為什么內部類實例變量不能修改外部類實例變量，而內部類局部變量可以

Question

為什么test變量在localClassMethod內部和tt方法外部發生編譯錯誤，而在tt方法中編譯是可以的。 這意味着內部類實例變量不能修改外部類實例變量，但內部類局部變量可以修改外部類實例變量。

public class Outer {
    int test = 0;

    void classMethod() {
        class localClassInMethod {
            int k = test;//compile ok
            test = 1;//compile error

            public void tt() {
                test++;//compile ok
                int m = test;//compile ok
            }
        }
    }
}

Answer 1

雖然線條看起來相似，但它們並不相同：

public class Outer {
    int test = 0; // This is a field declaration, with initializer.

    void classMethod() {
        class localClassInMethod {
            int k = test; // This is a field declaration, with initializer.
            test = 1;     // This is an assignment statement, and those are only
                          // valid inside a method body or initializer block.

            public void tt() {
                test++;       // This is a post-increment expression statement.
                int m = test; // This is a local variable declaration, with initializer.

                test = 2;     // Assignment statement is valid here.
            }
        }
    }
}

如果要在創建localClassInMethod的新實例時運行代碼以將值1分配給字段test ，請使用實例初始值設定項塊：

        class localClassInMethod {
            int k = test;
            { // Initializer block.
                test = 1; // Assignment statement is valid here.
            }

            public void tt() {
                ...
            }
        }

這與將語句放在每個構造函數中是一樣的：

        class localClassInMethod {
            int k = test;

            public localClassInMethod() {
                test = 1; // Assignment statement is valid here.
            }

            public void tt() {
                ...
            }
        }