Java内部类总结
 
Java内部类其实在J2EE编程中使用较少,不过在窗口应用编程中特别常见,主要用来事件的处理。其实,做非GUI编程,内部类完全可以不用。
 
内部类的声明、访问控制等于外部类有所不同,要灵活使用内部类来编写程序,还是有相当难度的,Java发明了这种难懂的玩意儿,在其他语言中是没有的,但是在Java中,内部类也相当的重要,尤其做GUI开发时候,事件的响应处理全靠内部类了。
 
内部类所做的功能使用外部类也同样可以实现,只是有时候内部类做的更巧妙些。
 
内部类按照其所在位置不同,可分为以下几种:
1、(普通的)内部类(最常见的内部类,内部类的定义与类成员平级,)
2、方法内部类
3、匿名类
4、静态内部类
5、接口内部类
 
一、内部类声明与访问
 
1、内部类直接在类的内部进行声明。可以声明为private、protected、public或者默认访问权限,这个访问权限约定和外部类完全一样。
 
2、内部类自动拥有对其外围类所有成员(方法、属性)的访问权。如果内部类和外部类成员的名字完全相同,在内部类方法中要访问外部类成员,则需要使用下面的方式来访问:外部类名.this.外部成员名,例如Outer.this.i++;  (看例子)
 
3、必须使用外部类对象来创建内部类对象,而不是直接去new一个。
格式为:外部对象名.new 内部类构造方法
 
比如要创建一个内部类iner对象,需要这么做:
        Outer outer = new Outer();
        Outer.Inner iner = outer.new Inner();
 
/**
* 内部类创建与初始化
*
* @author leizhimin 2009-7-17 13:51:52
*/

public class Outer {
        private int i = 10;
        private int y = 8;

        Outer() {
                System.out.println("调用Outer构造方法:outer");
        }

        public void sayMsg() {
                System.out.println("Outer class!");
        }

        class Inner {
                int i = 1000;

                Inner() {
                        System.out.println("调用Inner构造方法:inner");
                }

                void innerMsg() {
                        System.out.println(">>>>>Inner class!");
                        sayMsg();
                        //访问内部类自己的成员i,也可以写成 this.i++
                        this.i++;
                        //访问外部类的成员 i和y
                        Outer.this.i++;
                        y--;
                }

                int getI() {
                        return i;
                }
        }

        public void test() {
                Inner in = new Inner();
                in.innerMsg();
        }

        public int getI() {
                return i;
        }

        public void setI(int i) {
                this.i = i;
        }
}

class Test1 {
        public static void main(String[] args) {
                Outer outer = new Outer();
                outer.test();
                System.out.println(outer.getI());
                System.out.println("-------1--------");

                Outer.Inner iner = outer.new Inner();
                iner.innerMsg();
                System.out.println(iner.getI());
                System.out.println("-------2--------");

                System.out.println(outer.getI());
        }
}
 
运行结果:
调用Outer构造方法:outer
调用Inner构造方法:inner
>>>>>Inner class!
Outer class!
11
-------1--------
调用Inner构造方法:inner
>>>>>Inner class!
Outer class!
1001
-------2--------
12

Process finished with exit code 0
 
二、内部类与接口
 
1、内部类可以实现接口。
2、内部类之间相互可见,但并非内部类之间方法都可见。
 
public interface Foo{
         void say();
}
 
public interface Bar {
        void readme();
}
 
/**
* 内部类实现接口
*
* @author leizhimin 2009-7-17 14:57:50
*/

public class Test2 {
        public static void main(String[] args) {
                Outer outer = new Outer();
                Foo f = outer.genFoo();
                Bar b = outer.genBar();
                f.say();
                b.readme();
        }
}

class Outer {
        private class FooImpl implements Foo {
                public void say() {
                        System.out.println("say foo!");
                }
        }

        private class BarImpl implements Bar {
                public void readme() {
                        System.out.println("say bar!");
                }
        }

        public Foo genFoo() {
                return new FooImpl();
        }

        public Bar genBar() {
                return new BarImpl();
        }
}
 
输入结果:
say foo!
say bar!

