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。

posted @ 2017-05-16 01:01  明月镇魂  阅读(131)  评论(0编辑  收藏  举报