1. Default Method (Java 8)

Issue

Before Java8, Interface can have only Abstract method. And all child classes has to provide abstract method implementation.

public interface Bird{
    public void canFly();
}

public class Eagle implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }
}

public class Sparrow implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }
}

Any new method added in interface means need to change in all its implementation

public interface Bird{
    public void canFly();
    public int getMinimumFlyHeight();
}

public class Eagle implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }

    @Override
    public int getMinimumFlyHeight(){
        return 100;
    }
}

public class Sparrow implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }

    @Override
    public int getMinimumFlyHeight(){
        return 100;
    }
}

Fix using Default Method

public interface Bird{
    public void canFly();

    default int getMinimumFlyHeight(){
        return 100;
    }
}

public class Eagle implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }
}

public class Sparrow implements Bird{
    @Override
    public void canFly(){
        // Eagle fly implementation
    }
}

Usage in Main

public class Main {
    public static void main(String args[]){
        Eagle eagleobj = new Eagle();
        eagleObj.getMinimumFlyHeight();
    }
}

Why Default method was introduced?

To add functionality in existing Legacy Interface we need to use Default method. Example Stream() method in Collection.

Default and Multiple Inheritance, how to Handle?

Diamond problem with 2 inheritance

public interface Bird{
    default boolean canBreathe(){
        return true;
    }
}

public interface LivingThing{
    default boolean canBreathe(){
        return true;
    }
}

Error

public class Eagle implements Bird, LivingThing{
}

Fix

public class Eagle implements Bird, LivingThing{
    public boolean canBreathe(){
        return true;
    }
}

How to extend interface, that contains Default Method?

1st Way

public interface LivingThing{
    default boolean canBreathe(){
        return true;
    }
}

public interface Bird extends LivingThing{
}

public class Eagle implements Bird {
}

usage

public class Main {
    public static void main(String args[]){
        Eagle eagleobj = new Eagle();
        eagleObj.canBreathe();
    }
}

2nd Way

public interface LivingThing{
    default boolean canBreathe(){
        return true;
    }
}

Child Interface - Made it to abstract

public interface Bird extends LivingThing{
    default boolean canBreathe();
}

Not Possible: Error

public class Eagle implements Bird {
}

Fix

public class Eagle implements Bird {
    @Override
    public boolean canBreathe(){
        return true;
    }
}

3rd Way

public interface LivingThing{
    default boolean canBreathe(){
        return true;
    }
}

public interface Bird extends LivingThing{
    default boolean canBreathe(){
        boolean canBreatheOrNot = LivingThing.super.canBreathe();
        // do something else
        return canBreatheOrNot;
    }
}

public class Eagle implements Bird {
}

2. Static Method (Java8)

We can provide the implementation of the method in interface.
But it cannot be overridden by classes which implement the interface.
We can access it using interface name itself.
Its by default public.

public interface Bird {
    static boolean canBreathe(){
        return true;
    }
}

public class Eagle implements Bird {

    public void digestiveSystemTestMethod(){
        if(Bird.canBreathe()){
            // do something 
        }
    }
}

if you try to override it,

it will just be treated as new method in Eagle class

public class Eagle implements Bird {

    public boolean canBreathe(){
        System.out.println("in interface);
        return true;
    }
}

If you try to add @ override annotation

It will throw compilation error

public class Eagle implements Bird {
    @override
    public boolean canBreathe(){
        System.out.println("in interface);
        return true;
    }
}

3. Private Method and Private Static method (Java 9)

We can provide the implemenation of method but as a private access modifier in interface.
It brings more readability of the code. For example if multiple default method share some code, that this help.
It can be defined as static and non-static.
From static method, we can call only private static interface method.
Private static method, can be called from both static and non static method.
Private interface method can not be abstract. Means we have to provide the definition.
It can be used inside of the particular interface only. No other child classes/ interfaces can call the private methods.

public interface Bird {
    void canFly();

    public default void minimumFlyingHeight(){
        myStaticPublicMethod(); //Calling static method
        myStaticPrivateMethod(); //calling private static method
        myPrivateMethod(); //Calling private method
    }

    private void myPrivateMethod(){

    }

    static void myStaticPublicMethod(){
        myStaticPrivateMethod(); // from static we can call other static method only
    }

    private static void myStaticPrivateMethod() {
        //Private method implementation
    }
}

11 Java - Interfaces (Latest Versions)

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