Java Method Overriding with Example

In this post, I’ll explain what method overriding is, the rules of method overriding, and some examples. So let’s get started and learn about method overriding in Java.

We learned about inheritance in the previous tutorial. Inheritance is an OOP property that allows us to create a new class (subclass) by deriving from an existing class (superclass). The superclass’s attributes and methods are passed down to the subclass.

Contents

What is Method Overriding and how does it work?

When a method in a child class (subclass) has the same name, parameters, and return type (or covariant return type) as a method in its parent class (or super-class), the child class method has overridden the parent class method.

If the same method is defined in both the superclass and the subclass, the method defined in the subclass takes precedence over the method defined in the superclass. This is referred to as method overriding.

Usage of Java Method Overriding

Method overriding is used to provide a specific implementation of a method that its superclass already provides.
For runtime polymorphism, method overriding is used.

Rules for Java Method Overriding

The method’s name must be the same as it is in the parent class.
The method’s parameters must be the same as those in the parent class.
We cannot override the method declared as final and static.
There must be an IS-A connection (inheritance).

Method Overriding Example

class Teacher {
   public void displayInfo() {
      System.out.println("I am a Teacher.");
   }
}

class Student extends Teacher {
   @Override
   public void displayInfo() {
      System.out.println("I am a Student.");
   }
}

class Main {
   public static void main(String[] args) {
      Student S = new Student();
      S.displayInfo();
   }
}

Output:

I am a Student

The displayInfo() method is present in both the Teacher superclass and the Student subclass in the preceding program.
When we use the S object (subclass object) to call displayInfo(), the method inside the subclass Student is called. The subclass’s displayInfo() method overrides the superclass’s displayInfo() method.

In our example, the @Override annotation is used. Annotations are metadata in Java that we use to provide information to the compiler. The @Override annotation tells the compiler that the method following this annotation overrides the superclass method.

It is not required to use @Override. However, when we use this, the method must adhere to all of the overriding rules. Otherwise, an error will be generated by the compiler.

Access Specifiers in Method Overriding

Different access specifiers can be assigned to the same method declared in the superclass and its subclasses. However, there is a catch.
We can only use access specifiers in subclasses that provide greater access than the superclass access specifier. As an example:
Assume a method in the superclass, Sample(), is declared protected. Then, in the subclass, the same method Sample() can be public or protected, but not private.

Example: Access Specifier in Overriding

class Teacher {
   protected void displayInfo() {
      System.out.println("I am a Teacher.");
   }
}

class Student extends Teacher {
   @Override
   public void displayInfo() {
      System.out.println("I am a Student.");
   }
}

class Main {
   public static void main(String[] args) {
      Student S = new Student();
      S.displayInfo();
   }
}

Output:

I am a Student

In the preceding example, the subclass Student overrides the superclass Teacher method displayInfo().
When we use the S (subclass object) to call displayInfo(), the method inside the subclass is called.
It’s worth noting that the displayInfo() method in the Teacher superclass is marked as protected. In the Student subclass, the same method has the public access specifier. This is possible because the general public has greater access than the protected.

Example: Private Methods can not be Overridden

class Parent {
    //Access modifier of parent method is private
    // This method can not override the child class method
    private void display() {
        System.out.println("parent method is execute");
    }
}

//Subclass
class Child extends Parent {
    
    //Below method can not overrides the Parent display() method 
    //This method is unique to the child class
    private void display() {
        System.out.println("child method is executed");
    }
}
//Driver class 
public class Main {
    public static void main(String args[])
    {
        Parent P = new Parent(); 
        P.display(); // this line when execute will throw compiler error
  
        Parent P1 = new Child(); 
        P1.display(); // this line when execute will throw compiler error
    }
}

Output:

Another restriction with access modifiers is that you cannot override private methods in the parent class. If a subclass attempts to override a parent class’s private method, the program will throw a compile-time error.

Example: Final Methods can not be Overridden in Java

class Parent 
{ 
    // Can't be overridden 
    final void display() {  } 
} 
//Child class or Subclass  
class Base extends Parent 
{ 
    // This would produce compile time error 
    void display() {  } 
}

Output:

/Main.java:13: error: display() in Base cannot override display() in Parent
void display() { }
^
overridden method is final

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Example: Static Methods can not be Overridden

 //Base class or SuperClass
class Parent {
    
    static void display() {
        System.out.println("parent static method is execute");
    }
    
    void show() {
        System.out.println("parent non-static method is executed");
    }
}

//Derived class or SubClass
class Child extends Parent {
    
    //This method hides the Parent display() method 
    static void display() {
        System.out.println("child static method is execute");
    }
    // This method overrides the Parent show() method
    void show() {
        System.out.println("child non-static method is execute");
    }
}

//Driver Class
public class Main {
    public static void main(String args[])
    {
        Parent P = new Child();
        // static method can not be overridden,
        // so below line calls parent display() method
        P.display();
        // Expected child method will run
        P.show(); 
    }
}

Output:

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parent static method is execute
child non-static method is execute

A method marked static cannot be overridden. Method hiding occurs when a static method in a subclass (child class) has the same method signature as the super class (parent class).

Abstract method and Overriding

A class must be declared abstract if it has one or more abstract methods. An abstract class can have non-abstract methods (the usual type of method). A non-abstract child must define a method with the same signature and return type as each abstract method inherited from its parent.

Only the first concrete class (classes without abstract methods) can override abstract methods, otherwise, a compiler error will be thrown.

//Abstract class  
abstract class Parent
{
  public abstract void display( int x, String j);
}
//Subclass or Derived Class
class Child extends Parent
{
    @Override
    public void display( int x, String j )
    { 
        System.out.println("child method is execute");
    }
}
//Driver class
public class Main {
    public static void main(String args[])
    {
        Parent P = new Child(); 
        P.display(5, "DevelopersDome"); 
    }
}

Output:

child method is executed

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