Java內部類總結

Java內部類其實在J2EE編程中使用較少，不過在窗口應用編程中特別常見，主要用來事件的處理。其實，做非GUI編程，內部類完全可以不用。

 

內部類的聲明、訪問控制等於外部類有所不同，要靈活使用內部類來編寫程序，還是有相當難度的。

 

一、內部類聲明與訪問

 

1、內部類直接在類的內部進行聲明。可以聲明爲private、protected、public或者默認訪問權限，這個訪問權限約定和外部類完全一樣。

 

2、內部類自動擁有對其外圍類所有成員（方法、屬性）的訪問權。如果內部類和外部類成員的名字完全相同，在內部類方法中要訪問外部類成員，則需要使用下面的方式來訪問：外部類名.this.外部成員名，例如Outer.this.i++;  （看例子）

 

3、必須使用外部類對象來創建內部類對象，而不是直接去new一個。

格式爲：外部對象名.new 內部類構造方法

 

比如要創建一個內部類iner對象，需要這麼做：

        Outer outer = new Outer();
        Outer.Inner iner = outer.new Inner();

 

/**
* 內部類創建與初始化
*
* @author leizhimin 2009-7-17 13:51:52
*/
public class Outer {
        private int i = 10;
        private int y = 8;

        Outer() {
                System.out.println("調用Outer構造方法：outer");
        }

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

        class Inner {
                int i = 1000;

                Inner() {
                        System.out.println("調用Inner構造方法：inner");
                }

                void innerMsg() {
                        System.out.println(">>>>>Inner class!");
                        sayMsg();
                        //訪問內部類自己的成員i，也可以寫成 this.i++
                        this.i++;
                        //訪問外部類的成員 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());
        }
}

 

運行結果：

調用Outer構造方法：outer
調用Inner構造方法：inner
>>>>>Inner class!
Outer class!
11
-------1--------
調用Inner構造方法：inner
>>>>>Inner class!
Outer class!
1001
-------2--------
12

Process finished with exit code 0

 

二、內部類與接口

 

1、內部類可以實現接口。

2、內部類之間相互可見，但並非內部類之間方法都可見。

 

public interface Foo{
         void say();
}

 

public interface Bar {
        void readme();
}

 

/**
* 內部類實現接口
*
* @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();
        }
}

 

輸入結果：

say foo!
say bar!

Process finished with exit code 0

 

三、訪問權限

 

外部類分兩種：
一種嵌入了內部類聲明代碼外部類，稱爲直接外部類。
另一種是與內部類沒有任何關係的外部類，稱爲外部類。

 

在同一個直接外部類中，內部類之間所有的方法都是相互可見的，包含在直接外部類的main()中可見。

在外部類中，要看到一個類的內部類成員，則至少要求這個內部類的class和成員權限大於或等於protected。

 

/**
* 內部類實現接口
*
* @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、方法內部類

 

方法內部類只在該方法內部可見，方法內部類可以定義在方法中的任何位置。

/**
* 內部類實現接口
*
* @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() {
                //方法內的內部類
                class FooImpl implements Foo {
                        public void say() {
                                System.out.println("say foo!");
                        }
                }
                return new FooImpl();
        }

        public Bar genBar() {
                Bar b = null;
                if (true) {
                        //任意位置的內部類
                        class BarImpl implements Bar {
                                public void readme() {
                                        System.out.println("say bar!");
                                }
                        }
                        b = new BarImpl();
                }
                return b;
        }
}

 

運行結果：

say foo!
say bar!

Process finished with exit code 0

 

五、匿名類

 

匿名類不給出類名，直接定義一個類，通常這個類實現了某種接口或者抽象。匿名類的訪問權限更沒有討論價值了，看個例子就行了。

 

在一些多線程程序中比較常見，有點變態，呵呵。

/**
* 匿名類.
*
* @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();
}

 

運行結果：

O(∩_∩)O哈哈~!
say foo!

Process finished with exit code 0

 

/**
* 普通類的匿名初始化
*
* @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() {
                //把後面的一對大括號去掉呢，呵呵
                return new Fk("fk") {
                };
        }

        public static void main(String[] args) {
                Test4 t = new Test4();
                Fk f = t.hehe();
                System.out.println(f);
        }
}

 

運行結果：

Fk{x='fk'}

Process finished with exit code 0

 

還有一個不得不提的經典實例，來自thining in java，有改動：

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);
    }
}

 

這個應用給了我們很多思考，我就不說了，不同人看了會有不同的感受。

 

內部類的巧妙使用會讓你的代碼很牛，如果要形容下，那就是：沒看懂的時候感覺神出鬼沒，看懂後感覺鬼斧神工。不過這些代碼多了，別人想看懂都難，想看懂你思路就難上加難了。呵呵！

 

六、靜態內部類

 

靜態內部類是static class型的內部類，這種內部類特點是：它不能訪問外部類的非靜態成員。要創建靜態內部類對象時候，也不需要外部類對象了，直接可以：

new 外部類名.內部類構造方法

來創建，給個例子：

/**
* 靜態內部類
*
* @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);
        }
}

 

運行結果：

Hello 1111
501
Hello 2222
502

Process finished with exit code 0

 

七、接口內部類

 

接口內部類自動都是public static的，相當於爲接口定義了一種變量類型，這在java的設計中就有使用，比如在HashMap中，就有：

static class Entry<K,V> implements Map.Entry<K,V>

 

下面我給個例子，

/**
* 接口內部類
*
* @author leizhimin 2009-7-17 17:20:28
*/
public interface AInterface {
        void readme();

        class Inner1 implements AInterface {
                public void readme() {
                        System.out.println("我是一個接口內部類");
                }
        }
}

class Main {
        public static void main(String[] args) {
                AInterface.Inner1 in1 = new AInterface.Inner1();
                in1.readme();
        }
}

 

八、內部的類的嵌套

 

所謂內部類嵌套，就是內部類裏面再定義內部類。其實這種用法還真沒見過，試試寫個簡單例子看看吧：

 

/**
* 嵌套內部類
*
* @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();
        }
}

 

運行結果：

f0
a
b

Process finished with exit code 0