java - 为什么在这个简单的程序中 setBackground 不立即并一致地更新背景颜色

Question

我有一个非常简单的程序，要求用户单击其背景颜色与较大面板(又名显示面板)的背景颜色相匹配的四个面板之一，该面板随机将其背景颜色设置为四个面板之一。如果面板错误点击Joptionpane就出来了；显示面板背景暂时设置为黑色。如果单击"is"，显示面板应重新设置其背景颜色。问题是我并不总是看到背景被更新。相反，有时背景保持黑色。 ...更令人困惑的是，如果您将另一个窗口拖到该程序的窗口上，它会保持黑色，如果您看到颜色在显示面板上移动时部分更新，或者只是切换到不同的窗口并重新聚焦，那么您会看到完成更新的背景颜色。

那么为什么 setMethod 会被调用，但只是偶尔由幕后的 Paint 方法执行呢？为什么类型转换其他 window 或框架使其可见？它与正在处理的鼠标单击事件有什么关系吗？

我很感谢对这一切的任何解释，谢谢大家

public class MainPanel extends JPanel{


    Subpanel panel1;
    Subpanel panel2;
    Subpanel panel3;
    Subpanel panel4;
    Subpanel displaypanel;

    Color[] color; //stores all the  colors that display on the subpanels


    public static void main(String[] args) {
        JFrame window=new JFrame("This is a test");
        window.setContentPane(new MainPanel());
        window.setLocation(100,30);
        window.setSize(600,500);
        window.setVisible(true);

    }


    public MainPanel(){


        setLayout(new FlowLayout(FlowLayout.CENTER,80,30));


        panel1= new Subpanel();
        panel2= new Subpanel();
        panel3= new Subpanel();
        panel4= new Subpanel();

        //the big sub panel
        displaypanel= new Subpanel();

        color=new Color[4];
        color[0]=Color.BLUE;
        color[1]=Color.RED;
        color[2]=Color.YELLOW;
        color[3]=Color.GREEN;



        setBackground(Color.GRAY);
        displaypanel.setBackground(displayRandomColor());
        panel1.setBackground(color[0]);
        panel2.setBackground(color[1]);
        panel3.setBackground(color[2]);
        panel4.setBackground(color[3]); 

        displaypanel.setPreferredSize(new Dimension(400,250));
        panel1.setPreferredSize(new Dimension(70,70));
        panel2.setPreferredSize(new Dimension(70,70));
        panel3.setPreferredSize(new Dimension(70,70));
        panel4.setPreferredSize(new Dimension(70,70));

        add(displaypanel);
        add(panel1);
        add(panel2);
        add(panel3);
        add(panel4);





    }


    public void paintComponent(Graphics g){
        super.paintComponent(g);



    }

    public Color displayRandomColor(){
            Color i=Color.WHITE;
        switch ((int)(Math.random()*4)+1){
            case 1:
            i= Color.YELLOW;
            break;

            case 2:
            i= Color.BLUE;
            break;

            case 3:
            i= Color.GREEN;
            break;

            case 4:
            i= Color.RED;
        }   
        return i;
    }


    public class Subpanel extends JPanel{

        public Subpanel(){
                this.addMouseListener(new MouseAdapter(){
                public void mouseClicked(MouseEvent evt){
                    Component source=(Component)evt.getSource();

                        if((source.getBackground()).equals(displaypanel.getBackground())){
                            //do nothing for this test..
                    }
                        else{
                                displaypanel.setBackground(Color.BLACK);

                                //ask user to reset the background color
                                //**the following 2 lines introduces the problem
                            if(JOptionPane.showOptionDialog(null,"click Yes to see a new Color","Incorrect",JOptionPane.YES_NO_OPTION,JOptionPane.PLAIN_MESSAGE,null,null,null)==JOptionPane.YES_OPTION){
                                displaypanel.setBackground(displayRandomColor());

                            }
                        }

                        }





        });
        } //end of constructor
        public void paintComponent(Graphics g){
            super.paintComponent(g);

        }

    }

}

Answer 1

当synchronized correctly时，您的程序运行得相当好。 。一些注意事项:

• 在程序执行时尽可能早地初始化对象一次。

• 使用Random实例获取随机整数；请注意 displayRandomColor() 的实现要简单得多。

  private Color displayRandomColor() {
      return color[r.nextInt(color.length)];
  }
  

• Swing GUI 对象应该仅在 event dispatch thread 上构建和操作。 .

• 当您确实想要覆盖 getPreferredSize() 时，请勿使用 setPreferredSize()]，如建议 here .

• 使用layouts和 pack() 封闭窗口。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class MainPanel extends JPanel {

    private static final Random r = new Random();
    private final JPanel displayPanel;
    private Color[] color = {Color.RED, Color.GREEN, Color.BLUE, Color.YELLOW};
    private Subpanel panel1 = new Subpanel(color[0]);
    private Subpanel panel2 = new Subpanel(color[1]);
    private Subpanel panel3 = new Subpanel(color[2]);
    private Subpanel panel4 = new Subpanel(color[3]);

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame window = new JFrame("This is a test");
                window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                window.add(new MainPanel());
                window.pack();
                window.setLocationRelativeTo(null);
                window.setVisible(true);
            }
        });
    }

    public MainPanel() {
        setLayout(new BorderLayout());
        setBackground(Color.GRAY);

        //the big sub panel
        displayPanel = new JPanel() {

            @Override
            public Dimension getPreferredSize() {
                return new Dimension(320, 240);
            }
        };
        displayPanel.setBackground(displayRandomColor());

        //the control panel
        JPanel p = new JPanel();
        p.add(panel1);
        p.add(panel2);
        p.add(panel3);
        p.add(panel4);

        add(displayPanel, BorderLayout.CENTER);
        add(p, BorderLayout.SOUTH);
    }

    private Color displayRandomColor() {
        return color[r.nextInt(color.length)];
    }

    public class Subpanel extends JPanel {

        public Subpanel(Color color) {
            this.setBackground(color);
            this.addMouseListener(new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent evt) {
                    Component source = (Component) evt.getSource();
                    if ((source.getBackground()).equals(displayPanel.getBackground())) {
                        System.out.println(source.getBackground());
                    } else {
                        displayPanel.setBackground(Color.BLACK);
                        if (JOptionPane.showOptionDialog(null,
                            "Click Yes to see a new Color", "Incorrect",
                            JOptionPane.YES_NO_OPTION, JOptionPane.PLAIN_MESSAGE,
                            null, null, null) == JOptionPane.YES_OPTION) {
                            displayPanel.setBackground(displayRandomColor());
                        }
                    }
                }
            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(70, 70);
        }
    }
}