單擊按鈕后陣列為空的GUI元素-有解無憂

我相信問題出在我的 GameEngine 的滴答聲之間。換句話說，我認為在一個游戲中的緩沖區中，我認為 GUI 元素出于某種原因會變為 null。這是我的代碼：GameEngine 類：

package pong;

import static pong.Main.window;

public class GameEngine{
    static GameState gameState;
    private long timeElapsed;
    
    public GameEngine(){
        gameState=new GameState(0);//0 is for the menu
    }
    
    public void run(){
        long beginTime=System.nanoTime();
        //33 million is 30 fps
        if(timeElapsed>=33333333){
            gameState.buffer();
            timeElapsed-=33333333;
        }
        window.getPanel().repaint();
        
        timeElapsed =System.nanoTime()-beginTime;
    }
    
    public long getTimeElapsed(){
        return timeElapsed;
    }
}

GUIButton 類：

package pong;

import java.awt.Color;
import java.awt.Graphics2D;
import static pong.Panel.mouseClicked;
import static pong.Panel.mouseX;
import static pong.Panel.mouseY;

public abstract class GUIButton extends GUI{
    private boolean mouseOverButton,readyForButtonAction;
    @Override
    public void paint(Graphics2D g) {
        g.setColor(Color.white);
        if(mouseOverButton){
            g.fillRect(x-5,y-5,110,40);
            g.setColor(Color.black);
            g.fillRect(x, y, 100, 30);
        }
        else{
            g.fillRect(x,y,100,30);
            g.setColor(Color.black);
            g.fillRect(x 5, y 5, 90, 20);
        }
    }
    
    @Override 
    public void buffer(){
        mouseOverButton=mouseX-8>x&&mouseX-8<x 100&&
            mouseY-32>y&&mouseY-32<y 30;
        if(mouseOverButton&&mouseClicked)
            readyForButtonAction=true;
    }
    
    public void buttonAction(){
        readyForButtonAction=false;
        mouseClicked=false;
    }
    
    public boolean buttonActionIsReady(){
        return readyForButtonAction;
    }
}

GUIButtonMultiplayer（將您轉移到標題 GUI 到多人游戲選項 GUI 的按鈕）

package pong;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import static pong.GameState.GUI;

public class GUIButtonMultiPlayer extends GUIButton{

    public GUIButtonMultiPlayer(){
        x=300;
        y=300;
    }
    
    @Override
    public void paint(Graphics2D g) {
        super.paint(g);
        g.setColor(Color.white);
        g.setFont(new Font("Serif",Font.BOLD,12));
        g.drawString("Multiplayer",x 10,y 20);
    }
    @Override
    public void buttonAction() {
        super.buttonAction();
        GUI=new GUI[4];
        GUI[0]=new GUIButtonLocal();
        GUI[1]=new GUIButtonJoin();
        GUI[2]=new GUIButtonHost();
        GUI[3]=new GUIButtonBackMultiPlayer();
    }
}

Panel 類（包含 mouseClicked 變數）

package pong;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.JPanel;
import static pong.GameEngine.gameState;

public class Panel extends JPanel implements KeyListener,MouseListener,MouseMotionListener{
    static int mouseX,mouseY;
    static boolean mouseDown,mouseClicked;
    private boolean keyDown[];
    
    public Panel(){
        keyDown=new boolean[4];
    }
    
    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        gameState.paint((Graphics2D)g);
    }

    public boolean getKeyDown(int index){
        return keyDown[index];
    }
    
    @Override
    public void keyTyped(KeyEvent ke) {}

    @Override
    public void keyPressed(KeyEvent ke) {
        switch(ke.getKeyCode()){
            case KeyEvent.VK_DOWN:
                keyDown[0]=true;
                break;
            case KeyEvent.VK_UP:
                keyDown[1]=true;
                break;
            case KeyEvent.VK_W:
                keyDown[2]=true;
                break;
            case KeyEvent.VK_S:
                keyDown[3]=true;
                break;
        }
    }

    @Override
    public void keyReleased(KeyEvent ke) {
        switch(ke.getKeyCode()){
            case KeyEvent.VK_DOWN:
                keyDown[0]=false;
                break;
            case KeyEvent.VK_UP:
                keyDown[1]=false;
                break;
            case KeyEvent.VK_W:
                keyDown[2]=false;
                break;
            case KeyEvent.VK_S:
                keyDown[3]=false;
                break;
        }
    }

    @Override
    public void mouseClicked(MouseEvent me) {
        mouseClicked=true;
    }

    @Override
    public void mousePressed(MouseEvent me) {
        mouseDown=true;
    }

    @Override
    public void mouseReleased(MouseEvent me) {
        mouseDown=false;
    }

    @Override
    public void mouseEntered(MouseEvent me) {}

    @Override
    public void mouseExited(MouseEvent me) {}

    @Override
    public void mouseDragged(MouseEvent me) {}

    @Override
    public void mouseMoved(MouseEvent me) {
        mouseX=me.getX();
        mouseY=me.getY();
    }
}

