IndexOutOfBoundsException for Graphics

Question

I am attempting to write a simple game and whenever I run my code this error apears:

Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
    at java.util.ArrayList.rangeCheck(ArrayList.java:653)
    at java.util.ArrayList.get(ArrayList.java:429)
    at GameController.paint(GameController.java:41)

It still runs and displays the green rectangle, but paint runs before initObj and paint runs again after InitObj .

public class GameController extends JFrame{
    Land land;
    int counter;
    ArrayList<Land> Lands = new ArrayList<>();

    public GameController(){
        initGUI();
    }

    public void initGUI(){
        System.out.println("GUI");
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        setVisible(true);
        setSize(1000,1000);
        initObj();
    }
    public void initObj(){
        System.out.println("Obj");
        Lands.add(new Land(this,0,0,this.getSize().width, this.getSize().height));
    }
    public void step(){

    }
    public void paint(Graphics g){
        System.out.println("paint");
        g.setColor(new Color(25,150,50));
        g.fillRect(Lands.get(0).xPos,Lands.get(0).yPos, Lands.get(0).Width, Lands.get(0).Height);
    }
}

Answer 1

Since paint is being called before initObj , why don't you check to see if Lands has anything in it before trying to access it?

public void paint(Graphics g){
    System.out.println("paint");
    if (Lands.size() > 0) {
        g.setColor(new Color(25,150,50));
        g.fillRect(Lands.get(0).xPos,Lands.get(0).yPos, Lands.get(0).Width, Lands.get(0).Height);
    }
}

Answer 2

You have created

ArrayList<Land> Lands = new ArrayList<>();

If you did not populate the Lands object and try to access, you will definitely get IndexOutOfBounds exception.

Your current code should be modified :

public void paint(Graphics g){
        System.out.println("paint");
        g.setColor(new Color(25,150,50));
        g.fillRect(Lands.get(0).xPos,Lands.get(0).yPos, Lands.get(0).Width, Lands.get(0).Height);
    }

Currently you are looking for 0th element. But in future, it may be any index position.

The generic code would be like this:

public void paint(Graphics g){
        System.out.println("paint");
        g.setColor(new Color(25,150,50));
        Land land = null;
        try{
           land = Lands.get(index); // for your index is 0;
        }catch(Exception err){
            err.printStackTrace();
        }
        if ( land != null) {
            g.fillRect(land.xPos,land.yPos, land.Width, land.Height);
       }
    }