Why does this little program crash sometimes?

Question

I don't know if the code of the program is the most important here, I'm guessing I have to learn some extra things to help the program not crash anymore. But what?
My question remains, still: Why does this little program crash sometimes?, because I might guess wrong.
The program is a small application that let's you introduce two numbers in 2 text fields (or if you want, there is also a button that generates some random int numbers to complete the text fields for you), let's you introduces the result of the division of those numbers in another text field, and the result of the multiplication of those numbers in another text field. Then you have another 2 buttons that after being pushed, tell you if you resolved the operations right.
The application runs fine most of the times, but sometimes it just freezes when I push the button that generates new random numbers.
Why does this happen? Why does this happen only sometimes and not all the time?

Answer 1

package my.NumberAddition;
* @author  zi02
public class NumberAdditionUI extends javax.swing.JFrame {
    /** Creates new form NumberAdditionUI */
    public NumberAdditionUI() {
        initComponents();
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                         
    private void initComponents() {
        buttonGroup1 = new javax.swing.ButtonGroup();
        jPanel1 = new javax.swing.JPanel();
        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        jLabel3 = new javax.swing.JLabel();
        jTextField1 = new javax.swing.JTextField();
        jTextField2 = new javax.swing.JTextField();
        jTextField3 = new javax.swing.JTextField();
        jButton1 = new javax.swing.JButton();
        jButton2 = new javax.swing.JButton();
        jTextField4 = new javax.swing.JTextField();
        jButton4 = new javax.swing.JButton();
        jTextField5 = new javax.swing.JTextField();
        jLabel4 = new javax.swing.JLabel();
        jTextField6 = new javax.swing.JTextField();
        jButton5 = new javax.swing.JButton();
        jButton3 = new javax.swing.JButton();
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        jPanel1.setBorder(javax.swing.BorderFactory.createTitledBorder(new javax.swing.border.SoftBevelBorder(javax.swing.border.BevelBorder.RAISED), "Number Addition", javax.swing.border.TitledBorder.DEFAULT_JUSTIFICATION, javax.swing.border.TitledBorder.DEFAULT_POSITION, new java.awt.Font("Tahoma", 0, 11), new java.awt.Color(0, 153, 204))); // NOI18N
        jLabel1.setText("First Number:");
        jLabel2.setText("Second Number:");
        jLabel3.setText("Multiply:");
        jTextField3.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jTextField3ActionPerformed(evt);
        jButton1.setText("Clear");
        jButton1.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
        buttonGroup1.add(jButton1);
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
        jButton2.setText("Result");
        jButton2.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
        buttonGroup1.add(jButton2);
        jButton2.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton2ActionPerformed(evt);
        jButton4.setText("Result");
        jButton4.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
        buttonGroup1.add(jButton4);
        jButton4.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton4ActionPerformed(evt);
        jLabel4.setText("Divide:");
        jTextField6.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jTextField6ActionPerformed(evt);
        jButton5.setText("New Numbers");
        jButton5.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
        jButton5.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton5ActionPerformed(evt);
        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jLabel4, javax.swing.GroupLayout.Alignment.TRAILING)
                    .addComponent(jLabel3, javax.swing.GroupLayout.Alignment.TRAILING)
                    .addComponent(jLabel2, javax.swing.GroupLayout.Alignment.TRAILING)
                    .addComponent(jLabel1, javax.swing.GroupLayout.Alignment.TRAILING))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jTextField6, javax.swing.GroupLayout.Alignment.TRAILING)
                            .addComponent(jTextField3, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, 112, Short.MAX_VALUE))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jButton4, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                            .addComponent(jButton2, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jTextField4, javax.swing.GroupLayout.DEFAULT_SIZE, 41, Short.MAX_VALUE)
                            .addComponent(jTextField5, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, 65, Short.MAX_VALUE)))
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addGap(84, 84, 84)
                        .addComponent(jButton5)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jButton1))
