内部类

1. 链接外部类

// innerclasses/Sequence.java
// Holds a sequence of Objects
interface Selector {
    boolean end();
    Object current();
    void next();
}
public class Sequence {
    private Object[] items;
    private int next = 0;
    public Sequence(int size) {
        items = new Object[size];
    }
    public void add(Object x) {
        if(next < items.length)
            items[next++] = x;
    }
    private class SequenceSelector implements Selector {
        private int i = 0;
        @Override
        public boolean end() { return i == items.length; }
        @Override
        public Object current() { return items[i]; }
        @Override
        public void next() { if(i < items.length) i++; }
    }
    public Selector selector() {
        return new SequenceSelector();
    }
    public static void main(String[] args) {
        Sequence sequence = new Sequence(10);
        for(int i = 0; i < 10; i++)
            sequence.add(Integer.toString(i));
        Selector selector = sequence.selector();
        while(!selector.end()) {
            System.out.print(selector.current() + " ");
            selector.next();
        }
    }
}

简单来讲就是内部类访问外部数据，因为外部类是先初始化的，所以emm数据就可以拿了。当然这里指的是非static内部类。

2.  .this  .new

如果你需要生成对外部类对象的引用，可以使用外部类的名字后面紧跟圆点和 this。这样产生的引用自动地具有正确的类型，这一点在编译期就被知晓并受到检查，因此没有任何运行时开销。下面的示例展示了如何使用 .this：

// innerclasses/DotThis.java
// Accessing the outer-class object
public class DotThis {
    void f() { System.out.println("DotThis.f()"); }

    public class Inner {
        public DotThis outer() {
            return DotThis.this;
            // A plain "this" would be Inner's "this"
        }
    }

    public Inner inner() { return new Inner(); }

    public static void main(String[] args) {
        DotThis dt = new DotThis();
        DotThis.Inner dti = dt.inner();
        dti.outer().f();
    }
}

要想直接创建内部类的对象，你不能按照你想象的方式，去引用外部类的名字 DotNew，而是必须使用外部类的对象来创建该内部类对象，就像在上面的程序中所看到的那样。这也解决了内部类名字作用域的问题，因此你不必声明（实际上你不能声明）dn.new DotNew.Innero。

// innerclasses/Parcel3.java
// Using .new to create instances of inner classes
public class Parcel3 {
    class Contents {
        private int i = 11;
        public int value() { return i; }
    }
    class Destination {
        private String label;
        Destination(String whereTo) { label = whereTo; }
        String readLabel() { return label; }
    }
    public static void main(String[] args) {
        Parcel3 p = new Parcel3();
        // Must use instance of outer class
        // to create an instance of the inner class:
        Parcel3.Contents c = p.new Contents();
        Parcel3.Destination d =
                p.new Destination("Tasmania");
    }
}

小tip，只要private便只能在本类访问。

// innerclasses/TestParcel.java
class Parcel4 {
    private class PContents implements Contents {
        private int i = 11;
        @Override
        public int value() { return i; }
    }
    protected final class PDestination implements Destination {
        private String label;
        private PDestination(String whereTo) {
            label = whereTo;
        }
        @Override
        public String readLabel() { return label; }
    }
    public Destination destination(String s) {
        return new PDestination(s);
    }
    public Contents contents() {
        return new PContents();
    }
}
public class TestParcel {
    public static void main(String[] args) {
        Parcel4 p = new Parcel4();
        Contents c = p.contents();
        Destination d = p.destination("Tasmania");
        // Illegal -- can't access private class:
        //- Parcel4.PContents pc = p.new PContents();
    }
}

这段代码一开始看起来和前面的.new 类似，但是问题是此处并非在Parcel4类中啊，我一开始还没理解为什么最后行没法编译，小技巧就是先看public class因为一个java文件只能有一个public class

