java - 为什么编译器在内部类中接受非最终变量？

Question

我正在创建一个带有 seekbar 的 MediaPlayer。这里，mp 是一个 MediaPlayer 对象，seeker 是一个在 MainActivity 类中创建的 seekbar 对象。

我的理解是匿名内部类只能访问final成员，那么runnable是如何访问这些对象的(mp和seeker)

     h = new Handler();  // create a handler for the MainActivity

 //create a runnable activity to change the progress of seekbar in MainThread for every 1 sec
   Runnable r = new Runnable() {
        @Override
        public void run() {
            if (mp != null) {
                seeker.setProgress(mp.getCurrentPosition() / 1000);
            }
           h.postDelayed(this, 1000);
        }
   };

    this.runOnUiThread(r);

注意:代码运行完美。感谢您的帮助。

Answer 1

它发生了，因为有一种叫做有效最终的东西。

jls-8.1.3:

Any local variable, formal parameter, or exception parameter used but not declared in an inner class must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.

jls-4.12.4:

If a variable is effectively final, adding the final modifier to its declaration will not introduce any compile-time errors. Conversely, a local variable or parameter that is declared final in a valid program becomes effectively final if the final modifier is removed.

当变量只设置一次并在内部类中使用时，编译器将变量的修饰符变为final。

考虑这段代码 Tests.java:

public class Tests  {
   public static void main(String[] args) throws Throwable {
       String value = "Hello";

       Runnable runnable = new Runnable() {
           @Override
           public void run() {
               System.out.println(value);
           }
       };
   }
}

上面的代码在编译时会产生下面的代码。看到编译器把 value 的修饰符改成 final 了吗？

Tests.class(已解码):

public class Tests {
    public Tests() {
    }

    public static void main(String[] args) throws Throwable {
        final String value = "Hello";
        Runnable var10000 = new Runnable() {
            public void run() {
                System.out.println(value);
            }
        };
    }
}

但是，如果你改变变量的值(见下面的代码)，它不会被编译，因为编译器知道它不是一个有效的最终变量:

public class Tests  {
   public static void main(String[] args) throws Throwable {
       String value = "Hello";
       value = "world";

       Runnable runnable = new Runnable() {
           @Override
           public void run() {
               System.out.println(value);
           }
       };
   }
}