java - 为什么即使调用了 repaint()，该程序也不重新绘制小程序的某个部分

Question

这是一个简单的石头剪刀布小程序。有 3 个变量记录胜利、失败和平局。然后有一个标签用于显示它们。当我使用调试器时，正在调用重绘方法，因此我不明白为什么小程序的这部分没有更新。

import java.awt.Font;
import java.awt.Graphics;

import javax.swing.JApplet;
import javax.swing.JLabel;
import javax.swing.JButton;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.util.Random;

public class JRockPaperScissors extends JApplet {
    JLabel lblDescision = new JLabel("descision");
    JLabel lblWinner = new JLabel("winner");
    JLabel lblTally = new JLabel("tally");
    int tally_user = 0;
    int tally_comp = 0;
    int tally_ties = 0;



/**
 * Create the applet.
 */
public JRockPaperScissors() {
        setSize(500,500);
        getContentPane().setLayout(null);



    JLabel lblRockPaperScissors = new JLabel("Rock, Paper, Scissors");
    lblRockPaperScissors.setBounds(95, 50, 280, 48);
    getContentPane().add(lblRockPaperScissors);
    Font arial_1 = new Font("Arial", Font.BOLD, 25);
    lblRockPaperScissors.setFont(arial_1);

    JLabel lblChooseOneButton = new JLabel("Choose one button");
    lblChooseOneButton.setBounds(10, 93, 146, 25);
    getContentPane().add(lblChooseOneButton);
    Font arial_2 = new Font("Arial", Font.BOLD, 15);
    lblChooseOneButton.setFont(arial_2);

    JButton btnRock = new JButton("Rock");
    btnRock.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            determine_winner(0);
            repaint();
        }
    });
    btnRock.setBounds(166, 95, 89, 23);
    getContentPane().add(btnRock);

    JButton btnPaper = new JButton("Paper");
    btnPaper.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            determine_winner(1);
        }
    });
    btnPaper.setBounds(265, 95, 89, 23);
    getContentPane().add(btnPaper);

    JButton btnScissors = new JButton("Scissors");
    btnScissors.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            determine_winner(2);
        }
    });
    btnScissors.setBounds(361, 95, 89, 23);
    getContentPane().add(btnScissors);

    JLabel lblresults = new JLabel("------Results------");
    lblresults.setBounds(20, 114, 126, 25);
    getContentPane().add(lblresults);
    Font arial_3 = new Font("Arial", Font.BOLD, 15);
    lblresults.setFont(arial_3);

    lblDescision.setBounds(30, 150, 311, 14);
    getContentPane().add(lblDescision);

    lblWinner.setBounds(20, 175, 146, 14);
    getContentPane().add(lblWinner);

    JLabel lblTally = new JLabel("You:" + tally_user + "    Computer:" + tally_comp + 
            "    Ties:"+ tally_ties);
    lblTally.setBounds(20, 214, 201, 30);
    getContentPane().add(lblTally);


}

public void determine_winner(int user_choice){
    Random random = new Random();
    int computer_choice = random.nextInt(3);
    if(user_choice == 0 ){
        if(computer_choice ==0){
            lblDescision.setText("You picked Rock --- Computer picked Rock");
            lblWinner.setText("Winner: Tie");
            tally_ties +=1;
        }
        else if(computer_choice ==1){
            lblDescision.setText("You picked Rock --- Computer picked Paper");
            lblWinner.setText("Winner: Computer");
            tally_comp +=1;

        }
        else if(computer_choice ==2){
            lblDescision.setText("You picked Rock --- Computer picked Scissors");
            lblWinner.setText("Winner: You");
            tally_user +=1;

