接口

接口只能包含：constants, method signatures（abstract）, default methods, static methods, and nested types

方法体只存在于：default methods and static methods

接口不能实例化，只能被类实现，或者被其他接口继承（接口可以多继承）。

定义接口：

public interface OperateCar {

   // constant declarations, if any

   // method signatures
   
   // An enum with values RIGHT, LEFT
   int turn(Direction direction,
            double radius,
            double startSpeed,
            double endSpeed);
   int changeLanes(Direction direction,
                   double startSpeed,
                   double endSpeed);
   int signalTurn(Direction direction,
                  boolean signalOn);
   int getRadarFront(double distanceToCar,
                     double speedOfCar);
   int getRadarRear(double distanceToCar,
                    double speedOfCar);
         ......
   // more method signatures
}

类实现接口：

public class OperateBMW760i implements OperateCar {

    // the OperateCar method signatures, with implementation --
    // for example:
    public int signalTurn(Direction direction, boolean signalOn) {
       // code to turn BMW's LEFT turn indicator lights on
       // code to turn BMW's LEFT turn indicator lights off
       // code to turn BMW's RIGHT turn indicator lights on
       // code to turn BMW's RIGHT turn indicator lights off
    }

    // other members, as needed -- for example, helper classes not 
    // visible to clients of the interface
}

如果接口不是public，那么只能被同一个包里面的类实现。

实现接口

接口：

public interface Relatable {

    // this (object calling isLargerThan())
    // and other must be instances of 
    // the same class returns 1, 0, -1 
    // if this is greater than, 
    // equal to, or less than other
    public int isLargerThan(Relatable other);
}

实现：

public class RectanglePlus 
    implements Relatable {
    public int width = 0;
    public int height = 0;
    public Point origin;

    // four constructors
    public RectanglePlus() {
        origin = new Point(0, 0);
    }
    public RectanglePlus(Point p) {
        origin = p;
    }
    public RectanglePlus(int w, int h) {
        origin = new Point(0, 0);
        width = w;
        height = h;
    }
    public RectanglePlus(Point p, int w, int h) {
        origin = p;
        width = w;
        height = h;
    }

    // a method for moving the rectangle
    public void move(int x, int y) {
        origin.x = x;
        origin.y = y;
    }

    // a method for computing
    // the area of the rectangle
    public int getArea() {
        return width * height;
    }
    
    // a method required to implement
    // the Relatable interface
    public int isLargerThan(Relatable other) {
        RectanglePlus otherRect 
            = (RectanglePlus)other;
        if (this.getArea() < otherRect.getArea())
            return -1;
        else if (this.getArea() > otherRect.getArea())
            return 1;
        else
            return 0;               
    }
}

对于接口：

public interface DoIt {
   void doSomething(int i, double x);
   int doSomethingElse(String s);
}

如果想添加新方法，为了避免影响已经实现了该接口的所有类，有以下几种方式：

①继承

public interface DoItPlus extends DoIt {

   boolean didItWork(int i, double x, String s);
   
}

②default method

public interface DoIt {

   void doSomething(int i, double x);
   int doSomethingElse(String s);
   default boolean didItWork(int i, double x, String s) {
       // Method body 
   }
}

如果继承了接口，那么子接口的默认方法有可能出现：

1. 无声明，继承父接口的默认方法
2. 声明，变为abstract method，实现类，必须实现该方法（也就是父类默认，但是子类不默认了）
3. 声明并重写，以子类的默认方法为准

③static method

public interface TimeClient {
    // ...
    static public ZoneId getZoneId (String zoneString) {
        try {
            return ZoneId.of(zoneString);
        } catch (DateTimeException e) {
            System.err.println("Invalid time zone: " + zoneString +
                "; using default time zone instead.");
            return ZoneId.systemDefault();
        }
    }

    default public ZonedDateTime getZonedDateTime(String zoneString) {
        return ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));
    }    
}

接口的static method，就相当于把多个class的static method抽到了一个公共的地方，实现该接口的类，都具有static method。

接口类型

接口也可以作为类型：

public Object findLargest(Object object1, Object object2) {
   Relatable obj1 = (Relatable)object1;
   Relatable obj2 = (Relatable)object2;
   if ((obj1).isLargerThan(obj2) > 0)
      return object1;
   else 
      return object2;
}

接口类型的赋值对象，必须是实现该接口的类的实例。

参考资料：

https://dev.java/learn/interfaces/