How does the use of (this) relate to object orientated programming

General Discussion Other

wpkelley1

How does the use of (this) relate to object orientated programming

in General Discussion • Other • 10 months ago

I've been trying to understand the use of (this) when referring to the current object. Does (this) reference the sketch that is being written or does it refer to the last object encountered in the sketch? What is the current object, I'm new to the use of objects.

Replies(12)

GoToLoop

Re: How does the use of (this) relate to object orientated programming

10 months ago

If this is inside a class (that is, a code within a class block),
it refers to that class itself (or an instance of it).
If it's outside any class, it refers to the sketch.
And sketch is a class itself (extends PApplet's)!

wpkelley1

Re: How does the use of (this) relate to object orientated programming

10 months ago

Thanks for your answer, it makes sense to me now. I've looked at several books and the javadoc and it wasn't as clear as your answer.

Thanks

WPK

Chrisir

Re: How does the use of (this) relate to object orientated programming

10 months ago

argh...

I need a reference to the sketch/applet within a class.

When I use " this", it gives me the class, but I need the skecth...

timpulver

Re: How does the use of (this) relate to object orientated programming

10 months ago

What about this:

A a;
void setup() {
a = new A(this);
a.doStuff();
}
void draw() {}
public class A{
PApplet p;
A(PApplet p) {
this.p = p;
}
void doStuff() {
p.println("stuff");
}
}

GoToLoop

Re: Re: How does the use of (this) relate to object orientated programming

10 months ago

Excellent example timpulver!

Pass the this belonging to sketch, and when it's still inside one of the sketch's methods, as a PApplet parameter to the class which needs to know about it!

Chrisir

Re: How does the use of (this) relate to object orientated programming

10 months ago

Thanks, tim!

Chrisir

Re: How does the use of (this) relate to object orientated programming

10 months ago

this approach is quite longwinded, since my class is called from another class so I have to pass "this" on twice......

I thought it might exist a global var like displayHeight holding the reference to the sketch no matter whether you are in a class or not.

What is worse, my code doesn't work at all but it doesn't tell why or where....

Thanks!

PhiLho

Re: How does the use of (this) relate to object orientated programming

10 months ago

It might be longwinded, but that's the classical method: look at all the libraries — most of them have this in the constructor.
Note that nothing prevents you to make a global variable yourself, that you assign with the this value.

GoToLoop

Re: Re: How does the use of (this) relate to object orientated programming

10 months ago

Like this one?:

PApplet sketch = this;
println(sketch);

timpulver

Re: How does the use of (this) relate to object orientated programming

10 months ago

this approach is quite longwinded, since my class is called from another class so I have to pass "this" on twice......

Sometimes it will be enough to just import processing.core.PApplet, there are a lot of static functions you can use without needing an object of PApplet (this).

sketch.pde

A a;
void setup() {
a = new A();
a.doStuff();
}

A.pde

import processing.core.PApplet;
public class A{
public void doStuff() {
PApplet.println("stuff");
}
}

PhiLho

Re: How does the use of (this) relate to object orientated programming

9 months ago

This trick is useful in a .java file / tab, not necessary in a .pde tab (as A will be a class nested in the sketch class).

timpulver

Re: How does the use of (this) relate to object orientated programming

9 months ago

You're right, A.pde will work like this, too:

class A{
public void doStuff() {
println("stuff");
ellipse(50, 50, 50, 50);
}
}

Just wanted to show, that you can access static functions without a reference to the sketch.

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