simple GUI not showing up?

Question

Ok so, when I run it, it works all fine but it just doesn't pop up with the answer.

What did I do wrong?

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

public class AF implements ActionListener{

    double length;
    double width;
    double answer;

    JTextField twidth;
    JTextField tlength;

    void AFWindow() {

        JFrame AFwindow = new JFrame();
        JPanel pan2 = new JPanel(new GridBagLayout());
        AFwindow.setVisible(true);
        AFwindow.setSize(250, 150);
        AFwindow.setResizable(false);
        GridBagConstraints c = new GridBagConstraints();
        AFwindow.add(pan2);
        pan2.setBackground(Color.LIGHT_GRAY);

        c.gridx = 0;
        c.gridy = 5;
        tlength = new JTextField();
        tlength.setText("    Length    ");
        pan2.add(tlength, c);

        c.gridx = 0;
        c.gridy = 0;
        twidth = new JTextField();
        twidth.setText("     Width     ");
        pan2.add(twidth, c);

        JButton Find = new JButton("Ok");

        c.gridx = 0;
        c.gridy = -5;
        pan2.add(Find);
        Find.addActionListener(this);
        Find.setActionCommand("ok");
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equalsIgnoreCase("ok")){
            try {
            this.length = Double.parseDouble(tlength.getText());
            this.width = Double.parseDouble(twidth.getText());
        }
        catch(NumberFormatException ex){
            System.out.println("There was an issue!");
        }
        }
    }

    int area = (int) (length * width);
    public void answer(){
        JFrame answer = new JFrame();
        answer.setVisible(true);
        answer.setBackground(Color.yellow);
        JPanel pan2 = new JPanel();
        JLabel howanswer = new JLabel("Your answer is" + area + "We got this by multiplying the length and width");
        pan2.add(howanswer);
    }
}

Answer 1

That code has multiple issues. Forgot to document them, but look at this code for the beginnings of the fixes.

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

public class AF implements ActionListener{

    double length;
    double width;

    JTextField twidth;
    JTextField tlength;
    JFrame AFwindow;

    void AFWindow() {

        AFwindow = new JFrame();
        JPanel pan2 = new JPanel(new GridBagLayout());

        GridBagConstraints c = new GridBagConstraints();
        AFwindow.add(pan2);
        pan2.setBackground(Color.LIGHT_GRAY);
        pan2.setBorder(new EmptyBorder(100,100,100,100));

        c.gridx = 0;
        c.gridy = 5;
        tlength = new JTextField();
        tlength.setText("    Length    ");
        pan2.add(tlength, c);

        c.gridx = 0;
        c.gridy = 0;
        twidth = new JTextField();
        twidth.setText("     Width     ");
        pan2.add(twidth, c);

        JButton Find = new JButton("Ok");

        c.gridx = 0;
        c.gridy = -5;
        pan2.add(Find);
        Find.addActionListener(this);
        Find.setActionCommand("ok");
        AFwindow.pack();
        AFwindow.setLocationByPlatform(true);
        AFwindow.setVisible(true);
    }

    int area;

    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equalsIgnoreCase("ok")){
            try {
                this.length = Double.parseDouble(tlength.getText());
                this.width = Double.parseDouble(twidth.getText());
                area = (int) (length * width);
                answer();
            }
            catch(NumberFormatException ex){
                ex.printStackTrace();
                // System.out.println("There was an issue!");
                // Can you vague that up for me?!?!
                JOptionPane.showMessageDialog(AFwindow, ex);
            }
        }
    }

    public void answer(){
        JPanel pan2 = new JPanel();
        JLabel howanswer = new JLabel("Your answer is " + area + " We got this by multiplying the length and width");
        pan2.add(howanswer);
        JOptionPane.showMessageDialog(AFwindow, pan2);
    }

    public static void main(String[] args) {
        System.out.println("Hi!");
        new AF().AFWindow();
    }
}

Answer 2

Are you expecting the answer frame to pop up when you push the OK button? If so, you need to call the answer method:

    if (e.getActionCommand().equalsIgnoreCase("ok")){
        try {
            this.length = Double.parseDouble(tlength.getText());
            this.width = Double.parseDouble(twidth.getText());
            answer();       //  <--- This is missing
        }

Also, some other things you need to do:

• Generally, you should call JFrame.setVisible at the end of your GUI construction method, after you have added components to the frame. I have shown this in the example below, but you should move setVisible(..) to the bottom of the AFwindow() method as well.

• When you do int area = (int) (length * width); outside of any method, this statement will be interpreted as an instance variable initialization and occur when you instantiate the AF object. And when the object is instantiated, you have not yet assigned any values to length and with. Hence, area will always be 0. Move that statement to the inside of the answer function instead.

• You forgot to add the actual components to the frame in the answer method. Hence, when you set the frame to visible, only an empty frame will be displayed.

Here are examples of some suitable changes:

public void answer(){
    int area = (int) (length * width);  //  <--- Move this inside answer method
    JFrame answer = new JFrame();
    answer.setBackground(Color.yellow);
    JPanel pan2 = new JPanel();
    JLabel howanswer = new JLabel("Your answer is" + area + "We got this by multiplying the length and width");
    pan2.add(howanswer);
    answer.add(pan2); //  <--- Don't forget to add your components to the frame
    answer.pack();    //  <--- Resize the frame to a suitable size
    answer.setVisible(true); //  <--- Set the frame to visible AFTER you have added the components

}

Answer 3

This code works for me:

public class Test {
    public static void main(String[] args){
        JFrame test = new JFrame();
        test.setVisible(true);
        test.setSize(100,100);
        test.setResizable(false);
        test.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

I assume that you can try to reduce your code and try to find problem point such way. You can put some logging to ensure setVisible(true) was called. Also I recomend to use setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); to terminate thread after exit GUI (if do not want to perform some post-exit actions).