        }
    }
    else if(user_choice == 1){
        if(computer_choice ==0){
            lblDescision.setText("You picked Paper --- Computer picked Rock");
            lblWinner.setText("Winner: You");
            tally_user +=1;
        }
        else if(computer_choice ==1){
            lblDescision.setText("You picked Paper --- Computer picked Paper");
            lblWinner.setText("Winner: Tie");
            tally_ties +=1;
        }
        else if(computer_choice ==2){
            lblDescision.setText("You picked Paper --- Computer picked Scissors");
            lblWinner.setText("Winner: Computer");
            tally_comp +=1;
        }
    }
    else if(user_choice == 2){
        if(computer_choice ==0){
            lblDescision.setText("You picked Scissors --- Computer picked Rock");
            lblWinner.setText("Winner: Computer");
            tally_comp +=1;
        }
        else if(computer_choice ==1){
            lblDescision.setText("You picked Scissors --- Computer picked Paper");
            lblWinner.setText("Winner: You");
            tally_user +=1;
        }
        else if(computer_choice ==2){
            lblDescision.setText("You picked Scissors --- Computer picked Scissors");
            lblWinner.setText("Winner: Tie");
            tally_ties +=1;
        }
    }
revalidate();
repaint();
}

}

Answer 1

• 问题 1:您永远不会更改计数 JLabel、lblTally 中的文本。换句话说，要更改此 JLabel 和类似组件中的文本，您必须专门调用 lblTally.setText(someNewString);
• 问题 2:您还隐藏了 lblTally 变量——您在构造函数中重新声明并重新初始化该变量，这意味着 GUI 中显示的 JLabel 对象与在 GUI 中显示的 JLabel 对象不同。类字段指的是——不要这样做。声明变量并仅使用有效引用对其进行初始化一次。

其他附带问题(与您的问题不直接相关的问题)是:

• 您使用的是 null 布局。您需要避免使用 null 布局并使用 setBounds(...) 进行组件放置，因为这会导致 GUI 非常不灵活，虽然它们在一个平台上看起来不错，但在大多数其他平台上看起来很糟糕或屏幕分辨率，并且很难更新和维护。
• 您的小程序没有 init() 方法，该方法应该使小程序启动并运行。
• 更改 JLabel 中的文本后无需调用 revalidate() 或 repaint()。

<小时/>

简而言之，您正在做的是:

import java.awt.event.ActionEvent;
import javax.swing.*;

public class JRockPaperEtc extends JApplet {
   private static final String FORMAT_TXT = "Tally: %03d";
   private int tally = 0;
   private JLabel lblTally = new JLabel(String.format(FORMAT_TXT, tally));

   @Override
   public void init() {
      JPanel mainPanel = new JPanel();

      // *** Variable Shadowing here
      JLabel lblTally = new JLabel("Tally: 000");
      mainPanel.add(lblTally);

      JButton button = new JButton(new AbstractAction("Push") {

         @Override
         public void actionPerformed(ActionEvent e) {
            tally++;
            // *** lblTally's text is never changed
         }
      });
      mainPanel.add(button);

      add(mainPanel);
   }
}

什么时候你应该这样做:

import java.awt.event.ActionEvent;
import javax.swing.*;

public class JRockPaperEtc2 extends JApplet {
   private static final String FORMAT_TXT = "Tally: %03d";
   private int tally = 0;
   private JLabel lblTally = new JLabel(String.format(FORMAT_TXT, tally));

   @Override
   public void init() {
      JPanel mainPanel = new JPanel();

      // lblTally = new JLabel("Tally: 000"); // *** no shadowing
      mainPanel.add(lblTally);

      JButton button = new JButton(new AbstractAction("Push") {

         @Override
         public void actionPerformed(ActionEvent e) {
            tally++;
            lblTally.setText(String.format(FORMAT_TXT, tally));
            // *** lblTally's text is now changed
         }
      });
      mainPanel.add(button);

      add(mainPanel);
   }
}

另外，你问:

so what would you do instead of set bounds? I'm just starting to learn gui apps so excuse me if this is a very in depth question.

使用布局管理器代替 setBounds()，为您处理所有布局繁重的工作。谷歌搜索 Swing 布局管理器教程并查看。