java - 错误 : local variable a is accessed from within inner class; needs to be declared final

Question

class Outer
{
    void m1()
    {
        int a=12;
        class Inner
        {
            void show()
            {
                System.out.println(a);
            }
        }
        new Inner().show();
    }

}

在这里，当我编译这段代码时，我得到了从内部类中访问局部变量 a 的错误；需要声明为final。但是这里“int a”是一个局部变量，所以为什么我们需要声明为 final 以便在内部类中访问。

Answer 1

如果在内部类中使用局部变量，则需要将其声明为 final。对于要在此类内部类中使用的局部变量，Java 在幕后获取局部变量的副本并将其变成隐式实例变量，以便内部类可以访问它。因为它是副本，所以如果值发生变化，副本可能是错误的。因此，编译器会强制您使其成为 final。

请注意，在 Java 8 中，这会编译，因为 a 是“有效最终的”——未声明 final，但一旦初始化就永远不会改变。

Section 8.1.3 of the JLS状态:

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.

Section 4.12.4 of the JLS状态:

A local variable or a method, constructor, lambda, or exception parameter is effectively final if it is not declared final but it never occurs as the left hand operand of an assignment operator (§15.26) or as the operand of a prefix or postfix increment or decrement operator (§15.14, §15.15).

In addition, a local variable whose declaration lacks an initializer is effectively final if all of the following are true:

• It is not declared final.

• Whenever it occurs as the left-hand operand of an assignment operator, it is definitely unassigned and not definitely assigned before the assignment; that is, it is definitely unassigned and not definitely assigned after the right-hand operand of the assignment (§16 (Definite Assignment)).

• It never occurs as the operand of a prefix or postfix increment or decrement operator.