LibGDX instanceof and this

Question

Well I'm new to LibGDX and Java actually and I'm trying to create a game by watching tutorials about what I need to do.

So here's the question I have specific objects to check collision with and

    public void beginContact(Contact contact)
    {
        if((contact.getFixtureA().getBody().getUserData() == "player" && contact.getFixtureB().getUserData() instanceof InteractiveTileObjects) )
    {
        Gdx.app.log("Yeah","");
    }

that works perfect, but when I go to InteractiveTileObjects the last part of the code is fixture = body.createFixture(fdef); I use it to setUserData to that specific objects. Here is the code:

bdef.type = BodyDef.BodyType.DynamicBody;
    bdef.position.set((bounds.getX() + bounds.getWidth() / 2) / MainClass.PPM, (bounds.getY() + bounds.getHeight() / 2) / MainClass.PPM);
    body = world.createBody(bdef);
    shape.setAsBox((bounds.getWidth() / 2) / MainClass.PPM, (bounds.getHeight()/ 2) / MainClass.PPM);
    fdef.shape = shape;
    fdef.filter.categoryBits=MainClass.BIT_DCATCHER;
    fixture = body.createFixture(fdef);

And this is one of my specific objects below:

public class DreamCatcher extends InteractiveTileObjects {

    public DreamCatcher(World world, TiledMap map, Rectangle bounds)
    {
        super(world, map, bounds);

        fixture.setUserData(this);
        setCategoryFilter(MainClass.BIT_DCATCHER);
    }

As you can see I use

fixture.setUserData(this) 

and when I change this to

fixture.setUserData("DreamCatcher")

it doesnt work in my beginContact part because of the instanceof InteractiveTileObjects. But again in begin contact if I change

contact.getFixtureB().getUserData() == "DreamCatcher"

it works perfect again what is that "this" doing to work that instanceof code ? I mean why it's like that ?

I know it's long but I'd be glad if someone can answer these...

Answer 1

1) instanceof:

In java instanceof operator is used to test whether the object is an instance of the specified type (class or subclass or interface).

The instanceof in java is also known as type comparison operator because it compares the instance with type. It returns either true or false. If we apply the instanceof operator with any variable that has null value, it returns false.

example of instanceof operator:

class Animal{}  
class Dog1 extends Animal{//Dog inherits Animal  

 public static void main(String args[]){  
 Dog1 d=new Dog1();  
 System.out.println(d instanceof Animal);//true  
 }  
}  

2) this keyword:

There can be a lot of usage of java this keyword. In java, this is a reference variable that refers to the current object. Usage of java this keyword

Here is given the 6 usage of java this keyword.

• this keyword can be used to refer current class instance variable.
• this() can be used to invoke current class constructor.
• this keyword can be used to invoke current class method (implicitly)
• this can be passed as an argument in the method call.
• this can be passed as argument in the constructor call.
• this keyword can also be used to return the current class instance.

Changing fixture.setUserData(this) to fixture.setUserData("DreamCatcher") will not work as setUserData() method expecting object of type DreamCatcher and not String.