Converter in Swing help.

[c cross]

Hello All,
i'm trying to produce a temperature converter in swing that uses a menu to select between c to f or f to c, from here someone can enter in the value click go and then they receive the answer.
I'm struggling with my jlabel section as a null error.
What i want it to look like is:
Select a type : (drop down menu)
Type in number: (box to type in) (go button)
answer will appear on this line.

I keep getting an error i cant solve with this code:

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JMenuBar;
import javax.swing.JTextField;

import java.awt.GridLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class temperatureConverter extends JFrame
{
    private JLabel promptLabel;
    private JLabel inputLabel;
    private JMenu conversionMenu;
    private JMenuBar conversionMenuBar;
    private JPanel menuPanel;
    private JPanel inputPanel;
    private JPanel answerPanel;
    private JLabel answerLabel;
    private JButton goButton;
    private JTextField floatInput;
    
    public temperatureConverter ()
    {
        super();
        this.setSize(400, 100);
        this.setTitle("Temperature Converter Window");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        this.setLayout(new GridLayout(3,1));
        
        this.promptLabel = new JLabel("Select conversion type: ",JLabel.RIGHT);
        this.inputLabel = new JLabel("(select a conversion type:)", JLabel.RIGHT);
        this.answerLabel = new JLabel(" ",JLabel.CENTER);
        
        this.conversionMenu = new JMenu("(no conversion type selected)");
        
        
        
        JMenuItem produceFahrenheit = new JMenuItem("Centigrade to Fahrenheit");
        produceFahrenheit.addActionListener(new TypeListener());
        this.conversionMenu.add(produceFahrenheit);

        JMenuItem produceCentigrade = new JMenuItem("Fahrenheit to Centigrade");
        produceCentigrade.addActionListener(new TypeListener());
        this.conversionMenu.add(produceCentigrade);

        conversionMenuBar = new JMenuBar();
        conversionMenuBar.add(conversionMenu);

        this.menuPanel = new JPanel();
        this.menuPanel.add(this.promptLabel);
        this.menuPanel.add(this.conversionMenuBar);
        this.add(this.menuPanel);
        
        this.inputPanel = new JPanel();
        this.inputPanel.add(this.inputLabel);
        this.inputPanel.add(this.floatInput);
        this.inputPanel.add(this.goButton);
        this.goButton.addActionListener(new goButtonListener());
        this.add(this.inputPanel);
        
        this.answerPanel = new JPanel();
        this.answerPanel.add(this.answerLabel);
        this.add(this.answerPanel);
    }

The error occurs in the inputPanel line and i dont know waht to do.

[Jesper de Jong]

Welcome to the Ranch.

c cross wrote:
The error occurs in the inputPanel line and i dont know waht to do.

Is it a compile error or a runtime error? And what exactly is the error message? (Error messages contain very valuable information that gives a hint about what's wrong. It's much easier to help you if you tell us exactly what the error message is).

[c cross]

Hello,
When i get rid of the inputpanel area i get no error. However, when i place it back in i get this error:
"Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1045)
at java.awt.Container.add(Container.java:365)
at temperatureConverter.<init>(temperatureConverter.java:66)
at temperatureConverter.main(temperatureConverter.java:110)"

[Aditya Jha]

The line number shown in stack-trace is 66, whereas in your code there is no executable statement at line 66. Please paste the actual code, or equivalent error.

The error you have mentioned can only mean one thing - the component being passed in add() is null. Look closely in your actual code. Is there a copy-pasted reference which is not initialised yet?

[Mohamed Sanaulla]

More of Swing'ish query, hence moving to appropriate forum.

Try to find out the line at which the NullPointerException is being thrown, and see if the references being used in the line are initialized before being used?

[c cross]

this.inputPanel.add(this.floatInput); is the line 66 on my computer. Hope this helps
i can post the whole file if you wish.

[Mohamed Sanaulla]

c cross wrote:this.inputPanel.add(this.floatInput); is the line 66 on my computer. Hope this helps
i can post the whole file if you wish.

Are you initiaizing the floatInput? From the code you have posted, it doesn't look like

[c cross]

How would i go about initialising it?
Sorry if this sounds like a really stupid question.

[Mohamed Sanaulla]

this should help you with initializing the floatInput (which is a JTextField)