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：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制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
代码语言：javascript复制public interface DoItPlus extends DoIt {
boolean didItWork(int i, double x, String s);
}②default method
代码语言：javascript复制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：
- 无声明，继承父接口的默认方法
- 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）
- 声明并重写，以子类的默认方法为准
③static method
代码语言：javascript复制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：
代码语言：javascript复制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/
