// innerclasses/Parcel1.java
// Creating inner classes
public class Parcel1 {
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; }
}
// Using inner classes looks just like
// using any other class, within Parcel1:
public void ship(String dest) {
Contents c = new Contents();
Destination d = new Destination(dest);
System.out.println(d.readLabel());
}
public static void main(String[] args) {
Parcel1 p = new Parcel1();
p.ship("Tasmania");
}
}
// innerclasses/Parcel2.java
// Returning a reference to an inner class
public class Parcel2 {
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 Destination to(String s) {
return new Destination(s);
}
public Contents contents() {
return new Contents();
}
public void ship(String dest) {
Contents c = contents();
Destination d = to(dest);
System.out.println(d.readLabel());
}
public static void main(String[] args) {
Parcel2 p = new Parcel2();
p.ship("Tasmania");
Parcel2 q = new Parcel2();
// Defining references to inner classes:
Parcel2.Contents c = q.contents();
Parcel2.Destination d = q.to("Borneo");
}
}
// 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();
}
}
输出为：
DotThis.f()
有时你可能想要告知某些其他对象，去创建其某个内部类的对象。要实现此目的，你必须在 new 表达式中提供对其他外部类对象的引用，这是需要使用 .new 语法，就像下面这样：
// innerclasses/DotNew.java
// Creating an inner class directly using .new syntax
public class DotNew {
public class Inner {}
public static void main(String[] args) {
DotNew dn = new DotNew();
DotNew.Inner dni = dn.new Inner();
}
}
// 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");
}
}
// 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");
}
}
// 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();
}
}
TrackingSlip 类被嵌入在 if 语句的作用域内，这并不是说该类的创建是有条件的，它其实与别的类一起编译过了。然而，在定义 Trackingslip 的作用域之外，它是不可用的，除此之外，它与普通的类一样。
匿名内部类
下面的例子看起来有点奇怪：
// 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();
}
}
这种奇怪的语法指的是：“创建一个继承自 Contents 的匿名类的对象。”通过 new 表达式返回的引用被自动向上转型为对 Contents 的引用。上述匿名内部类的语法是下述形式的简化形式：
// innerclasses/Parcel7b.java
// Expanded version of Parcel7.java
public class Parcel7b {
class MyContents implements Contents {
private int i = 11;
@Override
public int value() { return i; }
}
public Contents contents() {
return new MyContents();
}
public static void main(String[] args) {
Parcel7b p = new Parcel7b();
Contents c = p.contents();
}
}
// innerclasses/Parcel8.java
// Calling the base-class constructor
public class Parcel8 {
public Wrapping wrapping(int x) {
// Base constructor call:
return new Wrapping(x) { // [1]
@Override
public int value() {
return super.value() * 47;
}
}; // [2]
}
public static void main(String[] args) {
Parcel8 p = new Parcel8();
Wrapping w = p.wrapping(10);
}
}
// innerclasses/Wrapping.java
public class Wrapping {
private int i;
public Wrapping(int x) { i = x; }
public int value() { return i; }
}
为了多样性，Wrapping 拥有一个要求传递一个参数的构造器。
在匿名类中定义字段时，还能够对其执行初始化操作：
// 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");
}
}
如果在定义一个匿名内部类时，它要使用一个外部环境（在本匿名内部类之外定义）对象，那么编译器会要求其（该对象）参数引用是 final 或者是 “effectively final”（也就是说，该参数在初始化后不能被重新赋值，所以可以当作 final）的，就像你在 destination() 的参数中看到的那样。这里省略掉 final 也没问题，但通常加上 final 作为提醒比较好。
// innerclasses/AnonymousConstructor.java
// Creating a constructor for an anonymous inner class
abstract class Base {
Base(int i) {
System.out.println("Base constructor, i = " + i);
}
public abstract void f();
}
public class AnonymousConstructor {
public static Base getBase(int i) {
return new Base(i) {
{ System.out.println(
"Inside instance initializer"); }
@Override
public void f() {
System.out.println("In anonymous f()");
}
};
}
public static void main(String[] args) {
Base base = getBase(47);
base.f();
}
}
输出为：
Base constructor, i = 47
Inside instance initializer
In anonymous f()
在此例中，不要求变量 i 一定是 final 的。因为 i 被传递给匿名类的基类的构造器，它并不会在匿名类内部被直接使用。
下例是带实例初始化的"parcel"形式。注意 destination() 的参数必须是 final 的，因为它们是在匿名类内部使用的（译者注：即使不加 final, Java 8 的编译器也会为我们自动加上 final，以保证数据的一致性）。
// innerclasses/Parcel10.java
// Using "instance initialization" to perform
// construction on an anonymous inner class
public class Parcel10 {
public Destination
destination(final String dest, final float price) {
return new Destination() {
private int cost;
// Instance initialization for each object:
{
cost = Math.round(price);
if(cost > 100)
System.out.println("Over budget!");
}
private String label = dest;
@Override
public String readLabel() { return label; }
};
}
public static void main(String[] args) {
Parcel10 p = new Parcel10();
Destination d = p.destination("Tasmania", 101.395F);
}
}
输出为：
Over budget!