3. 几个例子

第一个定义在方法内的内部类

// innerclasses/Parcel5.java
// Nesting a class within a method
public class Parcel5 {
    public Destination destination(String s) {
        final class PDestination implements Destination {
            private String label;

            private PDestination(String whereTo) {
                label = whereTo;
            }

            @Override
            public String readLabel() { return label; }
        }
        return new PDestination(s);
    }

    public static void main(String[] args) {
        Parcel5 p = new Parcel5();
        Destination d = p.destination("Tasmania");
    }
}

这叫局部内部类，很容易理解，还是private注意一下。

第二个例子是在任意作用域插入一个类

// innerclasses/Parcel6.java
// Nesting a class within a scope
public class Parcel6 {
    private void internalTracking(boolean b) {
        if(b) {
            class TrackingSlip {
                private String id;
                TrackingSlip(String s) {
                    id = s;
                }
                String getSlip() { return id; }
            }
            TrackingSlip ts = new TrackingSlip("slip");
            String s = ts.getSlip();
        }
        // Can't use it here! Out of scope:
        //- TrackingSlip ts = new TrackingSlip("x");
    }
    public void track() { internalTracking(true); }
    public static void main(String[] args) {
        Parcel6 p = new Parcel6();
        p.track();
    }
}

匿名内部类，以前非常常用的，不过现在基本都用lambda了

// innerclasses/Parcel7.java
// Returning an instance of an anonymous inner class
public class Parcel7 {
    public Contents contents() {
        return new Contents() { // Insert class definition
            private int i = 11;

            @Override
            public int value() { return i; }
        }; // Semicolon required
    }

    public static void main(String[] args) {
        Parcel7 p = new Parcel7();
        Contents c = p.contents();
    }
}

匿名内部类是一种很自由的方式，我感觉有点像面向过程，因为它更让我们随心所欲的操作。

匿名内部类不能操作外部数据偶

// innerclasses/Parcel9.java
public class Parcel9 {
    // Argument must be final or "effectively final"
    // to use within the anonymous inner class:
    public Destination destination(final String dest) {
        return new Destination() {
            private String label = dest;
            @Override
            public String readLabel() { return label; }
        };
    }
    public static void main(String[] args) {
        Parcel9 p = new Parcel9();
        Destination d = p.destination("Tasmania");
    }
}

建议去看看R大的讲解，其实就是java目前实现的只是capture by value ，不是 capture by reference

JVM的规范中允许编程语言语义中创建闭包(closure)吗？ - 知乎 https://www.zhihu.com/question/27416568

为什么Java闭包不能通过返回值之外的方式向外传递值？ - 知乎 https://www.zhihu.com/question/28190927

R大是个很变态的人物。

嵌套内部类

package Test;

class A {
    private void a() {};
    class B {
        private void b(){};
        public class C {
            void c() {
                b();
                a();
            }
            public A getA() {
                return A.this;
            }
        }
    }
}

public class MultiNestingAccess {
    public static void main(String[] args) {
        A a = new A();
        A.B b = a.new B();
        A.B.C c = b.new C();
        A aa = c.getA();
    }
}

