NullPointerException with java.io using a gui

Hello, this summer has become boring, so I decided to learn programming. I've had no problems until now. I'm learning the java.io class, and decided to use it in a gui. In the program I'm trying to test a number to see if it's prime then save it to a file if it is the largest prime tested.
I get a null pointer exception. Any help would be appreciated.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at sun.misc.FloatingDecimal.readJavaFormatString(Floa tingDecimal.java:989)
at java.lang.Double.parseDouble(Double.java:510)
at primeGui.actionPerformed(primeGui.java:65)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.jav a:6288)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3267)
at java.awt.Component.processEvent(Component.java:605 3)
at java.awt.Container.processEvent(Container.java:204 1)
at java.awt.Component.dispatchEventImpl(Component.jav a:4651)
at java.awt.Container.dispatchEventImpl(Container.jav a:2099)
at java.awt.Component.dispatchEvent(Component.java:44 81)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4168)
at java.awt.Container.dispatchEventImpl(Container.jav a:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478 )
at java.awt.Component.dispatchEvent(Component.java:44 81)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:643)
at java.awt.EventQueue.access$000(EventQueue.java:84)
at java.awt.EventQueue$1.run(EventQueue.java:602)
at java.awt.EventQueue$1.run(EventQueue.java:600)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:616)
at java.awt.EventQueue$2.run(EventQueue.java:614)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 613)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

Code :

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import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import java.io.*;
public class primeGui implements ActionListener {
    JFrame frame; JPanel panel; JButton button; JLabel label, answer, highPrime; JTextField enterInfo;
    public primeGui() {
        frame = new JFrame("Prime Number Check");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        panel = new JPanel();
        panel.setLayout(new GridLayout(0,2,10,5));
        panel.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
        label = new JLabel("Enter an integer:                       ");
        panel.add(label);
        enterInfo = new JTextField(10);
        panel.add(enterInfo);
        answer = new JLabel(" ");
        panel.add(answer);
        button = new JButton("check");
        button.setActionCommand("check");
        button.addActionListener(this);
        panel.add(button);
        highPrime = new JLabel(" ");
        panel.add(highPrime);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }
    public void actionPerformed(ActionEvent event) {
        String eventName = event.getActionCommand();
        int counter=2; double number = 0; String prime = enterInfo.getText();
        String numInFile;
        if (eventName.equals("check")) {
             number = Double.parseDouble(enterInfo.getText());
            do {
                counter++;
            } while (number%counter!= 0);
            if (counter == Double.parseDouble(prime)) {
                answer.setText("The number IS prime.");
            }else{
                answer.setText("The number is NOT prime.");
            }
        }
        File prime1 = new File("prime.txt");
        if(!prime1.exists()) {
            try {
            prime1.createNewFile();
        }catch (IOException e) {
            highPrime.setText("Problem creating file.");
        }
        }
        try {
            FileReader in = new FileReader(prime1);
            BufferedReader readFile = new BufferedReader(in);
            FileWriter out = new FileWriter(prime1);
            BufferedWriter writeFile = new BufferedWriter(out);
            if ((numInFile = readFile.readLine()) == null) {
                writeFile.write(String.valueOf(number));
            }else{
                numInFile = readFile.readLine();
                if (number > Double.parseDouble(numInFile)) {
                    writeFile.write(String.valueOf(number));
                }
            }
            highPrime.setText("The highest prime ever tested is " + Double.parseDouble(readFile.readLine()));
            writeFile.close();out.close();readFile.close();in.close();
        }catch (FileNotFoundException e) {
            highPrime.setText("File not found.");
        }catch (IOException e) {
            highPrime.setText("Problem setting data.");
        }
    }
    private static void runGui() {
        primeGui gui = new primeGui();
    }
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                runGui();
            }
        });
    }
}

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Quote:

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I get a null pointer exception

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It would help if you copy and pasted the full text of the error message here

Also please wrap your code in code tags. See the # icon above box.

Anyone? Please.

at java.lang.Double.parseDouble(Double.java:510)
at primeGui.actionPerformed(primeGui.java:65)

Looking thru the stack trace you see this reference to a line of your code on the bottom and above it a reference to the method that was called at that line.
What variable on line 65 is null? Then determine why it is null and change the logic so that it has a valid value.

Thanks. That helped. Now I have some other minor problems to work around, but they look simple so far.