GameState 類（實作）

package pong;

import java.awt.Color;
import java.awt.Graphics2D;
import static pong.Panel.mouseClicked;

public class GameState{
    static GUI GUI[];
    private int ID;
    private Ball ball;
    private Paddle paddles[];
    
    public GameState(int id){
        ID=id;
        switch(ID){
            case 0://title screen
                GUI=new GUI[4];
                GUI[0]=new GUITitle();
                GUI[1]=new GUIButtonSinglePlayer();
                GUI[2]=new GUIButtonMultiPlayer();
                GUI[3]=new GUIButtonExit();
                break;
            case 1://single player
                paddles=new Paddle[2];
                //true indicates that it is a left paddle
                paddles[0]=new PaddlePlayer(true);
                paddles[1]=new PaddleCPU();
                GUI=new GUI[1];
                GUI[0]=new GUIScores();
                ball=new Ball(paddles,(GUIScores)GUI[0]);
                ((PaddleCPU)paddles[1]).setBall(ball);
                break;
            case 2://local multiplayer
                paddles=new Paddle[2];
                //true indicates that it is a left paddle
                paddles[0]=new PaddlePlayer(true);
                paddles[1]=new PaddlePlayer(false);
                GUI=new GUI[1];
                GUI[0]=new GUIScores();
                ball=new Ball(paddles,(GUIScores)GUI[0]);
                break;
            case 3://host multiplayer
                
                paddles=new Paddle[2];
                paddles[0]=new PaddleMultiplayer();
                paddles[1]=new PaddleMultiplayer();
                GUI=new GUI[1];
                GUI[0]=new GUIScores();
                ball=new Ball(paddles,(GUIScores)GUI[0]);
                break;
        }
    }
    
    public void paint(Graphics2D g){
        g.setColor(Color.black);
        g.fillRect(0,0,800,600);
        switch(ID){
            case 0://title screen
                    System.out.println();
                for(GUI gui:GUI){
                        System.out.println(gui==null);
                    gui.paint(g);
                }
                break;
            case 1:case 2:case 3://single player and multiplayer
                g.setColor(Color.white);
                for(int a=10;a<600;a =20)
                    g.fillRect(380, a, 10, 10);
                ball.paint(g);
                for(Paddle paddle:paddles)
                    paddle.paint(g);
                GUI[0].paint(g);
                break;
        }
    }
    
    public void buffer(){
        switch(ID){
            case 0://title screen   
                for(GUI gui:GUI)
                    if(gui instanceof GUIButton && ((GUIButton)gui).buttonActionIsReady()){
                        ((GUIButton)gui).buttonAction();
                        break;
                    }
                    else gui.buffer();
                break;
            case 1:case 2:case 3://single player and multiplayer
                ball.buffer();
                for(Paddle paddle:paddles)
                    paddle.buffer();
                break;
        }
        mouseClicked=false;
    }
    
    public int getID(){
        return ID;
    }
}

此代碼在繪制 GUI 時生成 NullPointerException。如果我注釋掉該行

gui.paint(g);

然后它只是創建一個 NullPointerException ，而不是緩沖 GUI。這是輸出的一個片段：

false
true
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

false
false
false
false
    at pong.GameState.paint(GameState.java:62)
    at pong.Panel.paintComponent(Panel.java:25)

false
false
false
false

false
false
false
false

false
false
false
false

false
false
false
false
    at javax.swing.JComponent.paint(JComponent.java:1056)

false
false
false
false

false
false
false
false

false
false
false
false

false
false
false
false
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5158)

false
false
false
    at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
false
    at javax.swing.RepaintManager$4.run(RepaintManager.java:831)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)

哪個 GUI 元素顯示為 null 似乎是隨機的。

uj5u.com熱心網友回復：

對我來說，您似乎在為 GUI 元素使用靜態 GameState.GUI 陣列（如果我錯了，請糾正我）。

在代碼的其他地方，您將按順序訪問這個靜態 GUI 陣列。似乎有時其他代碼會改變這個靜態 GUI 陣列的內容（像這樣的代碼可能是罪魁禍首GUI=new GUI[1];），而其他代碼即將開始回圈遍歷該陣列。您必須更改代碼，以免發生這種情況。您可以同步訪問此靜態 GUI 陣列的方法（但這會阻止其他方法并使它們等待，這對于您的用例可能適用，也可能不適用）。

如果您確定這確實是問題的原因（可能不是），并且您想避免使用同步方法，那么您還有其他一些選擇。您可以研究雙緩沖的作業原理。使用 2 個陣列，始終從主要陣列讀取并始終修改次要陣列，并在它們之間進行交換。

或者，如果您想保留當前代碼，您可以更改這些模式：

GUI=new GUI[4];
GUI[0]=new GUITitle();
GUI[1]=new GUIButtonSinglePlayer();
GUI[2]=new GUIButtonMultiPlayer();
GUI[3]=new GUIButtonExit();

在您使用的代碼中隨處可見這種模式GUI=new GUI[...];：

GUI[] newGui=new GUI[4];
newGui[0]=new GUITitle();
newGui[1]=new GUIButtonSinglePlayer();
newGui[2]=new GUIButtonMultiPlayer();
newGui[3]=new GUIButtonExit();
GUI=newGui

這可能足以避免這個問題。

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