用内部类的原因

每个内部类都能独立地继承自一个（接口的）实现，所以无论外围类是否已经继承了某个（接口的）实现，对于内部类都没有影响。

闭包与回调，其实java目前并没有真正的闭包，但也处处是闭包

// innerclasses/Callbacks.java
// Using inner classes for callbacks
// {java innerclasses.Callbacks}
package innerclasses;
interface Incrementable {
    void increment();
}
// Very simple to just implement the interface:
class Callee1 implements Incrementable {
    private int i = 0;
    @Override
    public void increment() {
        i++;
        System.out.println(i);
    }
}
class MyIncrement {
    public void increment() {
        System.out.println("Other operation");
    }
    static void f(MyIncrement mi) { mi.increment(); }
}
// If your class must implement increment() in
// some other way, you must use an inner class:
class Callee2 extends MyIncrement {
    private int i = 0;
    @Override
    public void increment() {
        super.increment();
        i++;
        System.out.println(i);
    }
    private class Closure implements Incrementable {
        @Override
        public void increment() {
            // Specify outer-class method, otherwise
            // you'll get an infinite recursion:
            Callee2.this.increment();
        }
    }
    Incrementable getCallbackReference() {
        return new Closure();
    }
}
class Caller {
    private Incrementable callbackReference;
    Caller(Incrementable cbh) {
        callbackReference = cbh;
    }
    void go() { callbackReference.increment(); }
}
public class Callbacks {
    public static void main(String[] args) {
        Callee1 c1 = new Callee1();
        Callee2 c2 = new Callee2();
        MyIncrement.f(c2);
        Caller caller1 = new Caller(c1);
        Caller caller2 =
                new Caller(c2.getCallbackReference());
        caller1.go();
        caller1.go();
        caller2.go();
        caller2.go();
    }
}

Other operation
1
1
2
Other operation
2
Other operation
3

内部类还有个用处就是可以做一个控制框架，我们提供一个通用的接口，再实现一个controller，这个controller中用内部类的方式实现各种操作。

// innerclasses/controller/Event.java
// The common methods for any control event
package innerclasses.controller;
import java.time.*; // Java 8 time classes
public abstract class Event {
    private Instant eventTime;
    protected final Duration delayTime;
    public Event(long millisecondDelay) {
        delayTime = Duration.ofMillis(millisecondDelay);
        start();
    }
    public void start() { // Allows restarting
        eventTime = Instant.now().plus(delayTime);
    }
    public boolean ready() {
        return Instant.now().isAfter(eventTime);
    }
    public abstract void action();
}

// innerclasses/controller/Controller.java
// The reusable framework for control systems
package innerclasses.controller;
import java.util.*;
public class Controller {
    // A class from java.util to hold Event objects:
    private List<Event> eventList = new ArrayList<>();
    public void addEvent(Event c) { eventList.add(c); }
    public void run() {
        while(eventList.size() > 0)
            // Make a copy so you're not modifying the list
            // while you're selecting the elements in it:
            for(Event e : new ArrayList<>(eventList))
                if(e.ready()) {
                    System.out.println(e);
                    e.action();
                    eventList.remove(e);
                }
    }
}

// innerclasses/GreenhouseControls.java
// This produces a specific application of the
// control system, all in a single class. Inner
// classes allow you to encapsulate different
// functionality for each type of event.
import innerclasses.controller.*;
public class GreenhouseControls extends Controller {
    private boolean light = false;
    public class LightOn extends Event {
        public LightOn(long delayTime) {
            super(delayTime); 
        }
        @Override
        public void action() {
            // Put hardware control code here to
            // physically turn on the light.
            light = true;
        }
        @Override
        public String toString() {
            return "Light is on";
        }
    }
    public class LightOff extends Event {
        public LightOff(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            // Put hardware control code here to
            // physically turn off the light.
            light = false;
        }
        @Override
        public String toString() {
            return "Light is off";
        }
    }
    private boolean water = false;
    public class WaterOn extends Event {
        public WaterOn(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            // Put hardware control code here.
            water = true;
        }
        @Override
        public String toString() {
            return "Greenhouse water is on";
        }
    }
    public class WaterOff extends Event {
        public WaterOff(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            // Put hardware control code here.
            water = false;
        }
        @Override
        public String toString() {
            return "Greenhouse water is off";
        }
    }
    private String thermostat = "Day";
    public class ThermostatNight extends Event {
        public ThermostatNight(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            // Put hardware control code here.
            thermostat = "Night";
        }
        @Override
        public String toString() {
            return "Thermostat on night setting";
        }
    }
    public class ThermostatDay extends Event {
        public ThermostatDay(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            // Put hardware control code here.
            thermostat = "Day";
        }
        @Override
        public String toString() {
            return "Thermostat on day setting";
        }
    }
    // An example of an action() that inserts a
    // new one of itself into the event list:
    public class Bell extends Event {
        public Bell(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() {
            addEvent(new Bell(delayTime.toMillis()));
        }
        @Override
        public String toString() {
            return "Bing!";
        }
    }
    public class Restart extends Event {
        private Event[] eventList;
        public
        Restart(long delayTime, Event[] eventList) {
            super(delayTime);
            this.eventList = eventList;
            for(Event e : eventList)
                addEvent(e);
        }
        @Override
        public void action() {
            for(Event e : eventList) {
                e.start(); // Rerun each event
                addEvent(e);
            }
            start(); // Rerun this Event
            addEvent(this);
        }
        @Override
        public String toString() {
            return "Restarting system";
        }
    }
    public static class Terminate extends Event {
        public Terminate(long delayTime) {
            super(delayTime);
        }
        @Override
        public void action() { System.exit(0); }
        @Override
        public String toString() {
            return "Terminating";
        }
    }
}

