线程“主”中的异常java.lang.NullPointerException对于初学者

Question

我知道这已经被讲过很多次了，但是我已经用Java编程了大约3个星期，而且我刚刚在做一些有趣的东西。人们所做的大多数其他报告都没有解决我的问题。屏幕显示但不显示矩形。这是我的代码：

import java.awt.Rectangle;
import javax.swing.JFrame;
import java.awt.Color;

public class something {
    public static int SCREEN_WIDTH = 500;
    public static int SCREEN_HEIGHT = 500;
    public static int Y = 10;
    public static int X = 10;
    public static int Width = 50;
    public static int Height = 50;
    public static Color player_color = Color.black;
    public static Graphics g;

    public static void screen() {
        JFrame screen = new JFrame();
        screen.setVisible(true);
        screen.setSize(SCREEN_WIDTH, SCREEN_HEIGHT);
        screen.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        screen.setTitle("test");
        screen.setResizable(false);
    }

    public static void player() {
        Rectangle r = new Rectangle(Y, X, Width, Height);
        g.fillRect((int)r.getX(), (int)r.getY(), (int)r.getWidth(), (int)r.getHeight());
        g.setColor(player_color);
    }

    public static void main(String[] args) {
        screen();
        player();
    }
}

通过我使用Eclipse的方式，我认为该问题意味着我正在使用具有空值的值？如果我错了，请纠正我。

Answer 1

您的g为null，并且未初始化为任何东西，因此在访问fillRect（）方法时会得到nullpointer。

您可以只在类级别使用扩展Applet并覆盖paint method（）。

public class something extends Applet {
    public static int SCREEN_WIDTH = 500;
    public static int SCREEN_HEIGHT = 500;
    public static int Y = 10;
    public static int X = 10;
    public static int Width = 50;
    public static int Height = 50;
    public static Color player_color = Color.black;

    @Override
    public void paint(Graphics g) {
        JFrame screen = new JFrame();
        screen.setVisible(true);
        screen.setSize(SCREEN_WIDTH, SCREEN_HEIGHT);
        screen.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        screen.setTitle("test");
        screen.setResizable(false);
        Rectangle r = new Rectangle(Y, X, Width, Height);
        g.fillRect((int) r.getX(), (int) r.getY(), (int) r.getWidth(), (int) r.getHeight());
        g.setColor(player_color);
    }