Java error “Exception in thread ”main“ java.lang.NullPointerException” [closed]

Question

Error:"Exception in thread "main" java.lang.NullPointerException at com.vipgamming.Frytree.Game.main(Game.java:47)"

I am not a very good programmer. Just saying and bad Inglish.

Game.java:

package com.vipgamming.Frytree;

import java.awt.Canvas;
import java.awt.Dimension;

import javax.swing.JFrame;

public class Game extends Canvas implements Runnable {

private static final long serialVersionUID = 1L;
public static int width = 300;
public static int height = width /16 * 9;
public static int scale = 3;

private Thread thread;
private JFrame frame;
private boolean running = false;

public Game() {
    Dimension size = new Dimension(width * scale, height * scale);
    setPreferredSize(size);
}

public synchronized void start() {
    running = true;
    thread = new Thread(this, "Display");
    thread.start();
}

public synchronized void stop() {
    running = false;
    try {
        thread.join();
    }catch (InterruptedException e) {
        e.printStackTrace();
    }
}

public void run() {
    while (running) {
            System.out.println("FryTree...Loading...");
        }
    }

    public static void main(String [] args) {
        Game game = new Game();
        game.frame.setResizable(true);
        game.frame.setTitle("Frytree");
        game.frame.add(game);
        game.frame.pack();
        game.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        game.frame.setLocationRelativeTo(null);
        game.frame.setVisible(true);

        game.start();
    }
}

Sorry I didnt understand how to post code.(not Inglish.Portuguese)

Answer 1

You need to instantiate frame before using it:

game.frame = new JFrame();
game.frame.setResizable(true);
...

You could also put this in the constructor:

public Game() {
 this.frame = new JFrame();
 Dimension size = new Dimension(width * scale, height * scale);
 setPreferredSize(size);
}

If the settings defined in main are going to be the same every time then you could break that whole piece out into the constructor instead of just creating a blank one. If not, you could always overload the constructor like this:

public Game() {//base constructor
 this.frame = new JFrame();
 Dimension size = new Dimension(width * scale, height * scale);
 setPreferredSize(size);
}

public Game(JFrame jframe)//injected frame constructor
{
 this.frame = jframe;
 Dimension size = new Dimension(width * scale, height * scale);
 setPreferredSize(size);
}

or have Game create the settings

public Game()
{
 this.ConstructFrame();
 Dimension size = new Dimension(width * scale, height * scale);
 setPreferredSize(size);
}

private void ConstructFrame()
{
    this.frame = new JFrame();
    this.frame.setResizable(true);
    this.frame.setTitle("Frytree");
    this.frame.add(game);
    this.frame.pack();
    this.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.frame.setLocationRelativeTo(null);
    this.frame.setVisible(true);
}

Answer 2

Game.frame is not initialised anywhere hence the NPE. Better to keep this frame initialization in a separate init method:

private void init() {
    frame = new JFrame();
    frame.setSize(400, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setResizable(true);
    frame.setTitle("Frytree");
    frame.add(this);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

(Also better to extend JPanel instead of heavyweight AWT Canvas )