Java 使用JTextField进行用户输入

谢谢你们的帮助，伙计们…现在程序正常运行了。。但我还有两个问题。 1.如何将输出输入JTestField t4或t5 2.如何使用JButton Buton3关闭应用程序

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.JButton;
import javax.swing.JFrame;
public class TriangleFarfan{
JFrame Triangle = new JFrame("Triangle Calculator");
   JButton Button1 = new JButton ("Area");
   JButton Button2 = new JButton("Perimeter");
   JButton Button3 = new JButton("Close");
   JTextField t1 = new JTextField(20);
   String t1TextBox = t1.getText();
   double side1 = Double.parseDouble(t1TextBox);
   JPanel j1 = new JPanel (new FlowLayout());
   JLabel l1 = new JLabel("Enter side 1:");
   JTextField t2 = new JTextField();
   String t2TextBox = t2.getText();
   double side2 = Double.parseDouble(t2TextBox);
   JPanel j2 = new JPanel (new FlowLayout());
   JLabel l2 = new JLabel("Enter side 2:");
   JTextField t3 = new JTextField();
   String t3TextBox = t3.getText();
   double side3 = Double.parseDouble(t3TextBox);
   JPanel j3 = new JPanel (new FlowLayout());
   JLabel l3 = new JLabel("Enter side 3:");
   JTextField t4 = new JTextField();
   JPanel j4 = new JPanel (new FlowLayout());
   JLabel l4 = new JLabel("Area Result");
   JTextField t5 = new JTextField(20);
   JPanel j5 = new JPanel (new FlowLayout());
   JLabel l5 = new JLabel("Perimeter Result");
public TriangleFarfan()
    {
    j1.add(l1);
    j1.add(t1);
    j2.add(l2);
    j2.add(t2);
    j3.add(l3);
    j3.add(t3);
    j4.add(l4);
    j4.add(t4);
    j5.add(l5);
    j5.add(t5);
    Triangle.add(j1);
    Triangle.add(j2);
    Triangle.add(j3);
    Triangle.add(j4);
    Triangle.add(j5);
    Triangle.add(Button1);
    Button1.addActionListener(new ActionListener() {

           public void actionPerformed(ActionEvent e)
           {
               double Area = (side1 * side2)/2;
               //Execute when button is pressed
               System.out.println(Area);
           }
       });      
    Triangle.add(Button2);
    Button2.addActionListener(new ActionListener() {

           public void actionPerformed(ActionEvent e)
           {
               //Execute when button is pressed
               System.out.println("You clicked the Perimeter Button");
           }
       });      
    Triangle.add(Button3);
    Button3.addActionListener(new ActionListener() {

           public void actionPerformed(ActionEvent e)
           {
               //Execute when button is pressed
               System.out.println("You clicked the Close Button");
           }
       });      
    Triangle.setLayout(new FlowLayout());
    Triangle.setSize(450,400);
    Triangle.setVisible(true);
    Triangle.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

  }

---

}

添加

main

方法：

public static void main(String[] args) {
   SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
         new TriangleFarfan();
      }
   });
}

宣言

JTextField t1 = new JTextField(20);

不将

JTextField

中的值设置为

20

。相反，它设置

JTextComponent

的列数，但带有空

字符串。所以这条线

double side1 = Double.parseDouble(t1TextBox);

将在启动时抛出一个
NumberFormatException
。
除了缺少一个主方法之外，正如Reimeus指出的，您的指令顺序是错误的。您试图在屏幕上显示任何内容之前，甚至在创建对象之前读取用户输入。例如，这一行：

String t1TextBox=t1.getText（）

尝试从尚未添加到尚未创建的面板的文本框中获取文本

要解决这个问题，您需要重新思考程序的逻辑。以下是一些提示：


避免方法之外的赋值。而不是写作

JFrame Triangle = new JFrame("Triangle Calculator");

在类主体中声明变量，如下所示：

JFrame Triangle;

Triangle = new JFrame("Triangle Calculator");

 Button1.addActionListener(new ActionListener() {

   public void actionPerformed(ActionEvent e)
   {
       // get the size of side1 from the textbox
       String t1TextBox = t1.getText();
       double side1 = Double.parseDouble(t1TextBox);

       // get the size of side2 from the textbox
       String t2TextBox = t2.getText();
       double side2 = Double.parseDouble(t2TextBox);

       // now we can calculate the area
       double Area = (side1 * side2)/2;

       //Execute when button is pressed
       System.out.println(Area);
   }

并在构造函数中指定它，如下所示：

JFrame Triangle;

Triangle = new JFrame("Triangle Calculator");

 Button1.addActionListener(new ActionListener() {

   public void actionPerformed(ActionEvent e)
   {
       // get the size of side1 from the textbox
       String t1TextBox = t1.getText();
       double side1 = Double.parseDouble(t1TextBox);

       // get the size of side2 from the textbox
       String t2TextBox = t2.getText();
       double side2 = Double.parseDouble(t2TextBox);

       // now we can calculate the area
       double Area = (side1 * side2)/2;

       //Execute when button is pressed
       System.out.println(Area);
   }


构建整个UI，然后担心侦听器。通过这种方式，您可以确保在获取用户输入时没有引用不存在的UI元素

在侦听器中获取用户输入，如下所示：

JFrame Triangle;

Triangle = new JFrame("Triangle Calculator");

 Button1.addActionListener(new ActionListener() {

   public void actionPerformed(ActionEvent e)
   {
       // get the size of side1 from the textbox
       String t1TextBox = t1.getText();
       double side1 = Double.parseDouble(t1TextBox);

       // get the size of side2 from the textbox
       String t2TextBox = t2.getText();
       double side2 = Double.parseDouble(t2TextBox);

       // now we can calculate the area
       double Area = (side1 * side2)/2;

       //Execute when button is pressed
       System.out.println(Area);
   }

}))

感谢您的快速重播，但一旦我测试了无法打开的程序，它几乎会立即关闭。代码有什么问题吗？这是因为您试图解析
JTextField
中的空
字符串。看看这条线索。它显示了应用程序失败的地方。谢谢你的帮助，再见。我遵照你的指示，程序运行得很好。