分配新变量（setter）并调用repaint（）后，无法更改paintComponent的output

Question

我被困在同一个地方将近 1 天，但我仍然找不到我的代码错误在哪里。 需要你们的帮助，这是我的代码。 （有点长）

我的 JFrame class：

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

public class CircleProject extends JFrame implements ActionListener{
    JTextField xTextField1;
    JTextField yTextField1;
    JTextField textFieldRadiusLeft;
    JTextField xTextField2;
    JTextField yTextField2;
    JTextField textFieldRadiusRight;
    JButton redraw;

public static void main(String[] args) {
    CircleProject frame = new CircleProject();
    frame.setResizable(true);
    frame.setVisible(true);
    frame.setSize(580, 700);
    frame.setTitle("YONG JING PING FINAL PROJECT");
}

public CircleProject() {
    JPanel titlePanel = new JPanel(new FlowLayout(FlowLayout.CENTER));
    JLabel theTitle = new JLabel("Two Circles Intersect?");
    JLabel intersection = new JLabel("");
    titlePanel.add(theTitle);
    titlePanel.add(intersection);

    JPanel contentPanel = new JPanel(new GridLayout(2, 1));
    JPanel userInputPanel = new JPanel(new FlowLayout(FlowLayout.CENTER));
    JPanel gridPanel = new JPanel(new GridLayout(1, 2, 8, 0));

    JPanel leftInputPanel = new JPanel(new GridLayout(4, 1));
    JLabel inputTitle1 = new JLabel("Enter circle 1 info: ");
    inputTitle1.setHorizontalAlignment(JLabel.CENTER);

    JPanel xInputPanelLeft = new JPanel();
    xTextField1 = new JTextField(5);
    xInputPanelLeft.add(new JLabel("Center X: "));
    xInputPanelLeft.add(xTextField1);

    JPanel yInputPanelLeft = new JPanel();
    yTextField1 = new JTextField(5);
    yInputPanelLeft.add(new JLabel("Center Y: "));
    yInputPanelLeft.add(yTextField1);

    JPanel radiusLeft = new JPanel();
    textFieldRadiusLeft = new JTextField(5);
    radiusLeft.add(new JLabel("Radius: "));
    radiusLeft.add(textFieldRadiusLeft);

    leftInputPanel.add(inputTitle1);
    leftInputPanel.add(xInputPanelLeft);
    leftInputPanel.add(yInputPanelLeft);
    leftInputPanel.add(radiusLeft);
    leftInputPanel.setBorder(BorderFactory.createLineBorder(Color.BLACK));

    JPanel rightInputPanel = new JPanel(new GridLayout(4, 1));
    JLabel inputTitle2 = new JLabel("Enter circle 2 info: ");
    inputTitle2.setHorizontalAlignment(JLabel.CENTER);

    JPanel xInputPanelRight = new JPanel();
    xTextField2 = new JTextField(5);

    xInputPanelRight.add(new JLabel("Center X: "));
    xInputPanelRight.add(xTextField2);

    JPanel yInputPanelRight = new JPanel();
    yTextField2 = new JTextField(5);
    yInputPanelRight.add(new JLabel("Center Y: "));
    yInputPanelRight.add(yTextField2);

    JPanel radiusRight = new JPanel();
    textFieldRadiusRight = new JTextField(5);
    radiusRight.add(new JLabel("Radius: "));
    radiusRight.add(textFieldRadiusRight);

    rightInputPanel.add(inputTitle2);
    rightInputPanel.add(xInputPanelRight);
    rightInputPanel.add(yInputPanelRight);
    rightInputPanel.add(radiusRight);
    rightInputPanel.setBorder(BorderFactory.createLineBorder(Color.BLACK));

    gridPanel.add(leftInputPanel);
    gridPanel.add(rightInputPanel);
    userInputPanel.add(gridPanel);
    contentPanel.add(new DrawPanel());
    contentPanel.add(userInputPanel);

    redraw = new JButton("Redraw Circles");
    redraw.setEnabled(true);
    redraw.addActionListener(this);
    JPanel buttonPanel = new JPanel();
    buttonPanel.add(redraw);

    setLayout(new BorderLayout());
    add(titlePanel, BorderLayout.NORTH);
    add(contentPanel, BorderLayout.CENTER);
    add(buttonPanel, BorderLayout.SOUTH);

}

DrawPanel drawCircle = new DrawPanel();

@Override
public void actionPerformed(ActionEvent e) {
    this.setVisible(false);
    if (e.getSource() == redraw) {
        CircleProject frame2 = new CircleProject();
        frame2.setResizable(false);
        frame2.setVisible(true);
        frame2.setSize(580,700);
        frame2.setTitle("YONG JING PING FINAL PROJECT");


