Java Basic Problems

Java: I don’t want to allow Plus (+) or Minus (-) sign as a first entry of my JTextField.

In my previous example I have shown about How to add TextField (JTextField)in java.

Here I will try to show how to add validation in JTextField.

Suppose,I am trying to prepare a Calculator and I don’t want my user to give invalid input at the  first keypress entry,  for example, Plus (+) or Minus (-).

But, they can give any numeric entry at the first keyPress.So, I  need to put validation in my JTextField so that Plus (+) or Minus () sign will not be allowed at the first keypress /key entry, but  they are allowed after any numeric or digit entry.

In the given example, if users press (+) or () sign at first, our program will show error message but it will allow users to take numeric entry at first. After first entry, (+) or minus (-) will be allowed.


Code Example:

import java.awt.*;

import java.awt.event.*;

import java.awt.event.KeyEvent;

import javax.swing.*;

import java.util.*;

public class Calc extends JFrame


   JTextField ques, answer;

   private Toolkit toolkit;

   char ch;

   int plusOccurs=0;

        public Calc()


       JPanel panel =new JPanel();




          setTitle(“Easy Calculator”);



                    Dimension size=toolkit.getScreenSize();

                    setLocation(size.width/2-getWidth()/2, size.height/2-getHeight()/2);

                    ques=new JTextField();

                    answer=new JTextField();

        JButton calculate=new JButton(“Calculate”);


                   ques.setBounds(10,10,220,30);     // Positioning of TextField(JTextField)

          answer.setBounds(10,100,220,30); //Positioning  of TextField(JTextField)


          panel.add(ques );    // Adding TextField(JTextField) in JPanel

          panel.add(answer );  // Adding TextField(JTextField) in JPanel

                    ques.addKeyListener(new KeyAdapter()


                    public void keyTyped(KeyEvent e)



               if (ch==’+’ && ques.getText().length()==0)



   System.out.println(“Sorry, You have entered :”+ch+”:at first,Please Enter Numeric value, then try”);




                    else if (ch==’-‘&& ques.getText().length()==0)



                                         System.out.println(“Sorry, You have entered :”+ch+”:at first,Please Enter Numeric value at first”);












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