Java 2D NullPointerException

Question

Recently I've started coding some Java 2D.

I made this:

    public void paintComponent(Graphics comp) {
         Graphics2D comp2D = (Graphics2D) comp;
         Font fontx = new Font("Verdana", Font.BOLD, 5);
         comp2D.setFont(fontx);
         comp2D.drawString("Hello World!", 5, 50);
}  

I did import JFrame and java.awt.*, but there's still a problem.

When I run it, I get this:

    Exception in thread "main" java.lang.NullPointerException
    at game.Game.paintComponent(Game.java:41) - comp2D.setFont(fontx); - Sets Font
    at game.Game.next(Game.java:36) - paintComponent(null); - calls the paintComponent public void from the next() public void
    at game.Game.main(Game.java:26) - next.next(); - calls a public void called "next" using an object called "next" (this public void throws InterruptedException)
Java Result: 1

How can I solve it?

Answer 1

You state:

Exception in thread "main" java.lang.NullPointerException
at game.Game.paintComponent(Game.java:41) -  
    comp2D.setFont(fontx); - Sets Font

This means that comp2D is null and you're trying to call a method on a null variable.

at game.Game.next(Game.java:36) - paintComponent(null); 
     - calls the paintComponent public void from the next() public void

This means that you're calling paintComponent directly and passing in null!

So you're calling paintComponent directly and passing in null! It should come as no surprise that the Graphics object is null and will throw a NPE if you try to call methods on it.

Solution:

• You almost never call paintComponent directly.
• Instead have the JVM call it when you call repaint(). The JVM will pass in a valid Graphics object.
• Most important -- read the painting with Swing tutorial . You can't guess at this stuff and expect it to work.
• Be sure that your paintComponent method is held within a JPanel or other JComponent derived component.
• Be sure that your override is valid by using the @Override annotation for paintComponent.
• Don't neglect to call the super.paintComponent(...) method within your override.
• For example, have your other method change a class field, say have it change a String field called text, then call repaint() , and then have your paintComponent(...) method use the text field for the text to print in the JPanel. This is just an example. You can change any of the drawing component's fields, and then use them inside of paintComponent(...) .