Process finished with exit code 0
 
三、访问权限
 
外部类分两种:
一种嵌入了内部类声明代码外部类,称为直接外部类。
另一种是与内部类没有任何关系的外部类,称为外部类。
 
在同一个直接外部类中,内部类之间所有的方法都是相互可见的,包含在直接外部类的main()中可见。

在外部类中,要看到一个类的内部类成员,则至少要求这个内部类的class和成员权限大于或等于protected。
 
/**
* 内部类实现接口
*
* @author leizhimin 2009-7-17 14:57:50
*/

public class Test2 {
        public static void main(String[] args) {
                Outer o = new Outer();
                Outer.Bar b = o.genBar();
                b.readme();
        }
}

class Outer {

        protected class Foo {
                protected void say() {
                        System.out.println("say foo!");
                }

                private void test() {
                        System.out.println("----test------");
                }
        }

        protected class Bar {
                protected void readme() {
                        System.out.println("say bar!");
                        new Foo().test();
                }
        }

        public Foo genFoo() {
                return new Foo();
        }

        public Bar genBar() {
                return new Bar();
        }
}
 
四、方法内部类
 
方法内部类只在该方法内部可见,方法内部类可以定义在方法中的任何位置。
/**
* 内部类实现接口
*
* @author leizhimin 2009-7-17 14:57:50
*/

public class Test2 {
        public static void main(String[] args) {
                Outer outer = new Outer();
                Foo f = outer.genFoo();
                Bar b = outer.genBar();
                f.say();
                b.readme();
        }
}

class Outer {
        public Foo genFoo() {
                //方法内的内部类
                class FooImpl implements Foo {
                        public void say() {
                                System.out.println("say foo!");
                        }
                }
                return new FooImpl();
        }

        public Bar genBar() {
                Bar b = null;
                if (true) {
                        //任意位置的内部类
                        class BarImpl implements Bar {
                                public void readme() {
                                        System.out.println("say bar!");
                                }
                        }
                        b = new BarImpl();
                }
                return b;
        }
}
 
运行结果:
say foo!
say bar!

Process finished with exit code 0
 
五、匿名类
 
匿名类不给出类名,直接定义一个类,通常这个类实现了某种接口或者抽象。匿名类的访问权限更没有讨论价值了,看个例子就行了。
 
在一些多线程程序中比较常见,有点变态,呵呵。
/**
* 匿名类.
*
* @author leizhimin 2009-7-17 15:56:17
*/

public class Test3 {
        public Foo f = new Foo() {
                public void say() {
                        System.out.println("O(∩_∩)O哈哈~!");
                }
        };

        public Foo test() {
                return new Foo() {
                        public void say() {
                                System.out.println("say foo!");
                        }
                };
        }

        public static void main(String[] args) {
                Test3 t = new Test3();
                t.f.say();
                t.test().say();
        }
}

interface Foo {
        void say();
}
 
运行结果:
O(∩_∩)O哈哈~!
say foo!

Process finished with exit code 0
 
/**
* 普通类的匿名初始化
*
* @author leizhimin 2009-7-17 16:13:31
*/

public class Fk {
        private String x;

        public Fk(String x) {
                this.x = x;
        }

        @Override
        public String toString() {
                return "Fk{" +
                                "x='" + x + '\'' +
                                '}';
        }
}

class Test4 {
        public Fk hehe() {
                //把后面的一对大括号去掉呢,呵呵
                return new Fk("fk") {
                };
        }

        public static void main(String[] args) {
                Test4 t = new Test4();
                Fk f = t.hehe();
                System.out.println(f);
        }
}
 
运行结果:
Fk{x='fk'}

Process finished with exit code 0
 
还有一个不得不提的经典实例,来自thining in java,有改动:
interface Service {
    void method1();
    void method2();
}

interface ServiceFactory {
    Service getService();
}

class Implementation1 implements Service {
    private Implementation1() {}
    public void method1() {System.out.println("Implementation1 method1");}
    public void method2() {System.out.println("Implementation1 method2");}
    public static ServiceFactory factory = new ServiceFactory() {
            public Service getService() {
                return new Implementation1();
            }
        };
}

class Implementation2 implements Service {
    private Implementation2() {}
    public void method1() {System.out.println("Implementation2 method1");}
    public void method2() {System.out.println("Implementation2 method2");}
    public static ServiceFactory factory = new ServiceFactory() {
            public Service getService() {
                return new Implementation2();
            }
        };
}

public class Factories {
    public static void serviceConsumer(ServiceFactory fact) {
        Service s = fact.getService();
        s.method1();
        s.method2();
    }
    public static void main(String[] args) {
        serviceConsumer(Implementation1.factory);
        serviceConsumer(Implementation2.factory);
    }
}
 
这个应用给了我们很多思考,我就不说了,不同人看了会有不同的感受。
 
内部类的巧妙使用会让你的代码很牛,如果要形容下,那就是:没看懂的时候感觉神出鬼没,看懂后感觉鬼斧神工。不过这些代码多了,别人想看懂都难,想看懂你思路就难上加难了。呵呵!
 