在实例初始化操作的内部，可以看到有一段代码，它们不能作为字段初始化动作的一部分来执行（就是 if 语句）。所以对于匿名类而言，实例初始化的实际效果就是构造器。当然它受到了限制-你不能重载实例初始化方法，所以你仅有一个这样的构造器。
// innerclasses/Parcel11.java
// Nested classes (static inner classes)
public class Parcel11 {
private static class ParcelContents implements Contents {
private int i = 11;
@Override
public int value() { return i; }
}
protected static final class ParcelDestination
implements Destination {
private String label;
private ParcelDestination(String whereTo) {
label = whereTo;
}
@Override
public String readLabel() { return label; }
// Nested classes can contain other static elements:
public static void f() {}
static int x = 10;
static class AnotherLevel {
public static void f() {}
static int x = 10;
}
}
public static Destination destination(String s) {
return new ParcelDestination(s);
}
public static Contents contents() {
return new ParcelContents();
}
public static void main(String[] args) {
Contents c = contents();
Destination d = destination("Tasmania");
}
}
// innerclasses/TestBed.java
// Putting test code in a nested class
// {java TestBed$Tester}
public class TestBed {
public void f() { System.out.println("f()"); }
public static class Tester {
public static void main(String[] args) {
TestBed t = new TestBed();
t.f();
}
}
}
// innerclasses/MultiNestingAccess.java
// Nested classes can access all members of all
// levels of the classes they are nested within
class MNA {
private void f() {}
class A {
private void g() {}
public class B {
void h() {
g();
f();
}
}
}
}
public class MultiNestingAccess {
public static void main(String[] args) {
MNA mna = new MNA();
MNA.A mnaa = mna.new A();
MNA.A.B mnaab = mnaa.new B();
mnaab.h();
}
}
// innerclasses/mui/MultiInterfaces.java
// Two ways a class can implement multiple interfaces
// {java innerclasses.mui.MultiInterfaces}
package innerclasses.mui;
interface A {}
interface B {}
class X implements A, B {}
class Y implements A {
B makeB() {
// Anonymous inner class:
return new B() {};
}
}
public class MultiInterfaces {
static void takesA(A a) {}
static void takesB(B b) {}
public static void main(String[] args) {
X x = new X();
Y y = new Y();
takesA(x);
takesA(y);
takesB(x);
takesB(y.makeB());
}
}
// innerclasses/MultiImplementation.java
// For concrete or abstract classes, inner classes
// produce "multiple implementation inheritance"
// {java innerclasses.MultiImplementation}
package innerclasses;
class D {}
abstract class E {}
class Z extends D {
E makeE() {
return new E() {};
}
}
public class MultiImplementation {
static void takesD(D d) {}
static void takesE(E e) {}
public static void main(String[] args) {
Z z = new Z();
takesD(z);
takesE(z.makeE());
}
}
// 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();
}
}
输出为：
Other operation
1
1
2
Other operation
2
Other operation
3
// 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();
}
下面的文件包含了一个用来管理并触发事件的实际控制框架。Event 对象被保存在 List<Event> 类型（读作“Event 的列表”）的容器对象中，容器会在 集合 中详细介绍。目前读者只需要知道 add() 方法用来将一个 Event 添加到 List 的尾端，size() 方法用来得到 List 中元素的个数，foreach 语法用来连续获取 List 中的 Event，remove() 方法用来从 List 中移除指定的 Event。
// 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();
}
}
输出为：
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
// innerclasses/InheritInner.java
// Inheriting an inner class
class WithInner {
class Inner {}
}
public class InheritInner extends WithInner.Inner {
//- InheritInner() {} // Won't compile
InheritInner(WithInner wi) {
wi.super();
}
public static void main(String[] args) {
WithInner wi = new WithInner();
InheritInner ii = new InheritInner(wi);
}
}
// innerclasses/BigEgg.java
// An inner class cannot be overridden like a method
class Egg {
private Yolk y;
protected class Yolk {
public Yolk() {
System.out.println("Egg.Yolk()");
}
}
Egg() {
System.out.println("New Egg()");
y = new Yolk();
}
}
public class BigEgg extends Egg {
public class Yolk {
public Yolk() {
System.out.println("BigEgg.Yolk()");
}
}
public static void main(String[] args) {
new BigEgg();
}
}
// innerclasses/BigEgg2.java
// Proper inheritance of an inner class
class Egg2 {
protected class Yolk {
public Yolk() {
System.out.println("Egg2.Yolk()");
}
public void f() {
System.out.println("Egg2.Yolk.f()");
}
}
private Yolk y = new Yolk();
Egg2() { System.out.println("New Egg2()"); }
public void insertYolk(Yolk yy) { y = yy; }
public void g() { y.f(); }
}
public class BigEgg2 extends Egg2 {
public class Yolk extends Egg2.Yolk {
public Yolk() {
System.out.println("BigEgg2.Yolk()");
}
@Override
public void f() {
System.out.println("BigEgg2.Yolk.f()");
}
}
public BigEgg2() { insertYolk(new Yolk()); }
public static void main(String[] args) {
Egg2 e2 = new BigEgg2();
e2.g();
}
}
输出为：
Egg2.Yolk()
New Egg2()
Egg2.Yolk()
BigEgg2.Yolk()
BigEgg2.Yolk.f()
// 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());
}
}
输出为：
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