局部内部类——内部实现与外在形式一致

 package test;
public class TestFinal 
{
	//只有定义成 final User user 才不会报错为什么？
	public void test( User user)
	{
		(new Thread()
			{ 
				public void run()
					{   
						System.out.println("user.name-->"+user.name);
					}
			} 
		).start();  
	}
	public static void main(String[] args) 
	{
		User user=new User();
		user.setId(007);
		user.setName("zhaoyang");
		
		TestFinal testFinal=new TestFinal();
		testFinal.test(user);	
	}
}


class  User
{
	String name=null;
	int id;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	
}

<span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">1)从程序设计语言的</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">理论上</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">:</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">局部内部类</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">(即:定义在方法中的内部类),由于本身就是在方法内部(可出现在形式参数定义处或者方法体处),因而</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">访问方法中的局部变量</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">(形式参数或局部变量)是</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">天经地义的</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">.是很自然的</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">2)为什么JAVA中要加上一条限制:只能访问final型的局部变量?</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">3)JAVA语言的编译程序的设计者当然全实现:局部内部类能访问方法中的所有的局部变量(因为:从理论上这是很自然的要求),但是:</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">编译技术是无法实现的或代价极高</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">.</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">4)困难在何处?</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">到底难在哪儿</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">?</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">局部变量的生命周期与局部内部类的对象的生命周期的不一致性!</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">5)设方法f被调用,从而在它的调用栈中生成了变量i,此时产生了一个局部内部类对象inner_object,它访问了该局部变量i .当</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">方法f()运行结束后,局部变量i就已死亡了,不存在了.</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">但:局部内部类对象inner_object还可能一直存在(只能没有人再引用该对象时,它才会死亡),它不会随着方法f()运行结束死亡.这时:出现了</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">一个"荒唐"结果</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">:局部内部类对象inner_object</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">要访问一个已不存在的局部变量i!</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">6)</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">“当变量是final时,通过将final局部变量"复制"一份”</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">这不是final的理由，因为从编译实现上，当普通局部变量传给匿名类的时候同样可以“复制”。</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">其实就是传给匿名类的构造方法，使方法中的局部变量和匿名类中同时拥有这个对象的引用。</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">但真正的原因就在这里，当外面的方法重新赋值的时候，不影响内部类的句柄指向那个对象，</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">即外部的变量指向A而匿名类的变量却指向B了，这才是不能接受的结果。</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">因为从源程序上看它们是同一变量。</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);"></span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">当变量是final时,若是引用类型,由于其引用值不变(即:永远指向同一个对象),因而:其复制品与原始的引用变量一样,永远指向同一个对象(由于是final,从而保证:只能指向这个对象,再不能指向其它对象),达到:局部内部类中访问的复制品与方法代码中访问的原始对象,永远都是同一个即:语义效果是一样的.</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">否则</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">:当方法中改原始变量,而局部内部类中改复制品时,就</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">无法保证:复制品与原始变量保持一致了(因此:它们原本就应该是同一个变量.)</span><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><br style="box-sizing: content-box; color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);" /><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">一句话:这个规定是一种无可奈何.也说明:程序设计语言的</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">设计</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">是</span><span style="box-sizing: content-box; font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; color: rgb(255, 0, 0); background-color: rgb(245, 245, 245);">受到实现技术的限制</span><span style="color: rgb(51, 51, 51); font-family: Helvetica, Tahoma, Arial, sans-serif; font-size: 14px; line-height: 24px; background-color: rgb(245, 245, 245);">的.这就是一例. 因为:我就看到不少人都持这种观点:设计与想法是最重要的,实现的技术是无关紧要的,只要你作出设计与规定,都能实现.</span>