Java小白踩坑录 - Shadowing Obscuring 揭秘

Java 中有时候也会出现真假美猴王的事件，请看下面的程序后打印什么？

public class Pet {public final String name;public final String food;public final String sound;public Pet(String name, String food, String sound) {this.name = name;this.food = food;this.sound = sound;}public void eat() {System.out.println(name + ": Mmmmm, " + food);}public void play() {System.out.println(name + ": " + sound + " " + sound);}public void sleep() {System.out.println(name + ": Zzzzzzz...");}public void live() {new Thread() {public void run() {while (true) {eat();play();sleep();}}}.start();}public static void main(String[] args) {new Pet("Fido", "beef", "Woof").live();}
}

我们期望程序打印：

Fido: Mmmmm, beefFido: Woof WoofFido: Zzzzzzz…

实际上报编译错误。

The method sleep(long) in the type Thread is not applicable for the arguments ()

查看 Thread 的 sleep 方法：

 /*** Causes the currently executing thread to sleep (temporarily cease* execution) for the specified number of milliseconds, subject to* the precision and accuracy of system timers and schedulers. The thread* does not lose ownership of any monitors.** @param millis* the length of time to sleep in milliseconds** @throws IllegalArgumentException* if the value of {@code millis} is negative** @throws InterruptedException* if any thread has interrupted the current thread. The* <i>interrupted status</i> of the current thread is* cleared when this exception is thrown.*/public static native void sleep(long millis) throws InterruptedException;/*** Causes the currently executing thread to sleep (temporarily cease* execution) for the specified number of milliseconds plus the specified* number of nanoseconds, subject to the precision and accuracy of system* timers and schedulers. The thread does not lose ownership of any* monitors.** @param millis* the length of time to sleep in milliseconds** @param nanos* {@code 0-999999} additional nanoseconds to sleep** @throws IllegalArgumentException* if the value of {@code millis} is negative, or the value of* {@code nanos} is not in the range {@code 0-999999}** @throws InterruptedException* if any thread has interrupted the current thread. The* <i>interrupted status</i> of the current thread is* cleared when this exception is thrown.*/public static void sleep(long millis, int nanos)throws InterruptedException {if (millis < 0) {throw new IllegalArgumentException("timeout value is negative");}if (nanos < 0 || nanos > 999999) {throw new IllegalArgumentException("nanosecond timeout value out of range");}if (nanos >= 500000 || (nanos != 0 && millis == 0)) {millis++;}sleep(millis);}

等等！

我不是要调用 Thread 的 sleep 方法，而是要调用 Pet 的 sleep 方法。为什么出现这种情况呢？

JSL-6.4 定义了这种情况：

It is a compile-time error if the name of a formal parameter is used to declare a new variable within the body of the method, constructor, or lambda expression, unless the new variable is declared within a class declaration contained by the method, constructor, or lambda expression.

注：如果形参的名字用在方法体、构造器体或者 lambda 表示式体内声明的新变量，将会抛出编译时错误，除非该新变量是在方法体、构造器体或者 lambda 表示式所包含的类声明内声明的。

It is a compile-time error if the name of a local variable v is used to declare a new variable within the scope of v, unless the new variable is declared within a class whose declaration is within the scope of v.

注：如果局部变量v的名字用来在v的作用域内声明新的变量，将会抛出编译时错误，除非该新变量是其类声明在v作用域的类内声明的。

It is a compile-time error if the name of an exception parameter is used to declare a new variable within the Block of the catch clause, unless the new variable is declared within a class declaration contained by the Block of the catch clause.

注：如果表达式参数的名字用在catch子句的语句块内声明的新变量，将会抛出编译时错误，除非该新变量是在catch子句的语句块所包含的类声明内声明的。

It is a compile-time error if the name of a local class C is used to declare a new local class within the scope of C, unless the new local class is declared within another class whose declaration is within the scope of C.

注：如果局部类c的名字用来在c的作用域内声明新的局部类，将会抛出编译时错误，除非该新局部类是在其类声明在c的作用域内的另一个类内声明的。

Java 中有 Shadowing （遮蔽)）的描述，其中：

Shadowing：Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

简单的意思是：在作用域内，一个地方的声明可能被另一个同名的声明所遮蔽。在这种情况下不能简单的使用名字来引用他们所声明的实体。

变量 Shadowing 举例：

class Test1 {public static void main(String[] args) {int i;for (int i = 0; i < 10; i++)System.out.println(i);}
}

编译报错，但编译检测也不是万能的，也有一些 trick 来逃避：

class Test2 {public static void main(String[] args) {int i;class Local {{for (int i = 0; i < 10; i++)System.out.println(i);}}new Local();}
}

如果在不同 block，则不会出现 Shadowing 的问题：

class Test3 {public static void main(String[] args) {for (int i = 0; i < 10; i++)System.out.print(i + " ");for (int i = 10; i > 0; i--)System.out.print(i + " ");System.out.println();}
}

原因找到了，那该怎么解决呢？

问题解决

方式一：线程内调用，改成 Pet.this.sleep(); 限定具体的方法：

public void live() {new Thread() {public void run() {while (true) {eat();play();Pet.this.sleep();}}}.start();
}

方式二：将 sleep 名称改为其它不冲突的名称，如 petSleep，然后线程内调用该方法：

public void petSleep() {System.out.println(name + ": Zzzzzzz...");
}

方式三：也是最好的方式，使用 Thread(Runnable) 构造器来替代对 Thread 的继承。那个匿名类不会再继承Thread.sleep 方法，故也不会有冲突了。

public void live(){new Thread(new Runnable(){public void run(){while(true){eat();play();sleep();}}}).start();
}

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