using "this" in java

Question

Hello i'm still kind of new to java.. I get the concept of "this" when it comes to instance variables but when i use it in a constructor with no parameters i get a little confused. So my question is how does something like this work?

private double x;
private double y;

public static final double EPSILON = 1e-5;
public static boolean debug = false;


public Point(double x, double y){
    this.x=x;
    this.y=y;  // Done sets the x,y private types to the x,y type provided in the ()
}

public Point(){
    this(0.0,0.0);  //Sets, x and y to doubles of 0.0,0.0?? 
}                   //How does this work?

Would my point() constructor create an origin of (0.0,0.0) by calling the point (x,y) constructor? Any clarification on this would help me out a lot!

*"Would my point() constructor create an origin of (0.0,0.0) by calling the point (x,y) constructor?"* - Yes, that's the point. `this(...)` allows you to chain constructor calls together to insure the state of the object when it is created — MadProgrammer, Jun 25 '14 at 02:21

score 7 · Answer 1 · answered Jun 25 '14 at 02:22

this(arguments) is a special syntax only available inside constructors. What it does is call a another constructors with the given arguments. So calling this(0.0, 0.0) will invoke the constructor Point(double, double) with the values (0.0, 0.0). This, in turn, will set x and y to 0.0.

score 1 · Answer 2 · answered Jun 25 '14 at 02:21

When calling this(), you redirect the call of that constructor to another constructor (in this case the first constructor). So you create a Point (0,0).

You basically states that whenever one calls new Point(), it is replaced by Java with new Point(0.0,0.0)

It can sometimes be useful to do the opposite (call a constructor with less parameters). In that case each constructor simply handles its additional parameters which is more oriented to "separation of concerns".

For instance:

public class Point {

    private double x = 0.0d;
    private double y = 0.0d;

    public Point () {
    }

    public Point (double x) {
        this();
        this.x = x;
    }

    public Point (double x, double y) {
        this(x);
        this.y = y;
    }

}

score 0 · Answer 3 · answered Jun 25 '14 at 02:25

Poine() will init x and y with 0.0

Point(double x, double y) will init x and y with x and y in the params of the function.

this in Point(double x, double y) was a pointer of the Point class

this in Point() you can see as a Constructor for the Point class , it will

call Point(double x , double y).

using "this" in java

3 Answers3