六、静态内部类
 
静态内部类是static class型的内部类,这种内部类特点是:它不能访问外部类的非静态成员。要创建静态内部类对象时候,也不需要外部类对象了,直接可以:
new 外部类名.内部类构造方法
来创建,给个例子:
/**
* 静态内部类
*
* @author leizhimin 2009-7-17 16:53:05
*/

public class Outer {
        public static int i =500;
        protected static class Inner {
                int i =100;
                String name;

                Inner(String name) {
                        this.name = name;
                }

                void sayHello() {
                        System.out.println("Hello " + name);
                        Outer.i++;
                }
        }

        public Inner genInner(String name) {
                return new Inner(name);
        }
}

class Test {
        public static void main(String[] args) {
                Outer.Inner in1 = new Outer.Inner("1111");
                in1.sayHello();
                System.out.println(Outer.i);

                Outer.Inner in2 = new Outer().genInner("2222");
                in2.sayHello();
                System.out.println(Outer.i);
        }
}
 
运行结果:
Hello 1111
501
Hello 2222
502

Process finished with exit code 0
 
七、接口内部类
 
接口内部类自动都是public static的,相当于为接口定义了一种变量类型,这在java的设计中就有使用,比如在HashMap中,就有:
static class Entry<K,V> implements Map.Entry<K,V>
 
下面我给个例子,
/**
* 接口内部类
*
* @author leizhimin 2009-7-17 17:20:28
*/

public interface AInterface {
        void readme();

        class Inner1 implements AInterface {
                public void readme() {
                        System.out.println("我是一个接口内部类");
                }
        }
}

class Main {
        public static void main(String[] args) {
                AInterface.Inner1 in1 = new AInterface.Inner1();
                in1.readme();
        }
}
 
八、内部的类的嵌套
 
所谓内部类嵌套,就是内部类里面再定义内部类。其实这种用法还真没见过,试试写个简单例子看看吧:
 
/**
* 嵌套内部类
*
* @author leizhimin 2009-7-17 17:33:48
*/

public class Outer {
        private void f0() {
                System.out.println("f0");
        }

        class A {
                private void a() {
                        f0();
                        System.out.println("a");
                }

                class B {
                        protected void b() {
                                a();
                                System.out.println("b");
                        }
                }
        }
}
class Test{
        public static void main(String[] args) {
                Outer o = new Outer();
                Outer.A    a =     o.new A();
                Outer.A.B b = a.new B();
                b.b();
        }
}
 
运行结果:
f0
a
b

Process finished with exit code 0
 
八、内部类的继承
 
内部类的继承,可以继承内部类,也可以继承外部类。
/**
* 内部类的继承,可以继承内部类,也可以继承外部类
*
* @author leizhimin 2009-7-22 13:50:01
*/

public class Outer {
        class Inner {
                void doSomething() {
                        System.out.println("Inner doing ...");
                }
        }

        class Inner2 extends Inner {
                void doSomething() {
                        System.out.println("Inner2 doing ...");
                }

                void readme() {
                        System.out.println("HeHe!");
                }
        }
}

class Test {
        public static void main(String[] args) {
                Outer outer = new Outer();
                Outer.Inner in = outer.new Inner();
                Outer.Inner2 in2 = outer.new Inner2();
                in.doSomething();
                in2.doSomething();
                in2.readme();
        }
}
 
运行结果:
Inner doing ...
Inner2 doing ...
HeHe!

Process finished with exit code 0
 
 
总结:
 
内部类是Java中最复杂深奥的概念之一,而且内部类在访问控制,修饰符,继承,实现,抽象,序列化等等很多方面都是一个很让人迷惑的问题,在实际中,这些问题也许永远没机会没时间搞清,但是一般说来,懂得以上的内部类的知识就足够用了。
 
内部类的设计也许是弥补Java语言本身的先天不足吧,作为语言来说,这个特性太变态了点,难道就没别的法了?
 
以上的总结完全是建立在实践基础上的,所列举的例子也许偏颇,不能全面反映问题的本质,希望有兴趣的博友多多发表自己的看法与观点。