Why do we use this.A_Class

Question

public class HexBoard extends Canvas implements MouseListener {
    public Matrix white = null;
    public Matrix black = null;
    public void init() {
        makeMove(0, this.white, this.black);
        this.white.setValue(20, 10, 1);
       //makeMove(0, white, black)
       //white.setValue(20, 10, 1);
}command code gives me the same result, what does "this" acturally do in java?

Answer 1

"this" is a reference to the object whose init() method is being invoked. You also see it used in constructors where it refers to the object being constructed.
Some people use it to make it clear that white as an instance variable as you observe it adds no meaning in that case. It is handy if white also occurs as an argument and in this case it does alter the meaning because there are two different variables involved and "this" differentiates between them:
public void init(Matrix white, Matrix black) {
        // "essential" use of this
    this.white = white;
    this.black = black;
    makeMove(0, this.white, this.black);
    this.white.setValue(20, 10, 1);
}