                    .addComponent(jTextField2)
                    .addComponent(jTextField1))
                .addContainerGap())
        jPanel1Layout.linkSize(javax.swing.SwingConstants.HORIZONTAL, new java.awt.Component[] {jButton1, jButton2, jButton4});
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(jTextField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(jTextField2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel3)
                    .addComponent(jButton2)
                    .addComponent(jTextField3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jTextField5, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jButton4)
                    .addComponent(jTextField4, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jLabel4)
                    .addComponent(jTextField6, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jButton1)
                    .addComponent(jButton5))
                .addContainerGap(15, Short.MAX_VALUE))
        jButton3.setText("Exit");
        jButton3.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
        jButton3.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton3ActionPerformed(evt);
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jPanel1, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, 345, Short.MAX_VALUE)
                    .addComponent(jButton3, javax.swing.GroupLayout.Alignment.TRAILING))
                .addContainerGap())
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jButton3)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        pack();
    }// </editor-fold>                       
private void jButton3ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    System.exit(0);
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    jTextField1.setText("");
    jTextField2.setText("");
    jTextField3.setText("");
    jTextField4.setText("");
    jTextField5.setText("");
    jTextField6.setText("");
    @SuppressWarnings("empty-statement")
private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    // First we define float variables.
    float num1, num2, num3,  result;
    // We have to parse the text to a type float.
    num1 = Float.parseFloat(jTextField1.getText());
    num2 = Float.parseFloat(jTextField2.getText());
    // Now we can perform the addition.
    num3 = Float.parseFloat(jTextField3.getText());
    // We will now pass the value of result to jTextField3.
    // At the same time, we are going to
    // change the value of result from a float to a string.
    result = num1*num2;
    if (num3==result)
    jTextField5.setText("Correct");
    else jTextField5.setText("Wrong");
private void jButton4ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    // First we define float variables.
    float num1, num2, num4, result;
    // We have to parse the text to a type float.
    num1 = Float.parseFloat(jTextField1.getText());
    num2 = Float.parseFloat(jTextField2.getText());
    // Now we can perform the addition.
    num4 = Float.parseFloat(jTextField6.getText());
    // We will now pass the value of result to jTextField3.
    // At the same time, we are going to
    // change the value of result from a float to a string.
    result = num1/num2;
    if (num4==result)
    jTextField4.setText("Correct");
    else jTextField4.setText("Wrong");
private void jTextField6ActionPerformed(java.awt.event.ActionEvent evt) {                                           
// TODO add your handling code here:
private void jTextField3ActionPerformed(java.awt.event.ActionEvent evt) {                                           
// TODO add your handling code here:
private void jButton5ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    int number1,number2;
    number1 = (int)(Math.random()*100);
    number2 = (int)(Math.random()*100);
    while ((number1 < number2) || (number2 == 0) ||(number2==1)
            || (number1 % number2 !=0))
        number2 = (int)(Math.random()*10);       
    jTextField1.setText(String.valueOf(number1));
    jTextField2.setText(String.valueOf(number2));
    jTextField3.setText("");
    jTextField4.setText("");
    jTextField5.setText("");
    jTextField6.setText("");
    * @param args the command line arguments
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NumberAdditionUI().setVisible(true);
    // Variables declaration - do not modify                    
    private javax.swing.ButtonGroup buttonGroup1;
    private javax.swing.JButton jButton1;
    private javax.swing.JButton jButton2;
    private javax.swing.JButton jButton3;
    private javax.swing.JButton jButton4;
    private javax.swing.JButton jButton5;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JTextField jTextField1;
    private javax.swing.JTextField jTextField2;
    private javax.swing.JTextField jTextField3;
    private javax.swing.JTextField jTextField4;
    private javax.swing.JTextField jTextField5;
    private javax.swing.JTextField jTextField6;
    // End of variables declaration                  
}