core14InterfacesDemo

1package pkg1core;2 3/*4 * core14InterfacesDemo.java5 * -------------------6 * Interfaces: contracts, default & static methods, multiple inheritance of type,7 * private interface methods, and functional interfaces.8 *9 * EXPLANATION:10 *  - An interface defines a contract (abstract methods).11 *  - `default` methods provide an implementation (added in Java 8) so interfaces12 *    can evolve without breaking implementers.13 *  - A class can implement MANY interfaces (multiple inheritance of TYPE).14 */15public class core14InterfacesDemo {16 17    interface Greeter {18        String name();                                  // abstract19        default String greet() {                        // default method20            return prefix() + name();                   // can call private helper21        }22        private String prefix() { return "Hello, "; }   // private interface method23        static Greeter of(String n) { return () -> n; } // static factory; lambda impl24    }25 26    interface Swimmer { default String act(){ return "swims"; } }27    interface Flyer   { default String fly(){ return "flies"; } }28 29    // Multiple interfaces30    static class Duck implements Swimmer, Flyer {31        String describe() { return "Duck " + act() + " and " + fly(); }32    }33 34    public static void main(String[] args) {35        Greeter g = Greeter.of("World");        // implemented via lambda (functional)36        System.out.println(g.greet());37 38        System.out.println(new Duck().describe());39 40        // Interfaces enable programming to an abstraction41        Greeter custom = () -> "Custom";42        System.out.println(custom.greet());43    }44}