Help setting up input error checking

This is for a class assignment.

I am finishing up an assignment that requires data input checking. I have a JtextField that the user is supposed to input his weight in puonds. I need to code something that would allow me to use a JOptionPane to show if the user inputs anything other than an interger. Here is my code so far:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;

public class Earth extends JFrame
{
private ImagePanel imagePanel;

JLabel label = new JLabel();
JLabel textLabel = new JLabel("What planet do you want your weight on?");

JLabel fatLabel = new JLabel("Enter your weight on Earth: ");

JTextField fatField = new JTextField(25);

JButton button = new JButton("ENTER");

JComboBox comboBox = new JComboBox();
//JScrollPane scrollingArea = new JScrollPane(logArea);

JTextArea logArea = new JTextArea(10, 20);


private String names[] =
{ "Mercury.jpg", "Venus.jpg",
"Earth.jpg", "Mars.jpg", "Jupiter.jpg", "Saturn.jpg", "Uranus.jpg", "Neptune.jpg", "Pluto.jpg" };
private Icon icons[] =
{
new ImageIcon( names[ 0 ] ),
new ImageIcon( names[ 1 ] ),
new ImageIcon( names[ 2 ] ),
new ImageIcon( names[ 3 ] ),
new ImageIcon( names[ 4 ] ),
new ImageIcon( names[ 5 ] ),
new ImageIcon( names[ 6 ] ),
new ImageIcon( names[ 7 ] ),
new ImageIcon( names[ 8 ] )};



public Earth(String title)
{
super(title);

Container c = getContentPane();

c.setLayout(new BorderLayout(10,20));

JPanel centerPanel = new JPanel();
JPanel southPanel = new JPanel();

centerPanel.setLayout(new GridLayout(12,12,1,1));
southPanel.setLayout(new BorderLayout());
southPanel.add(label, "North");
southPanel.add(logArea, "South");
logArea.setEditable ( false );
logArea.setText( "Start here " );
JScrollPane scrollingArea = new JScrollPane(logArea);


centerPanel.add(fatLabel);
centerPanel.add(fatField);
centerPanel.add(button);
centerPanel.add( textLabel);

//fatField.setText (150);

button.setBackground(Color.orange);

comboBox.addItem("Mercury");
comboBox.addItem("Venus");
comboBox.addItem("Earth");
comboBox.addItem("Mars");
comboBox.addItem("Saturn");
comboBox.addItem("Uranus");
comboBox.addItem("Jupiter");
comboBox.addItem("Neptune");
comboBox.addItem("Pluto");
centerPanel.add(comboBox);
label.setIcon(
icons[2] );


comboBox.addActionListener(
new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
double weight = 0;
label.setIcon(
icons[ comboBox.getSelectedIndex() ] );
String selectedItem = (String) comboBox.getSelectedItem();
String text = logArea.getText();

//if (fatField == (null) ){
// JOptionPane.showMessageDialog(null, "Please enter your weight on Earth.", "Stupid Alert!", JOptionPane.ERROR_MESSAGE);

//}
if (selectedItem=="Mercury"){
weight = Integer.parseInt(fatField.getText()) * 0.38;
}
else if (selectedItem=="Venus"){
weight = Integer.parseInt(fatField.getText()) * 0.91;
//}
//else if (selectedItem=="Earth"){
// weight = Integer.parseInt(fatField.getText());
//text = "You already know what you weight on Earth!";
}
else if (selectedItem=="Mars"){
weight = Integer.parseInt(fatField.getText()) * 0.38;
}
else if (selectedItem=="Jupiter"){
weight = Integer.parseInt(fatField.getText()) * 2.54;
}
else if (selectedItem=="Saturn"){
weight = Integer.parseInt(fatField.getText()) * 0.93;
}
else if (selectedItem=="Uranus"){
weight = Integer.parseInt(fatField.getText()) * 0.8;
}
else if (selectedItem=="Neptune"){
weight = Integer.parseInt(fatField.getText()) * 1.2;
}
else if (selectedItem=="Pluto"){
JOptionPane.showMessageDialog(null, "There is not enough known about Pluto to know your answer.", "Danger Will Robinson!", JOptionPane.ERROR_MESSAGE);
}


//if (weight <= 0 ){
// JOptionPane.showMessageDialog(null, "Please enter your weight on Earth.", "Stupid Alert!", JOptionPane.ERROR_MESSAGE);
//}

text = text+ "Your weight on "+selectedItem + " is: "+ Double.toString(weight)+ " \n";

logArea.setText(text);
}
}
);
comboBox.setSelectedIndex(2);
//JTextField.setText( );

southPanel.add(logArea);

c.add(centerPanel, BorderLayout.CENTER);
c.add(southPanel, BorderLayout.SOUTH);
logArea.add(scrollingArea);

button.addActionListener(
new ActionListener()
{
public void actionPerformed(ActionEvent e)
{

String text = logArea.getText();
text = text+
fatField.getText()+" is what you say you weight on Earth.\n" ;
logArea.setText(text);
}
}
);
}

// Main
public static void main(String[] args)
{
Earth g2 = new Earth("Earth/Planet weight converter");
g2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
g2.setSize(700,700);
g2.setVisible(true);
}
}


Any guidance would be appreciated.

Thank you

There are at least two basic ways of handling this. One is to use the Character.isDigit() method to check if each character that the user entered is in fact a digit. This would require that you parse through the user's input one character at a time and check that all meet the isDigit requirement.

A second way is to use a try/catch block around the code you already have that uses the Integer.parseInt() method. If you look at the Integer class API you'll see that the parseInt() method throws a NumberFormatException if it fails. You can then catch this exception and tell the user that they didn't provide valid input.

Use whichever you feel is easiest for you.