Override vs Overload Methods in Java

In my previous blog I have written about IS-A and HAS-A relationship in Java which is based on the inheritance in Java.

If class have satisfied IS-A relation with other class than you have opportunity to override and overload class method in your class. Let’s see details in below section.

Overridden Methods

What is override method ? - Define a super class method in sub-class with a specific behavior to a particular subclass. Same method is present in super class as well as in sub class but with some extra code or some additional functionality.

For example    

public class Animal {

       public void eat() {

              System.out.println("Animal Eat Method");

       }

}

class Cat extends Animal {

       public void eat() {

              System.out.println("Cat Eat Method");

       }

}

Rules for Override Method

1) Arguments list must exactly same as overridden method. If they don’t match it is not override 

public class Animal {

       public void eat(String food) {...}

}

class Cat extends Animal {

       @Override

       public void eat(String food) {...}

}

2) Return type is same or subtype of the return type declare in the overridden method in super class

class Animal {

       public Animal eat(String food) {...}

}

class Cat extends Animal {

       @Override

       public Cat eat(String food) {...} // here return type is subtype of the Animal class

}

3)  Access level of override method in sub class can’t be more restrictive than overridden method in super class. It can be less restrictive.

     
            public class Animal {

       protected void eat(String food) {...}

}

class Cat extends Animal {

       @Override

       private void eat(String food) {...} // This is illegal access level

       public void eat(String food) {...} // This is legal access level

}

4)     You cannot override static method. If you do this than it is not override because override is depend on the instance of the class and static method is impendent on the class instance.

5)      Instance methods are override only if class is inherited by subclass.

6)      Subclass can override methods of superclass if methods cannot marked private or final in same package. Subclass in different package can override only non-final method marked with public or protected.

7)      The override method can throw any unchecked / runtime exception whether overridden method define it or not.

class Animal {

       public void eat(String food) { }

}

class Cat extends Animal {

       @Override

       public void eat(String food) throws ClassCastException { }

}

8)      Override method cannot throw Checked exception or broader than that declare in the overridden method. 

For example you can’t declare override method with FileNotFoundException unless overridden method declare exception that is super class of FileNotFoundException (i.e. IOException )

class Animal {

       public void eat(String food) throws IOException { }

}

class Cat extends Animal {

       @Override

       public void eat(String food) throws FileNotFoundException { } // this is llegal because FileNotFoundException is subclass of IOException

       public void eat(String food) throws Exception { } // this is illegal because Exception is super class of the IOException

}

9)   You cannot override final method.

10)  You cannot override method unless you inherited that class, if not that is not override it is simply class method define in same in two difference class.

 Overloaded Methods

Overload method let you define method with same name but with different argument (it may have different return type) in the same class or in subclass.

For Example

class Addition

{

       public void add(int a,int b){}

       public int add(float a,float b){return 0;}

       public void add(int a,double b){}

       protected void add(String a,int b) throws ArithmeticException{}

}

All add method define in class Addition is legal Overload method.

Rules for Overload Method

1.  Overload Methods name must be same (methods name “add” in above example are same).
2.  Overload methods are within the same class or in subclass.
3.  Overload methods must change argument list, it can one or more or none.
4.  Overload method may change return type.
5.  Overload method may declare any Exception.