Numberformat exception for converting jtextfield to int

Question

I've been looking at answers to other posts and they seem to point to what I'm doing, but I'm still getting NumberFormatException. I'm trying to build a simple calculator from a JFrame by pulling jtextfield value and converting it into int, but I'm getting the following exception.

package com.calculator.app;

import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class CalculatorApp extends JFrame {

    private JPanel contentPane;
    private JTextField textField = new JTextField();
    private JTextField textField_1 = new JTextField();
    private JTextField textField_2 = new JTextField();
    int num1 = Integer.parseInt(textField.getText());
    int num2 = Integer.parseInt(textField_1.getText());
    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    CalculatorApp frame = new CalculatorApp();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame.
     */
    public CalculatorApp() {

    //  int num2 = Integer.parseInt(value2);    

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 450, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        JLabel lblCalculatorApp = new JLabel("Calculator App");
        lblCalculatorApp.setBounds(141, 37, 138, 23);
        contentPane.add(lblCalculatorApp);

        JButton btnNewButton = new JButton("Add");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

            }
        });
        btnNewButton.setBounds(74, 112, 131, 31);
        contentPane.add(btnNewButton);

        JLabel lblNewLabel = new JLabel("");
        lblNewLabel.setBounds(169, 181, 82, 23);
        contentPane.add(lblNewLabel);

        textField = new JTextField();
        textField.setBounds(55, 73, 54, 23);
        contentPane.add(textField);
        textField.setColumns(10);

        textField_1 = new JTextField();
        textField_1.setColumns(10);
        textField_1.setBounds(151, 70, 54, 23);
        contentPane.add(textField_1);

        textField_2 = new JTextField();
        textField_2.setColumns(10);
        textField_2.setBounds(109, 162, 124, 55);
        contentPane.add(textField_2);
    //  textField_2.setText(String.valueOf(output));
        try{

            int output = Integer.parseInt(textField.getText());
                }
                catch(NumberFormatException e){
                    System.out.println("Error");    
                }

    }
}

Exception

java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at com.calculator.ap.CalculatorApp.<init>(CalculatorApp.java:23)
    at com.calculator.ap.CalculatorApp$1.run(CalculatorApp.java:32)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

This line:

int num1 = Integer.parseInt(textField.getText());

is equivalent to

int num1 = Integer.parseInt("");

because you don't initialize the textField with a specific value in this line:

private JTextField textField = new JTextField();

So, you are trying to parse an empty string as integer, but it throws NumberFormatException, because it's not an integer.

If you just want to get over those problems, you could put 3 integers there:

private JTextField textField = new JTextField("1");
private JTextField textField_1 = new JTextField("2");
private JTextField textField_2 = new JTextField("3");

or (much better) you can remove the following lines, because it seems that you need to parse them later (when the button is clicked):

int num1 = Integer.parseInt(textField.getText());
int num2 = Integer.parseInt(textField_1.getText());

---

The explanation above helps you to avoid that exception.

But if you want to sum those numbers, you should fill in the body of this method:

    btnNewButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            String text1 = textField.getText();
            String text2 = textField_1.getText();
            textField_2.setText(getIntegerValue(text1) + getIntegerValue(text2) + "");
        }
    });

where getIntegerValue can look like this:

private int getIntegerValue(String s) {
    if ( "".equals(s) ) {
        return 0;
    }
    return Integer.parseInt(s);
}

or shorter:

private int getIntegerValue(String s) {
    return "".equals(s) ? 0 : Integer.parseInt(s);
}

This method:

• returns 0 if the string is empty (no value)
• returns the integer if the string represents an integer
• throws NumberFormatException if the string is not an integer or an empty string
• doesn't need to catch NumberFormatException, because it's a RuntimeException

Of course, you can remove that if / ternary operator from the last method if you want to throw an exception even if the string is empty.