Interfaces

A contract of behavior

• interface declares what a type can do. A class implements one or more interfaces and must implement the abstract methods.
• Before Java 8, interfaces had only public abstract methods and public static final constants. Java 8 added default and static methods; Java 9 added private helpers.
• A variable can have an interface type: List xs = new ArrayList<>();.

interface Printer {
    void print(String msg);

    default void printLine(String msg) {
        print(msg + System.lineSeparator());
    }

    static Printer stdout() {
        return System.out::print;
    }
}

public class InterfaceDemo implements Printer {
    @Override
    public void print(String msg) {
        System.out.print("[out] " + msg);
    }

    public static void main(String[] args) {
        Printer p = new InterfaceDemo();
        p.printLine("hello");
        Printer.stdout().print("via factory\n");
    }
}

Multiple interfaces and default conflicts

• class C implements A, B is legal. If A and B both declare default void f(), C must override f() and may call A.super.f().
• A class method always wins over a default method with the same signature (class wins).

Functional interfaces

• A functional interface has exactly one abstract method (SAM). @FunctionalInterface documents that. Lambdas implement them (Runnable, Comparator, Function).
• default and static methods do not count toward the SAM count.

Evolution and marker interfaces

• Default methods let library authors add methods without breaking every implementor (List.sort).
• Marker interfaces (Serializable, RandomAccess) have no methods; they tag a type for the runtime or algorithms.
• Interface fields are constants. int MAX = 10; in an interface is public static final.
• A class can extend one class and implement many interfaces: class Dog extends Animal implements Pet, Trainable.

Two interfaces, one class

interface Camera {
    default String kind() {
        return "camera";
    }
}

interface Gps {
    default String kind() {
        return "gps";
    }
}

public class Phone implements Camera, Gps {
    @Override
    public String kind() {
        return Camera.super.kind() + "+" + Gps.super.kind();
    }

    public static void main(String[] args) {
        System.out.println(new Phone().kind());
    }
}