DefaultMethodsExample.java

package com.kiroule.ocpupgradejava8.topic7_2;

import static java.lang.String.format;

/**
 * @author Igor Baiborodine
 */
public class DefaultMethodsExample {

  public static void main(String... args) {

    Greeting defaultGreeting = new Greeting() {
      @Override
      public String getGreeting(String name) {
        return format(getTemplate(), name);
      }
    };
    System.out.println("Default greeting: " + defaultGreeting.getGreeting("Igor"));

    UpperCaseGreeting upperCaseGreeting = new UpperCaseGreeting() {
      @Override
      public String getGreeting(String name) {
        return format(getTemplate(), name);
      }

      @Override
      public String getGreetingInUpperCase(String name) {
        return getGreeting(name).toUpperCase();
      }
    };
    System.out
        .println("\nUpper case greeting: " + upperCaseGreeting.getGreetingInUpperCase("Igor"));

    Greeting abstractGreeting = new AbstractGreeting() {
      @Override
      public String getTemplate() {
        return "Abstract hello, %s!";
      }

      @Override
      public String getGreeting(String name) {
        return format(getTemplate(), name);
      }
    };
    System.out.println("\nAbstract greeting: " + abstractGreeting.getGreeting("Igor"));

    Greeting overriddenGreeting = new OverriddenGreeting() {
      @Override
      public String getGreeting(String name) {
        return format(getTemplate(), name);
      }
    };
    System.out.println("\nOverridden greeting: " + overriddenGreeting.getGreeting("Igor"));
  }
}

interface Greeting {

  String getGreeting(String name);

  // default method in question
  default String getTemplate() {
    return "Hello, %s!";
  }
}
// Inheritance rules when extending interfaces that contain default methods:

// 1) Not mention the default method at all, which lets your extended interface inherit the default method.
interface UpperCaseGreeting extends Greeting {

  String getGreetingInUpperCase(String name);
}

// 2) Redeclare the default method, which makes it abstract.
interface AbstractGreeting extends Greeting {

  String getTemplate();
}

// 3) Redefine the default method, which overrides it.
interface OverriddenGreeting extends Greeting {

  @Override
  default String getTemplate() {
    return "Overridden hello to you, %s!";
  }
}