Java: Class.this

Question

I have a Java program that looks like this.

public class LocalScreen {

   public void onMake() {
       aFuncCall(LocalScreen.this, oneString, twoString);
   }
}

What does LocalScreen.this means in aFuncCall?

Answer 1

LocalScreen.this refers to this of the enclosing class.

This example should explain it:

public class LocalScreen {

    public void method() {

        new Runnable() {
            public void run() {
                // Prints "An anonymous Runnable"
                System.out.println(this.toString());

                // Prints "A LocalScreen object"
                System.out.println(LocalScreen.this.toString());

                // Won't compile! 'this' is a Runnable!
                //onMake(this);

                // Compiles! Refers to enclosing object
                onMake(LocalScreen.this);
            }

            public String toString() {
                return "An anonymous Runnable!";
            }
        }.run();
    }

    public String toString() { return "A LocalScreen object";  }

    public void onMake(LocalScreen ls) { /* ... */ }

    public static void main(String[] args) {
        new LocalScreen().method();
    }
}

Output:

An anonymous Runnable!
A LocalScreen object

---

This post has been rewritten as an article here.

Answer 2

I know what is your confusion.I am encounter the problem just now, it should have special scene to distinguish them.

class THIS {
  def andthen = {
    new THIS {
      println(THIS.this.## + ":inner-THIS.this.##")
      println(this.## + ":inner-this.##")
      new THIS {
        println(THIS.this.## + ":inner-inner-THIS.this.##")
        println(this.## + ":inner-this.##")
      }
    }
  }
  def getInfo = {
    println(THIS.this.## + ":THIS.this.##")
    println(this.## + ":this.##")
  }
}

You can see the diff between THIS.this and this in new THIS operation by hashcode( .## )

test in scala console :

scala> val x = new THIS
x: THIS = THIS@5ab9b447

scala> val y = x.andthen
1522119751:inner-THIS.this.##
404586280:inner-this.##
1522119751:inner-inner-THIS.this.##
2027227708:inner-this.##
y: THIS = THIS$$anon$1@181d7f28

scala> x.getInfo
1522119751:THIS.this.##
1522119751:this.##

THIS.this always point to outer THIS class which is refer by val x,but this is beyond to anonymous new operation.

Answer 3

Class.this allows access to instance of the outer class. See the following example.

public class A
{
  final String name;
  final B      b;
  A(String name) {
    this.name = name;
    this.b = new B(name + "-b");
  }

  class B
  {
    final String name;
    final C      c;
    B(String name) {
      this.name = name;
      this.c = new C(name + "-c");
    }

    class C
    {
      final String name;
      final D      d;
      C(String name) {
        this.name = name;
        this.d = new D(name + "-d");
      }

      class D
      {
        final String name;
        D(String name) {
          this.name = name;
        }

        void printMe()
        {
          System.out.println("D: " + D.this.name); // `this` of class D
          System.out.println("C: " + C.this.name); // `this` of class C
          System.out.println("B: " + B.this.name); // `this` of class B
          System.out.println("A: " + A.this.name); // `this` of class A
        }
      }
    }
  }
  static public void main(String ... args)
  {
    final A a = new A("a");
    a.b.c.d.printMe();
  }
}

Then you will get.

D: a-b-c-d
C: a-b-c
B: a-b
A: a

Answer 4

It means the this instance of the outer LocalScreen class.

Writing this without a qualifier will return the instance of the inner class that the call is inside of.

Answer 5

The compiler takes the code and does something like this with it:

public class LocalScreen 
{
    public void method() 
    {
        new LocalScreen$1(this).run;
    }

    public String toString() 
    {
        return "A LocalScreen object"; 
    }

    public void onMake(LocalScreen ls) { /* ... */ }

    public static void main(String[] args) 
    {
        new LocalScreen().method();
    }
}

class LocalScreen$1
     extends Runnable
{
    final LocalScreen $this;

    LocalScreen$1(LocalScreen $this)
    {
        this.$this = $this;
    }

    public void run() 
    {
        // Prints "An anonymous Runnable"
        System.out.println(this.toString());

        // Prints "A LocalScreen object"
        System.out.println($this.toString());

        // Won't compile! 'this' is a Runnable!
        //onMake(this);

        // Compiles! Refers to enclosing object
        $this.onMake($this);
    }

    public String toString() 
    {
        return "An anonymous Runnable!";
    }
}

As you can see, when the compiler takes an inner class it converts it to an outer class (this was a design decision made a LONG time ago so that VMs did not need to be changed to understand inner classes).

When a non-static inner class is made it needs a reference to the parent so that it can call methods/access variables of the outer class.

The this inside of what was the inner class is not the proper type, you need to gain access to the outer class to get the right type for calling the onMake method.