        String text = xTextField1.getText();
        int x1 = Integer.parseInt(text);

        String text1 = xTextField2.getText();
        int x2 = Integer.parseInt(text1);

        String text2 = yTextField1.getText();
        int y1 = Integer.parseInt(text2);

        String text3 = yTextField2.getText();
        int y2 = Integer.parseInt(text3);

        String text4 = textFieldRadiusLeft.getText();
        int r1 = Integer.parseInt(text4);

        String text5 = textFieldRadiusRight.getText();
        int r2 = Integer.parseInt(text5);

        drawCircle.setCenterColumn1(x1, y1, r1);
        drawCircle.setCenterColumn2(x2, y2, r2);
        }
    }
}

我的绘图板 Class：

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

public class DrawPanel extends JPanel{
    //    the coordinate's variables is the start point of it.
    int coordinateX1=50, coordinateX2=300, coordinateY1=50, coordinateY2=50, radius1=0, radius2=0, length1=200, length2=200;

public void setCenterColumn1(int x, int y, int radius1){
    this.coordinateX1 = x-radius1;
    this.coordinateY1 = y-radius1;
    this.radius1 = radius1;
    length1 = radius1*2;
    System.out.println(coordinateX1);
    System.out.println(coordinateY1);
    System.out.println(length1);
    System.out.println(coordinateX2);
    System.out.println(coordinateY2);
    System.out.println(length2);
    System.out.println("-------1-------");
    repaint();
}
public void setCenterColumn2(int x, int y, int radius2){
    this.coordinateX2=x-radius2;
    this.coordinateY2=y-radius2;
    this.radius2=radius2;
    length2 = radius2*2;
    System.out.println(coordinateX1);
    System.out.println(coordinateY1);
    System.out.println(length1);
    System.out.println(coordinateX2);
    System.out.println(coordinateY2);
    System.out.println(length2);
    System.out.println("-------2-------");
    repaint();
}

public DrawPanel(){
    setBackground(Color.WHITE);
}

//    error from here, when redraw pressed this paintComponent does not
//    show up the new circle.
    @Override
    public void paintComponent (Graphics g){
        super.paintComponent(g);
        g.setColor(Color.BLACK);

//        Circle for column 1
        g.drawOval(coordinateX1,coordinateY1,length1,length1);

//        Circle for column 2
        g.drawOval(coordinateX2,coordinateY2,length2,length2);

    System.out.println(coordinateX1);
    System.out.println(coordinateY1);
    System.out.println(length1);
    System.out.println(coordinateX2);
    System.out.println(coordinateY2);
    System.out.println(length2);
    System.out.println("-------3-------");
    }
}

我的预期结果是用户输入圆的中心坐标并单击重绘 btn 后，圆将发生变化。

Answer 1

问题是你没有使用你的变量drawCircle (line: DrawPanel drawCircle = new DrawPanel();)

当您使用内部actionPerformed方法时，您永远不会使用该变量，因为它创建了 Circle Project 的另一个实例。 （行：CircleProject frame2 = new CircleProject();）

drawCircle变量来自您正在运行的当前 class，因此，当您再次实例化 CircleProject 时，该变量内部的更改将不起作用。

要解决此问题，无需再次实例化，您只需使用该变量即可。

您必须在构造函数CircleProject()中做的第一件事：

....
userInputPanel.add(gridPanel);
contentPanel.add(drawCircle);
contentPanel.add(userInputPanel);
....

这里的变化只有一个，因为你有contentPanel.add(new DrawPanel()); 这是不正确的（正如我所说，您从未使用过变量drawCircle ），因此，您必须将其更改为contentPanel.add(drawCircle);

另一方面，为避免实例化CircleProject ，您必须删除actionPerformed方法中的一些行，例如：

@Override
public void actionPerformed(ActionEvent e) {

if (e.getSource() == redraw) {
  
    String text = xTextField1.getText();
    int x1 = Integer.parseInt(text);

    String text1 = xTextField2.getText();
    int x2 = Integer.parseInt(text1);

    String text2 = yTextField1.getText();
    int y1 = Integer.parseInt(text2);

    String text3 = yTextField2.getText();
    int y2 = Integer.parseInt(text3);

    String text4 = textFieldRadiusLeft.getText();
    int r1 = Integer.parseInt(text4);

    String text5 = textFieldRadiusRight.getText();
    int r2 = Integer.parseInt(text5);

    drawCircle.setCenterColumn1(x1, y1, r1);
    drawCircle.setCenterColumn2(x2, y2, r2);
    }
}

我试过这个程序，它对我有用：我希望这也对你有用：D