Question

Though I’m trying to understand why “this” is needed, I’m very confused about its purpose. For instance, I coded the following:

public static void main (String args[])
{
    SandboxClass1 temp = new SandboxClass1(1,2,3);  
    System.out.println(temp.getX());
    System.out.println(temp.getY());
    System.out.println(temp.getZ());

    System.out.println("----------------------------");

    SandboxClass1 temp2 = new SandboxClass1(4,5,6);
    System.out.println(temp2.getX());
    System.out.println(temp2.getY());
    System.out.println(temp2.getZ());
}

public class SandboxClass1 
{

private int x = 1;
private int y = 1;
private int z = 0;

public SandboxClass1(int x, int y, int zz)
{
    this.x = x;
    this.y = y;
    z = zz;
}

public int getX()
{
    return(this.x);
}

public int getY()
{
    return(this.y);
}

public int getZ()
{
    return(this.z);
}

}

Why do I need to code “this.z = zz”
when I could just as well write, “z = zz”?

Answer 1

In your SandboxClass1 constructor, you have two pairs of variables each called x and y. There’s the x and y declared on the object itself (“private int x = 1”), and the separate x and y that are parameters to the constructor (“int x”).

The local (parameter) variable shadows the class variable. So if in the constructor you just did

x = x;

the assignment would have no effect.

The keyword this is a reference to the object that the method/constructor was called on. In the statement

this.x = x;

you’re using it to assign to the other x at class level. By qualifying the name, you can tell them apart.

It’s not necessary to use this with the z/zz assignment because they have different names.

It’s also not necessary in the getX/Y/Z methods because there are no local variables in those methods shadowing the relevant class variables. It does no harm though.