Default and Static Method in Interface

Up to Java 7, it was nearly impossible to add new methods or functionality in Interface due to backwards compatibility issues. Java 8 allows default and static methods in Interface which will have an implementation in Interface only. So from Java 8 onwards, Interfaces supports 3 types of methods.

abstract method
default method
static method

An abstract method has declaration only, the implementing class will override the method and provide the definition.

Default methods allow adding new functionality in the Interface only which if required then can be overridden by implementing class.

Static methods are part of the Interface only and are bound with them, it cannot be overridden.

package org.wesome.dsalgo.java8;

interface Fruit {
    /*  normal abstract method  */
    void likedFruit(String appleName);

    /*  default method  */
    default void likedApple(String appleName) {
        System.out.println("i love Apple, my favourite apple is " + appleName);
    }

    /*  static method  */
    static void likedMango(String mangoName) {
        System.out.println("i love Mango, my favourite mango is " + mangoName);
    }
}

class Apple implements Fruit {
    @Override
    public void likedFruit(String fruitName) {
        System.out.println("i love fruit, my favourite fruit is " + fruitName);
    }

    public static void main(String[] args) {
        Fruit apple = new Apple();
        apple.likedFruit("apple");
        apple.likedApple("Fuji");
        Fruit.likedMango("Alphonso");
    }
}

some differences between static and default methods are

	static method	default method
1	static methods belong to the interface and cannot be override	a default method can be overridden by implementing class
2	a static method can be invoked using interface name only	a default method can be invoked using the class instance
3	the static method has a static prefix	default method has a default prefix
4	static methods are by default public	default methods are by default public
5	multiple inheritances create a diamond problem which is resolved using the super keyword.	default method belongs to Interface so the diamond problem never arises.
6	Interface and implementing class both can have the same name method<	implementing class cannot have the same name method as the default method of the interface.
7	Mostly used as a utility method	mostly used for providing a default implementation

with the introduction of static and default methods in Interface, there is not much difference left between abstract class and Interface now except an abstract class can have constructor, variables etc which Interface cannot.

an implementing class can extend only 1 abstract class where as can implement any number of interfaces required.

Default and Static Method in Interface

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