getGraphics（）阻止我的游戏启动

Question

我正在学习双缓冲，我从老师那里得到的书告诉我写下我写的东西。但是当java试图从图像中获取图形对象时，它会崩溃。

frame = Image
engine = Graphics

错误

Exception in thread "main" java.lang.NullPointerException
at Engine.<init>(Engine.java:30)
at Game.<init>(Game.java:35)
at Game.main(Game.java:22)

Game.java

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import javax.swing.*;
import java.util.*;
import java.util.Timer;

public class Game implements ActionListener {

private static Game hosmos2;
private JFrame frmMain;
private AntiCheat holyanticheat;
private Dimension screen;
private Engine holyengine;
private int sx, sy;
private Timer timCheat;

public static void main(String[] args) throws FileNotFoundException, InterruptedException {

    hosmos2 = new Game();
}

private Game() throws FileNotFoundException, InterruptedException {

    frmMain = new JFrame("Hosmos 2");
    frmMain.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frmMain.setSize(1366, 768);
    screen = Toolkit.getDefaultToolkit().getScreenSize();
    sx = (int) ((screen.getWidth() - frmMain.getWidth()) / 2);
    sy = (int) ((screen.getHeight() - frmMain.getHeight()) / 2);
    frmMain.setLocation(sx, sy);
    holyanticheat = new AntiCheat();
    holyengine = new Engine();
    timCheat = new Timer();
    frmMain.setVisible(true);
    timCheat.schedule(holyanticheat, 500);
    Thread.sleep(1000);
    timCheat.cancel();
}

public void actionPerformed(ActionEvent e) {


}

}

Engine.java

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import javax.swing.*;
import java.util.*;

public class Engine extends JPanel {

private boolean countdown, quality;
private Font text, text2;
private Graphics engine;
private Image frame;
private ImageIcon background, button1, button21, button22, button3, button4, button51, button52, header;
private int weedbackground, fps, fps2;
public Rectangle box1, box2, box3, box4, box5;

public static void main(String[] args) {


}

public Engine() {

    this.setBackground(Color.black);
    frame = createImage(getSize().width, getSize().height);
    text = new Font("Microsoft Sans Serif", Font.BOLD, 24);
    text2 = new Font("Microsoft Sans Serif", Font.BOLD, 8);
    engine = frame.getGraphics(); // Error
    background = new ImageIcon("textures/background.png");
    //background = background.GetThumbnailImage(source.ClientSize.Width, source.ClientSize.Height, null, IntPtr.Zero);
    button1 = new ImageIcon("textures/button 1.png");
    button21 = new ImageIcon("textures/button 2-1.png");
    button22 = new ImageIcon("textures/button 2-2.png");
    button3 = new ImageIcon("textures/button 3.png");
    button4 = new ImageIcon("textures/button 4.png");
    button51 = new ImageIcon("textures/button 5-1.png");
    button52 = new ImageIcon("textures/button 5-2.png");
    header = new ImageIcon("textures/header.png");
    box1 = new Rectangle(20, 20, 87, 37);
    box2 = new Rectangle(20, 60, 96, 37);
    box3 = new Rectangle(20, 103, 182, 43);
    box4 = new Rectangle(20, 146, 83, 43);
    box5 = new Rectangle(20, 189, 206, 43);
}

public Rectangle GetBox1() {

    return box1;
}

public Rectangle GetBox2() {

    return box2;
}

public Rectangle GetBox3() {

    return box3;
}

public Rectangle GetBox4() {

    return box4;
}

public Rectangle GetBox5() {

    return box5;
}

public void FPS()
{
    fps2 = fps;
    fps = 0;
}

public void Quality(boolean quality2)
{
    quality = quality2;
}

public void Render(boolean music, boolean effect, int time, int spawned, int points, int cracktime, int crackend, int shieldtime, int shieldend, int weedtime, int weedend, ImageIcon malcolm, int malcolmX, int malcolmY, int item, ImageIcon crack, int crackX, int crackY, ImageIcon nuke, int nukeX, int nukeY, ImageIcon shield, int shieldX, int shieldY, ImageIcon skull, int skullX, int skullY, ImageIcon trollface, int trollfaceX, int trollfaceY, ImageIcon weed, int weedX, int weedY, LinkedList<Hosmin> hosmins) {

    engine = frame.getGraphics();
    super.paintComponent(engine);
    if (effect == false) {

        if (quality == false) {

            background.paintIcon(this, engine, 0, 0);
        }
        else {

            this.setBackground(Color.black);
        }
    }
    else {

        if (weedbackground == 1) {

            this.setBackground(Color.green);
            weedbackground++;
        }
        else if (weedbackground == 2) {

