected void move() {
		// animal moving code...
	}
}

public class Dog extends Animal {

	public void move() {
		// Dog moving code...
	}
}

However, in the following example, the Dog class won’t compile:

public class Dog extends Animal {

	void move() {
		// Dog moving code...
	}
}

It is because the move() method now has default access, which is more restrictive than the protected access of the superclass’ version.

 

Rule #6: The overriding method must not throw new or broader checked exceptions.

In other words, the overriding method may throw fewer or narrower checked exceptions, or any unchecked exceptions.

Consider the following superclass - Animal:

public class Animal {

	protected void move() throws IOException {
		// animal moving code...
	}
}

The following subclass - Dog, correctly overrides the move() method because the FileNotFoundException is a subclass of the FileIOException:

public class Dog extends Animal {

	protected void move() throws FileNotFoundException {
		// Dog moving code...
	}
}

The following example shows an illegal overriding attempt because the InterruptedException is a new and checked exception:

public class Dog extends Animal {

	protected void move() throws IOException, InterruptedException {
		// Dog moving code...
	}
}

However, the following example is a legal overriding, because the IllegalArgumentException is an unchecked exception:

public class Dog extends Animal {

	protected void move() throws IOException, IllegalArgumentException {
		// Dog moving code...
	}
}

And in the example below, the Dog class won’t compile because its move() method throws Exception which is superclass (broader) of the IOException:

public class Dog extends Animal {

	protected void move() throws Exception {
		// Dog moving code...
	}
}

 

5. Invoking the overridden method

Rule #7:Use the super keyword to invoke the overridden method from a subclass.

It’s very common that a subclass extends a superclass’ behavior rather than re-implementing the behavior from scratch. In such case, invoke the superclass’ method in the following form:

super.overriddenMethodName()

Consider the following example:

public class Animal {

	protected void move() {
		// animal moving code...
	}
}

public class Dog extends Animal {

	protected void move() {
		super.move();	// Animal movement

		// Dog-specific moving code...
	}
}

Here, the Dog class overrides the move() method from the Animal class. Then in the Dog’s move() method, it calls the superclass’ version of the method first, then add behaviors specific to only dogs.

 

6. Overriding and constructor

Rule #8:Constructors cannot be overridden.

Because constructors are not methods and a subclass’ constructor cannot have same name as a superclass’ one, so there’s nothing relates between constructors and overriding.

Learn more: 9 Rules about Constructors in Java

 

7. Overriding and abstract method

Rule #9: Abstract methods must be overridden by the first concrete (non-abstract) subclass.

Consider the following interface:

public interface Animal {
	void move();
}

If an abstract class implements the above interface, then it doesn’t require the subclass to override the move() method, as shown in the following AbstractDog class:

public abstract class AbstractDog implements Animal {

	protected abstract void bark();

}

But if a concrete (non-abstract) class, says BullDog, is a subclass of the AbstractDog class or the Animal interface, then it must override all the inherited abstract methods, as shown below:

public class BullDog extends AbstractDog {

	public void move() {

		// Bulldog moves...

	}

	protected void bark() {

		// Bulldog barks...

	}

}

In this respect, the BullDog class is said to implement the move() and bark() abstract methods of its supertypes - the Animal interface and the AbstractDog class. Although all the rules of overriding must be obeyed in this context, the term implement is more exact then the term override, since the overridden method is abstract. 

 

8. Overriding and static method

Rule #10: A static method in a subclass may hide another static one in a superclass, and that’s called hiding.

Consider the following example:

public class Animal {

	static void sleep() {
		System.out.println("Animal sleeps");
	}

}

public class Dog extends Animal {

	static void sleep() {
		System.out.println("Dog sleeps");
	}
}

Here, the sleep() method of the Dog class is said to hide the sleep() method of the Animal class. When a static method of the superclass is hidden, it requires the subclass to use a fully qualified class name of the superclass to invoke the hidden method, as shown in the doSomething() method of the Dog class below:

public class Dog extends Animal {

	static void sleep() {
		System.out.println("Dog sleeps");
	}

	void doSomething() {
		sleep();	// this calls the hiding method

		// because the Animal's sleep() is hidden, it requires to use
		// a fully qualified class name to access it.
		Animal.sleep();
	}
}

Note that the rules of overriding are still applied for the hiding method.

 

9. Overriding and synchronized method

Rule #11: The synchronized modifier has no effect on the rules of overriding.

The synchronized modifier relates to the acquiring and releasing of a monitor object in multi-threaded context, therefore it has totally no effect on the rules of overriding. That means a synchronized method can override a non-synchronized one and vice versa.

 

10. Overriding and strictfp method

Rule #12: The strictfp modifier has no effect on the rules of overriding.

That means the presence or absence of the strictfp modifier has absolutely no effect on the rules of overriding: it’s possible that a FP-strict method can override a non-FP-strict one and vice-versa.

 

Related Tutorials:

• Differences between Overriding and Overloading
• What is Overloading in Java and Examples
• @Override annotation examples
• Polymorphism in Java The WHAT, HOW and WHY
• 12 Rules and Examples About Inheritance in Java