在单击按钮时执行JTextField验证会引发NullPointerException

Question

I would like to validate few text fields on button click.

The main concept is to let user input few numbers in few text fields and when he clicks a button to run some method to validate inputs.

• Check if there are only numbers in textfields.
• Check if there is duplicate numbers in textfields.
• Check if all 6 fields contain some value.
• Check if numbers are in range 1-100

I would like to run validation on button click

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

} 

These are textfields I would like to validate:

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;  

I have found some piece of code online but I can't seem to get it working. Here is the code:

JTextField[] fields;

void validation() { //call when appropriate(ie ActionListener of JButton)     
    System.out.println( "Called validation!");
    int[] nums = new int[fields.length];
    Set<Integer> set = new HashSet<>();
    System.out.println( "Called validation!" + nums);

    for (int i = 0; i < fields.length; i++) {            
        try {
            nums[i] = Integer.parseInt(fields[i].getText());
        } catch (NumberFormatException ex) {
            //not a valid number tell user of error
            JOptionPane.showMessageDialog(null,"Not a valid format");
            return;
        }

        if (nums[i] < 1 || nums[i] > 48) {
            //out of number range tell user of error
            JOptionPane.showMessageDialog(null,"Range error");
            return;
        }

        if (!set.add(nums[i])) {
            //duplicate element tell user of error
            JOptionPane.showMessageDialog(null,"Duplicate");
            return;
        }
    }

    JOptionPane.showMessageDialog(null,"A, OK");
}

How can i make this method to work on buton click. When i run this method netbeans shows error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException...

Answer 1

You could try this code:

try{
    int nums[] = new int[6];
    nums[0] = Integer.parseInt(jTextField1.getText());
    nums[1] = Integer.parseInt(jTextField2.getText());
    nums[2] = Integer.parseInt(jTextField3.getText());
    nums[3] = Integer.parseInt(jTextField4.getText());
    nums[4] = Integer.parseInt(jTextField5.getText());
    nums[5] = Integer.parseInt(jTextField6.getText());
} catch (NumberFormatException e){
    System.out.println("Some field does not contain a number");
    return;
}

for(int i=0; i<5; i++){
    if(nums[i]==nums[i+1]){
        System.out.println("Repeated number");
        return;
    }
}

For floating point numbers: Float.parseFloat();

Answer 2

At this line:

int[] nums = new int[fields.length];

Based on your code it seems fields is never initialized and consequently you get a NPE.

Some tips

These requirement:

• Check if there are only numbers in textfields.
• Check if all 6 fields contain some value.
• Check if numbers are in range 1-100

Can be easily accomplished by using formatted text fields or spinners instead of plain text fields. By using one of these you won't have to care about parsing strings since you can make users have input restricted to integers. You can also give them a default value so all of them will have a value. And finally you can set an allowed range of values.

Take a look to How to Use Formatted Text Fields and How to Use Spinners tutorials.

This other requirements:

• Check if there is duplicate numbers in textfields.

Can be done by iterating over the formatted text fields or spinners, requesting their values and validating those conditions.

Example

This example is based on formatted text fields but it can also be accomplished with spinners.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JButton;
import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.text.NumberFormatter;

public class Demo {

    private void createAndShowGUI() {

        NumberFormat  format = NumberFormat.getInstance();
        format.setParseIntegerOnly(true);

        NumberFormatter formatter = new NumberFormatter(format);
        formatter.setMinimum(1);
        formatter.setMaximum(100);
        formatter.setAllowsInvalid(false);
        formatter.setOverwriteMode(true);        

        final JPanel gridPanel = new JPanel(new GridLayout(2, 3));

        for (int i = 0; i < 6; i++) {
            JFormattedTextField textField = new JFormattedTextField(formatter);
            textField.setColumns(5);
            textField.setValue(1);
            gridPanel.add(textField);
        }

        JButton button = new JButton("Validate");
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                List list = new ArrayList();
                for(Component comp : gridPanel.getComponents()) {
                    JFormattedTextField textField = (JFormattedTextField)comp;
                    Integer value = (Integer)textField.getValue();
                    Color foreground = list.contains(value) ? Color.RED : Color.BLACK;
                    textField.setForeground(foreground);
                    list.add(value);
                }
            }
        });

        JFrame frame = new JFrame("Demo");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.add(gridPanel, BorderLayout.CENTER);
        frame.add(button, BorderLayout.SOUTH);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

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