05.Java Basics - Interface
基本概念
接口（interface），在软件工程中，接口泛指供别人调用的方法或者函数。它使抽象的概念更进一步。
在 Java 中通过关键字 interface 定义一个接口，通过关键字 implements 实现接口。
我们可以接口理解为是一个极度抽象的类，因为它比抽象类更抽象。
实例探究
下面通过例子来看接口的特点：
定义一个接口
// 1.接口，访问权限只能是 默认（包访问权限）或 publicpublic interface Parent {
// 2.成员变量，准确来说应该叫常量。具有以下特点：
// 2.1.访问权限只能是 public （缺省也代表 public） ，修饰符只能是 final & static
public final static String WORD = "a"; // 2.2.即使不指定，默认也是被 public fainl static 修饰
String NAME = "b"; // 2.3.不能存在空的 final 变量 ，如 int num; 但是可以被非常量表达式初始化
int num = new Random().nextInt(100); // 3.抽象方法，访问权限只能是 默认（包访问权限）或 public
public abstract void print(); abstract void print(int i); // 4.普通方法，访问权限只能是 默认（包访问权限）或 public
public void play(); void play(int i);
}实现一个接口
public class Son implements Parent {
@Override
public void print() {
System.out.println("I am Son");
} @Override
public void play() {
System.out.println("Son is playing");
}
}嵌套接口
嵌套接口有两种形式：
在类中定义一个接口；
在接口中定义一个接口。
1.在类中定义接口
接口嵌套在类中，具有以下特点：
接口的访问权限没了限制，可以使用 private 修饰
方法/抽象方法/变量的访问权限与外部接口一致
下面通过实例来验证：
public class Demo {
// 内部接口
private interface A {
void f();
} // 内部类
class AImpl implements A { @Override
public void f() {
System.out.println("AImpl.f()");
}
}
A getA(){ return new AImpl();
} private A a; public void receive(A a){ this.a = a;
a.f();
}
}再来看它的调用过程：
Demo demo = new Demo();// 错误，因为 A 是私有接口，不可被外部访问 // Demo.A a = demo.getA(); // 因为 A 接口不可视，所以只能通过内部类来访问 Demo.AImpl a = (Demo.AImpl) demo.getA();a.f();// 或者是这样 demo.receiveA(demo.getA());
2.在接口中定义接口
在接口中定义接口，除了调用之外，均于普通接口一致。
public interface Demo {
// 内部接口
interface A { void play();
} void print();
}public class DemoImpl implements Demo,Demo.A{
@Override
public void play() {
System.out.println("DemoImpl.play()");
} @Override
public void print() {
System.out.println("DemoImpl.print()");
}
}public class Test{
public static void main(String[] args) { // 向上转型，只能调用 print（）
Demo demo = new DemoImpl(); // 向上转型，只能调用 play（）
Demo.A a = new DemoImpl();
}
}接口&抽象类
上面说过，我们可以把接口理解为是一个极度抽象的类，因为它比抽象类更抽象。
在实际运用中，两者还是有区别的。
1.语法区别
抽象类可以包含非抽象方法，而接口中只能存在抽象方法。
抽象类中的成员变量可以是各种类型的，而接口中的成员变量只能是公共的静态常量（public static final）。
抽象类可以有静态代码块和静态方法，接口中不能含有静态代码块以及静态方法。
一个类只能继承一个抽象类，而一个类却可以实现多个接口。
2.运用区别
抽象类是对事物整体（属性，行为）的抽象，而接口是对事物局部（行为）的抽象。
举个简单的例子，飞机和鸟是不同类的事物，但是它们都有一个共性，就是都会飞。
在设计的时候，可以将飞机设计为一个类 Airplane，将鸟设计为一个类 Bird。
但是不能将飞行这个特性也设计为类，因此它只是一个行为特性，并不是对一类事物的抽象描述。
此时可以将飞行 设计为一个接口 Fly，包含方法fly( )，然后 Airplane 和 Bird 分别根据自己的需要实现Fly 这个接口。
然后至于有不同种类的飞机，比如战斗机、民用飞机等直接继承 Airplane 即可，对于鸟也是类似的，不同种类的鸟直接继承 Bird 类即可。
从这里可以看出，继承是一个 “是不是”的关系，而接口实现则是 “有没有”的关系。
如果一个类继承了某个抽象类，则子类必定是抽象类的种类，而接口实现则是有没有、具备不具备的关系，比如鸟是否能飞（或者是否具备飞行这个特点），能飞行则可以实现这个接口，不能飞行就不实现这个接口。
以上就是05.Java 基础 - 接口的内容，更多相关内容请关注PHP中文网（www.php.cn）！
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