Why is the value incremented when assigned?

Question

I'm making a GUI for the game (Open Gl). That is, I have a Gui Screen on which other components are added. So I wanted to make all other GUIs move to the left relative to the original position of GuiScreen. It works but not as I wanted simply contrary to logic.

Here i have this situation (this code runs in onUpdate(), just do every frame):

private void moveRelativeContent(Gui guiContent)
{
    guiContent.position.x = guiContent.startPosition.x + this.position.x();
    guiContent.position.y = guiContent.startPosition.y + this.position.y();
}

As you can see I'm just using assignment. That is, I assign the content position from the List<Gui> to the sum of the original content position plus the current GuiScreen position. That is guiContent.position needs each frame always be guiContent.startPosition + this.position;. It's logical. But for me it's work very strange. I have every frame guiContent.position increases. It's like he's using the += command instead of the = command.

Values before method:

Before: ScreenPosX: 0.02
Before: Current Gui PosX: 0.0039999485
Before: Current Gui Start PosX: 0.0039999485

Values after method:

After: ScreenPos: 0.02
After: Current Gui PosX: 0.023999948
After: Current Gui Start PosX: 0.023999948

First look in my initialized GameObject code (include Guis):

public GameObject(float posX, float posY, float posZ, 
                    float rotX, float rotY, float rotZ, 
                        float scaleX, float scaleY, float scaleZ) 
{
    this();
    ...

    this.startPosition = new Vector3f(posX, posY, posZ);
    this.startRotation = new Vector3f(rotX, rotY, rotZ);
    this.startScale    = new Vector3f(scaleX, scaleY, scaleZ);
    this.position = this.startPosition;
    this.rotation = this.startRotation;
    this.scale    = this.startScale;

    ...
}

Here another version's of this constructor:

public GameObject(Vector3f position, Vector3f rotation, Vector3f scale) 
{
    this(position.x(), position.y(), position.z(), 
         rotation.x(), rotation.y(), rotation.z(), 
           scale.x() ,   scale.y() ,  scale.z() );
}

public GameObject(Vector2f position, Vector2f rotation, Vector2f scale) 
{
    this(position.x(), position.y(), 
         rotation.x(), rotation.y(), 
           scale.x() ,  scale.y() );
}

Next where started updated GuiScreen. This code, from my Scene and WorldScene.class in main onUpdate():

@Override
public abstract void onUpdate(); // in Scene

@Override
public void onUpdate() // in WorldScene
{
    if(!this.isInGamePause)
    {
        ... this some in-game code
        this.client.selectedGuiScreen.close(); // close once

    }
    else if(this.isInGamePause)
    {
        ...
        this.client.selectedGuiScreen.open();  // open once
        this.client.selectedGuiScreen.onUpdate();
    }

    ...

    this.updateFrameTime();
    this.client.getGameWindow().updateViewport();
    this.client.getGameWindow().updateWindow();

    ...
}

Next step its my GuiScreen.class ``onUpdate()``` method:

public GameObject onUpdate() 
{
    for(Gui guiContent : this.allContent)
    {
        guiContent.onUpdate();

        if(guiContent.getId() == 0)
        {
            SimplePrint.message("Before: ScreenPosX: " + this.position.x);
            SimplePrint.message("Before: Current Gui PosX: " + guiContent.position.x);
            SimplePrint.message("Before: Current Gui Start PosX: " + guiContent.startPosition.x);
        }

        this.moveRelaviteContent(guiContent);

        if(guiContent.getId() == 0)
        {
            SimplePrint.message("After: ScreenPos: " + this.position.x);
            SimplePrint.message("After: Current Gui PosX: " + guiContent.position.x);
            SimplePrint.message("After: Current Gui Start PosX: " + guiContent.startPosition.x);
        }

        System.exit(0);
    }

    return this;
}

And finally if who interested my guiContent.onUpdate() :

@Override
public GameObject onUpdate() 
{
    if(this.scale.x() <= 0) this.scale.x = 0.2f;
    if(this.scale.y() <= 0) this.scale.y = 0.2f;

    if(this.scale.x() >= 2) this.scale.x = 2f;
    if(this.scale.y() >= 2) this.scale.y = 2f;

    //this.rotateGui();
    return super.onUpdate();
}

Ps: Immediately say that mistakes in other methods I have no. I just double-checked 100 times.

And most interesting the that I in test project tested and guiContent.position = guiContent.startPosition + this.position; worked as I intended. Please help and the I already 3 day not can understand in than the problem.

Answer 1

I've reduced your constructor to a minimum to show the problem with this.position and this.startPosition. The same problem exists with this.rotation and this.scale - you have to apply the solution for this.position also to these two fields...

public GameObject(float posX, float posY, float posZ, 
                float rotX, float rotY, float rotZ, 
                float scaleX, float scaleY, float scaleZ) 
{
    this();
    this.position = new Vector3f(0, 0, 0); // 1
    this.startPosition = new Vector3f(posX, posY, posZ); // 2
    this.position = this.startPosition;  // 3
}

On line // 1 you create a Vector3f (A) and assign a reference to it to this.position.

On line // 2 you create another Vector3f (B) and assign a reference to it to this.startPosition.

On line // 3 you overwrite the reference to Vector3f (A) that has been stored in this.position with the reference to Vector3f (B) that you stored in this.startPosition.

From this point on this.position references the same Vector3f as this.startPosition, so this.position.x is just another name for this.startPosition.x (both refer to the same memory location).

---

To fix your problem you must assign a clone of this.startPosition to this.position

public GameObject(float posX, float posY, float posZ, 
                float rotX, float rotY, float rotZ, 
                float scaleX, float scaleY, float scaleZ) 
{
    this();
    this.position = new Vector3f(0, 0, 0); // 1
    this.startPosition = new Vector3f(posX, posY, posZ); // 2
    this.position = new Vector3f(this.startPosition);  // 3
}

Now, line // 3 creates a new, independent Vector3f that happens to have the same starting values as the one referenced by this.startPosition.