Java錯誤“線程“ main”中的異常java.lang.NullPointerException”

Question

錯誤：“線程“主”中的異常”位於com.vipgamming.Frytree.Game.main（Game.java:47）上的java.lang.NullPointerException”

我不是一個很好的程序員。 只是說和不好的英語。

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();
    }
}

抱歉，我不知道如何發布代碼。（不是Inglish.Portuguese）

Answer 1

您需要在使用之前實例化框架：

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

您也可以將其放在構造函數中：

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

如果main中定義的設置每次都相同，那么您可以將整個部分分解到構造函數中，而不僅僅是創建一個空白。 如果沒有，您總是可以像這樣重載構造函數：

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);
}

或讓游戲創建設置

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不會在任何地方初始化Game.frame 。 最好將此幀初始化保存在單獨的init方法中：

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);
}

（也最好擴展JPanel而不是重量級的AWT Canvas ）