            this.setBackground(Color.yellow);
            weedbackground++;
        }
        else if (weedbackground == 3) {

            this.setBackground(Color.red);
            weedbackground = 1;
        }
    }
    header.paintIcon(this, engine, 547, 10);
    button1.paintIcon(this, engine, 20, 20);
    if (music == false) {

        button21.paintIcon(this, engine, 20, 60);
    }
    else {

        button22.paintIcon(this, engine, 20, 60);
    }
    button3.paintIcon(this, engine, 20, 103);
    button4.paintIcon(this, engine, 20, 146);
    if (quality == false) {

        button51.paintIcon(this, engine, 20, 189);
    }
    else {

        button52.paintIcon(this, engine, 20, 189);
    }
    engine.setFont(text2);
    engine.setColor(Color.green);
    engine.drawString("FPS: " + fps2, 1293, 9);
    engine.setFont(text);
    engine.setColor(Color.red);
    engine.drawString("Time: " + time, 1178, 22);
    engine.drawString("Hosmins: " + spawned, 1124, 59);
    engine.drawString("Points: " + points, 1160, 96);
    engine.setFont(text2);
    engine.setColor(Color.green);
    engine.drawString("Developer: HolyDuFF", 1229, 708);
    malcolm.paintIcon(this, engine, malcolmX, malcolmY);
    if (countdown == true) {

        if (crackend - cracktime == 0) {

            countdown = false;
        }
        else if (cracktime >= 0) {

            engine.setFont(text);
            engine.setColor(Color.red);
            crackend = crackend - cracktime;
            if (crackend != 10) {

                engine.drawString("" + crackend, malcolmX + 17, malcolmY - 40);
            }
            else {

                engine.drawString("" + crackend, malcolmX + 7, malcolmY - 40);
            }
        }
        else if (shieldtime >= 0) {

            engine.setFont(text);
            engine.setColor(Color.red);
            shieldend = shieldend - shieldtime;
            if (shieldend != 10) {

                engine.drawString("" + shieldend, malcolmX + 17, malcolmY - 40);
            }
            else {

                engine.drawString("" + shieldend, malcolmX + 7, malcolmY - 40);
            }
        }
        else if (weedtime >= 0) {

            engine.setFont(text);
            engine.setColor(Color.blue);
            weedend = weedend - weedtime;
            if (weedend != 10) {

                engine.drawString("" + weedend, malcolmX + 17, malcolmY - 40);
            }
            else {

                engine.drawString("" + weedend, malcolmX + 7, malcolmY - 40);
            }
        }
    }
    for (int i = 0; i < hosmins.size(); i++) {

        Hosmin temp = hosmins.get(i);
        temp.Draw().paintIcon(this, engine, temp.GetX(), temp.GetY());
    }
    if (item == 1) {

        crack.paintIcon(this, engine, crackX, crackY);
    }
    else if (item == 2) {

        nuke.paintIcon(this, engine, nukeX, nukeY);
    }
    else if (item == 3) {

        shield.paintIcon(this, engine, shieldX, shieldY);
    }
    else if (item == 4) {

        skull.paintIcon(this, engine, skullX, skullY);
    }
    else if (item == 5) {

        trollface.paintIcon(this, engine, trollfaceX, trollfaceY);
    }
    else if (item == 6) {

        weed.paintIcon(this, engine, weedX, weedY);
    }
    engine.dispose();
    repaint();
    fps++;
}

public void paintComponent(Graphics g) {

    super.paintComponent(g);
    g.drawImage(frame, 0, 0, this);
}

public void SetBackground(int background2) {

    weedbackground = background2;
}

public void SetCountdown(boolean countdown2) {

    countdown = countdown2;
}

public void SetBox2(int mode) {

    if (mode == 1) {

        box2.width = 96;
        box2.height = 37;
    }
    else {

        box2.width = 76;
        box2.height = 43;
    }
}

public void SetBox5(int mode) {

    if (mode == 1) {

        box5.width = 206;
    }
    else {

        box5.width = 213;
    }
}

}

Answer 1

您在null Image变量上调用getGraphics()。如果您查看Component createImage()的API，您会明白为什么会这样：

  

返回屏幕外可绘制图像，可用于双缓冲。 如果组件不可显示，则返回值可能为null 。如果GraphicsEnvironment.isHeadless（）返回true，则总会发生这种情况。

引擎尚未在其构造函数中显示。必须首先通过在顶级窗口中调用pack()或setVisible(true)来呈现GUI，然后才能显示此JPanel。