Java 更改组合框时显示文本字段

我正在创建一个程序来计算某些形状的面积。当我运行程序时，选择一个形状并输入面积值，它会正确计算所有形状的面积。但是，如果我想在不关闭程序的情况下计算另一个形状的面积，它只会在上一个形状之后添加框，而不是切换框。我想知道是否有办法让它在更改形状时只显示一组文本字段？我的代码如下：

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class MainPanel extends JPanel implements ActionListener {

JComboBox shapeList;

JTextField answer;
JTextField shapeSelected;
JTextField circle;
JTextField square;
JTextField triangle;
JTextField triangle3;

JLabel circle2;
JLabel square2;
JLabel triangle2;
JLabel triangle4;
JLabel triangle5;
JLabel squareAnswer;
JLabel circleAnswer;
JLabel triangleAnswer;

JButton compute;

MainPanel()
{
    String [] shapes = {"circle", "square", "triangle"};

    shapeList = new JComboBox(shapes);
    shapeList.addActionListener(this);
    add(shapeList);

    compute = new JButton("Compute");
    compute.addActionListener(this);
    add(compute);
}

@Override
public void actionPerformed(ActionEvent e) 
{
    Object o = e.getSource();

    if (o == shapeList)
    {   
        if (shapeList.getSelectedItem().equals("circle")) 
        {
            circle2 = new JLabel();
            circle2.setText("Radius:");
            add(circle2);

            circle = new JTextField(10);
            add(circle);

            this.revalidate();
            this.repaint();     
        }

        if (shapeList.getSelectedItem().equals("square")) 
        {
            square2 = new JLabel();
            square2.setText("Width/Height:");
            add(square2);

            square = new JTextField(10);
            add(square);

            this.revalidate();
            this.repaint();
        }

        if (shapeList.getSelectedItem().equals("triangle")) 
        {
            triangle4 = new JLabel();
            triangle4.setText("Base:");
            add(triangle4);

            triangle = new JTextField(10);
            add(triangle);

            triangle5 = new JLabel();
            triangle5.setText("Height:");
            add(triangle5);

            triangle3 = new JTextField(10);
            add(triangle3);

            this.revalidate();
            this.repaint();
        }
    }

    if (o == compute)
    {
        if (shapeList.getSelectedItem().equals("square"))
        {
            squareAnswer = new JLabel();
            squareAnswer.setText("Area:");
            add(squareAnswer);

            answer = new JTextField(10);
            add(answer);

            this.revalidate();
            this.repaint();

            double sum = Double.parseDouble(square.getText()) * Double.parseDouble(square.getText());
            answer.setText(String.valueOf(sum));
        }

        if (shapeList.getSelectedItem().equals("triangle"))
        {
            triangleAnswer = new JLabel();
            triangleAnswer.setText("Area:");
            add(triangleAnswer);

            answer = new JTextField(10);
            add(answer);

            this.revalidate();
            this.repaint();

            double sum = (Double.parseDouble(triangle.getText()) * Double.parseDouble(triangle3.getText())) / 2;
            answer.setText(String.valueOf(sum));
        }

        if (shapeList.getSelectedItem().equals("circle"))
        {
            circleAnswer = new JLabel();
            circleAnswer.setText("Area:");
            add(circleAnswer);

            answer = new JTextField(10);
            add(answer);

            this.revalidate();
            this.repaint();

            double sum = (Double.parseDouble(circle.getText()) * Double.parseDouble(circle.getText())) * Math.PI;
            answer.setText(String.valueOf(sum));
        }
    }
}
}

使用卡片布局。从组合框中选择形状时，可以为指定形状交换面板

有关更多信息和工作示例，请阅读上Swing教程的部分

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工作示例显示了在组合框中选择项目时如何摆动面板。

@hovercraftfullofels，谢谢。这令人沮丧。同一个问题在两天内发布了3次，但没有采取任何措施来实施已经提出的建议。