使用JTextField进行用户输入

Question

Thanks for your help guys...now the program works and runs like it should.. but I have 2 more question. 1.How can I get the output into a JTestField t4 or t5 2.How can I close the application using the JButton Buton3

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);

  }

}

Answer 1

In addition to missing a main method, as Reimeus pointed out, your order of instructions is wrong. You are trying to read the user input before anything is even shown on the screen, and even before an object is created. For example, this line:

String t1TextBox = t1.getText();

tries to obtain a text from a TextBox that wasn't even added to a Panel that wasn't yet created.

To solve this, you need to rethink the logic of your program. Here are a few hints:

• avoid assignments outside methods. Instead of writing

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

  declare the variable in the class body like this:

   JFrame Triangle; 

  and assign it inside the constructor like this:

   Triangle = new JFrame("Triangle Calculator"); 

• Build the whole UI, then worry about listeners. This way you can be sure that you are not referencing an UI element that does not exist when getting the user input.

• Get the user input inside the listeners, like this:

    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); } 

  });

Answer 2

Add a main method:

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

The declaration

JTextField t1 = new JTextField(20);

doesn't set the value in the JTextField to 20 . Instead it sets the number of columns for the JTextComponent but with an empty String . Therefore the line

double side1 = Double.parseDouble(t1TextBox);

will throw an NumberFormatException on startup.