Java的内部静态类

Java的内部类语法比较复杂，只有内部静态类，而没有静态类。

public class Hello {
public class Child {
public Child() {}
public void sayHello() {
System.out.println("Hello from child!");
}
}

public static void main(String[] args) {
System.out.println("Hello, World!");
Hello h = new Hello();
Child child = h.new Child();
child.sayHello();
}
}

上面的例子用到了.new的语法，因为main是个静态方法，所以Hello类不会被初始化，Child类也不会被加载到内存中，必须显示地初始化Hello类，然后使用.new的语法来初始化内部类。当然还可以使用内部静态类。

public final class Hello {
public static class Child {
public Child() {
}

public void sayHello() {
System.out.println("Hello from child!");
}
}

public static void main(String[] args) {
System.out.println("Hello, World!");
Child child = new Hello.Child();
child.sayHello();
}
}

这里因为内部的Child类是static，所以即使Hello没有初始化，Child也已经加载到内存中了。

这样的处理方式和类中的static成员是一致的，内部类可以随意的访问外部类，这是内部类最大的特点。

public class Outter {
public class Inner {
private int in = 3;
public void print() {
System.out.println(Outter.this.out);
}
}

private int out = 4;

public void print() {
System.out.println(new Inner().in);
}

public static void main(String[] args) {
new Outter().new Inner().print();
}
}

诡异的内部类语法

内部类的使用，下面是一个command模式的例子，摘自Java编程思想（Thinking in java)

Event.java

public abstract class Event {
private long eventTime;
protected final long delayTime;
public Event(long delayTime) {
super();
this.delayTime = delayTime;
start();
}

public void start() {
eventTime = System.nanoTime() + delayTime;
}

public boolean ready() {
return System.nanoTime() >= eventTime;
}

public abstract void action();
}

Controller.java

import java.util.ArrayList;
import java.util.List;

public class Controller {
private List<Event> eventList = new ArrayList<Event>();
public void addEvent(Event c) {
eventList.add(c);
}
public void run() {
while (eventList.size() > 0) {
for (Event e : new ArrayList<Event>(eventList)) {
if (e.ready()) {
System.out.println(e);
e.action();
eventList.remove(e);
}
}
}
}
}

GreenhouseControls.java

public class GreenhouseControls extends Controller {
private boolean light = false;

public class LightOn extends Event {
public LightOn(long delayTime) {
super(delayTime);
}

@Override
public void action() {
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() {
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() {
water = true;
}

@Override
public String toString() {
return "Water is on.";
}
}

public class WaterOff extends Event {
public WaterOff(long delayTime) {
super(delayTime);
}

@Override
public void action() {
water = false;
}

@Override
public String toString() {
return "Water is off.";
}
}

// 恒温器 空调
private String thermostat = "Day";
public class ThermostatNight extends Event {

public ThermostatNight(long delayTime) {
super(delayTime);
}

@Override
public void action() {
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() {
thermostat = "Day";
}

@Override
public String toString() {
return "Thermostat on day setting.";
}
}

public class Bell extends Event {

public Bell(long delayTime) {
super(delayTime);
}

@Override
public void action() {
addEvent(new Bell(delayTime));
}

@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();
addEvent(e);
}
start();
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.";
}
}
}

GreenhouseController.java

public class GreenhouseController {
public static void main(String[] args) {
GreenhouseControls gc = new GreenhouseControls();
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));

if (args.length == 1) {
gc.addEvent(new GreenhouseControls.Terminate(new Integer(args[0])));
}
gc.run();
}
}

上述的内部类实现command模式，还是比较优雅的。