JAVA

  minte9
REMEMBERS




Last update:   29-10-2021

Contract

p225 Every class that implements an interfece must override all interface methods.
 
/**
 * An interface is forcing the developer to implement methods ...
 * it is like a contract.
 * 
 * In an interface everything must be an abstract method.
 */

package com.minte9.oop.interfaces;

public class Contract {
    public static void main(String[] args) {

        Dog d = new Dog();
        d.play(); 
            // Dog is playing
    }
}

class Dog implements Canine { // Look Here
    
    @Override 
    public void play() {
        System.out.println("Dog is playing");
    }
    
    @Override
    public void eat() {
        System.out.println("Dog is eating");
    }
}

interface Canine { 
        // no class keyword

    public abstract void play();
    public void eat(); 
        // abstract by default

    // public void move() {}; 
        // Error: Interface methods cannot have body
}

interface Feline {
    public abstract void play();
}

Legacy

Extending interfaces is not recommended.
 
/**
 * When you add methods to an interface, current related classes will break.
 * You'll need to create more interfaces that extend initial interface.
 * 
 * Where is possible, it is better to createa a new interface.
 */

package com.minte9.oop.interfaces;

public class Legacy {
    public static void main(String[] args) {

        new LA().setvalue(); // LA - setvalue
        new LB().settype(); // LB - settype
    }
}

class LA implements A {
    @Override public void setvalue() {
        System.out.println("LA - setvalue");
    }
}

class LB implements B {
    @Override public void setvalue() {
        System.out.println("LB - setvalue");
    }
    @Override public void settype() {
        System.out.println("LB - settype");
    }
}

interface B extends A { // Look Here
    public void settype();
}

interface A {
    public void setvalue();
}

Default Method

Alternatively, you can define your new methods as default methods.
 
/**
 * Default keyword allows you to add implementation in interfaces.
 * Non default methods must be all define as abstract.
 */

package com.minte9.oop.interfaces;

public class DefaultMethod {
    public static void main(String[] args) {

        new AC().output(); // A - output
        new BC().output(); // B - output

        new AC().output_new(); // C - default output_new
        new BC().output_new(); // C - default output_new
    }
}

class AC implements C {
    @Override public void output() {
        System.out.println("A - output");
    }
}

class BC implements C {
    @Override public void output() {
        System.out.println("B - output");
    }
}

interface C {
    void output();

    default void output_new() { // Look Here
        System.out.println("C - default output_new");
    };
}

Questions     github OOP
Constructors
        A B C D E F
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