// innerclasses/GreenhouseController.java
// Configure and execute the greenhouse system
import innerclasses.controller.*;
public class GreenhouseController {
    public static void main(String[] args) {
        GreenhouseControls gc = new GreenhouseControls();
        // Instead of using code, you could parse
        // configuration information from a text file:
        gc.addEvent(gc.new Bell(900));
        Event[] eventList = {
                gc.new ThermostatNight(0),
                gc.new LightOn(200),
                gc.new LightOff(400),
                gc.new WaterOn(600),
                gc.new WaterOff(800),
                gc.new ThermostatDay(1400)
        };
        gc.addEvent(gc.new Restart(2000, eventList));
        gc.addEvent(
                new GreenhouseControls.Terminate(5000));
        gc.run();
    }
}

Thermostat on night setting
Light is on
Light is off
Greenhouse water is on
Greenhouse water is off
Bing!
Thermostat on day setting
Bing!
Restarting system
Thermostat on night setting
Light is on
Light is off
Greenhouse water is on
Bing!
Greenhouse water is off
Thermostat on day setting
Bing!
Restarting system
Thermostat on night setting
Light is on
Light is off
Bing!
Greenhouse water is on
Greenhouse water is off
Terminating

局部内部类

// innerclasses/LocalInnerClass.java
// Holds a sequence of Objects
interface Counter {
    int next();
}
public class LocalInnerClass {
    private int count = 0;
    Counter getCounter(final String name) {
        // A local inner class:
        class LocalCounter implements Counter {
            LocalCounter() {
                // Local inner class can have a constructor
                System.out.println("LocalCounter()");
            }
            @Override
            public int next() {
                System.out.print(name); // Access local final
                return count++;
            }
        }
        return new LocalCounter();
    }
    // Repeat, but with an anonymous inner class:
    Counter getCounter2(final String name) {
        return new Counter() {
            // Anonymous inner class cannot have a named
            // constructor, only an instance initializer:
            {
                System.out.println("Counter()");
            }
            @Override
            public int next() {
                System.out.print(name); // Access local final
                return count++;
            }
        };
    }
    public static void main(String[] args) {
        LocalInnerClass lic = new LocalInnerClass();
        Counter
                c1 = lic.getCounter("Local inner "),
                c2 = lic.getCounter2("Anonymous inner ");
        for(int i = 0; i < 5; i++)
            System.out.println(c1.next());
        for(int i = 0; i < 5; i++)
            System.out.println(c2.next());
    }
}

LocalCounter()
Counter()
Local inner 0
Local inner 1
Local inner 2
Local inner 3
Local inner 4
Anonymous inner 5
Anonymous inner 6
Anonymous inner 7
Anonymous inner 8
Anonymous inner 9

内部类理解并不困难，主要在于使用，个人而言很少使用复杂的内部类使用，原因一是不好维护，二是水平的确有限，一般来讲，内部类经常用于框架搭建和配合设计模式。