匿名线程类无法访问非静态实例变量

Question

I am trying to access instance variable inside a Thread anonymous class . I am getting an error here saying to make it static . The point here is if i can access "this" keyword inside the anonymous class which treats it as its current object holder, then why is it not able to access the instance variables in a non static way .

public class AnonymousThreadDemo {
    int num;

    public AnonymousThreadDemo(int num) {
        this.num = num;
    }

    public static void main(String[] args) {
        Thread thread = new Thread() {
            @Override
            public void run() {
                System.out.println("Anonymous " + num); // Why cant we access num instance variable
                System.out.println("Anonymous " + this); // This can be accessed in a nonstatic way
            }
        };
        thread.start();
    }
}

Answer 1

num is a non static field, it belongs to a specific instance. You can not reference it in static main directly, because a static method can be called without creating an instance.

this is actually referencing thread , it is a local variable, when you execute run , the thread must have been created.

If you try reference AnonymousThreadDemo.this in main , you will get same result:

public static void main(String[] args) {
    Thread thread = new Thread() {

        @Override
        public void run() {
            System.out.println("Anonymous " + AnonymousThreadDemo.this); // compile error
        }
    };
    thread.start();
}

---

This is ok, you can reference a local variable in local class:

public static void main(String[] args) {
    int num = 0;

    Thread thread = new Thread() {
        @Override
        public void run() {
            System.out.println("Anonymous " + num);
        }
    };
    thread.start();
}

---

This is ok, you can reference a non static local class field in its method:

public static void main(String[] args) {
    Thread thread = new Thread() {
        int num = 0;

        @Override
        public void run() {
            System.out.println("Anonymous " + num);
        }
    };
    thread.start();
}

---

Check this for more.

Answer 2

num is non static that means it will come after static main in the memory. Hence when main will try to point num it won't be available in the memory ie. It still won't be declared yet.