为什么我不能在声明方法之外访问本地类？

Question

I recently bumped into a rare -- but perfectly legal syntax: Local Classes .

I was wondering why can't I access a local class outside that method? With what is it different from an inner class which can be access in the outer class or with an enclosed object ( outer.new Inner() )?

Example: this is perfectly valid,

//this is valid
public class Outer {
    int outer_x = 100;

    public void test() {
        Inner inner = new Inner();
        inner.display();

    }

    public class Inner {
        public void display() {
            System.out.println("Outer x is: " + outer_x);
        }
    }
}

This is valid as well

 //this is valid as well
public class Outer {
    int outer_x = 100;

    public void test() {
        Inner inner = new Inner();
        inner.display();
    }

    public class Inner {
        public void display() {
            System.out.println("Outer x is: " + outer_x);
        }
    }

    public void test2() {

        Inner inner2 = new Inner();
        inner2.display();
    }
}

But this will not compile:

public class Outer {
    int outer_x = 100;

    public void test() {

        class Inner {

            public void display() {
                System.out.println("Outer x is: " + outer_x);
            }
        }
        Inner inner = new Inner();
        inner.display();

    }

    public void test2() {

        Inner inner2 = new Inner(); // error here
        inner2.display();
    }
}

Why is this so?

Answer 1

Because every call to test() creates a complete new version of the class. Therefore it may access (final or effectively final) local variables!

Here's a rundown what's going on.

public class Outer {

    public void test(int i) {
        class Inner {
            private int x = i; // i may be different on each call of test

            public void display() {
                System.out.println("Inner x is: " + x);
            }
        }
        Inner inner = new Inner();
        inner.display();
    }

    public void test2() {
        test(1); // prints 1
        test(2); // prints 2

        //now imagine this is valid
        Inner inner2 = new Inner();
        inner2.display();// what's the value of x??
    }
}