“this” keyword: Working mechanism in Java

Question

After learning Java for sometime, its the first time the use of this keyword has confused me so much.

Here is how I got confused. I wrote the following

Answer 1

1. Why x and this.x point to the x of base class and not the Child class?

we can see this example:

class TestBase {
    private int x;
    public void a() {
        this.x++;
    }
    public int getX() {
        return x;
    }
}
public class Test extends TestBase{
    private int x;
    public int getX() {
        return this.x;
    }
}

and generated bytecode:

public class Test extends TestBase{
public Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method TestBase."":()V
   4:   return

public int getX();
  Code:
   0:   aload_0
   1:   getfield        #2; //Field x:I
   4:   ireturn

public void a();
  Code:
   0:   aload_0
   1:   invokespecial   #3; //Method TestBase.a:()V
   4:   return

}

In there the Test extends TestBase and the method a is compiled into the Test class, it will call it's father 1: invokespecial #3; //Method TestBase.a:()V.

the Test's getX method will call 1: getfield #2; //Field x:I from it's own constant pool table, http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings

the TestBase class bytecode:

class TestBase extends java.lang.Object{
TestBase();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

public void a();
  Code:
   0:   aload_0
   1:   dup
   2:   getfield        #2; //Field x:I
   5:   iconst_1
   6:   iadd
   7:   putfield        #2; //Field x:I
   10:  return

public int getX();
  Code:
   0:   aload_0
   1:   getfield        #2; //Field x:I
   4:   ireturn

}

the method a() also will get x from it's own constant pool by getfield #2; //Field x:I.

so there is another thing: the Java's getter and setter is evil.