如何检查字符串是否为int

Question

Ok guys, so I basically made a GUI Java program, a basic one. Just adds/subs/divides orr multiplies numbers that are in the textfields, nothing big as I have only started learning Java. It works but there are some bugs, such as when you execute it and if you click on one of the radiobutton without entering a number in the textfields then the programme will not work. How can I check whether the user entered an integer whenenver the user clicks on the radiobuttons? Heres the code:

import java.awt.event.*;
import java.text.NumberFormat;
import javax.swing.*;
import java.awt.*;


public class GUI extends JFrame{

    Button button1;
    TextField num1;
    TextField num2;
    JRadioButton add,sub,mul,div;
    boolean isnumber = false;


    int x, y, sum;

    public static void main(String[] args){


        new GUI();

    }

    public GUI(){

        thehandler handle = new thehandler();

        JPanel panel = new JPanel();
        this.setLocationRelativeTo(null);
        this.setVisible(true);
        this.setSize(800, 70);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("Calc");
        this.add(panel);



        button1 = new Button("Calculate");
        button1.addActionListener(handle);
        panel.add(button1);

        num1 = new TextField("Enter a number here");
        num1.addActionListener(handle);
        num2 = new TextField("Enter a number here");
        num2.addActionListener(handle);
        panel.add(num1);
        panel.add(num2);

        add = new JRadioButton("Add");
        add.addActionListener(handle);
        sub = new JRadioButton("Subtract");
        sub.addActionListener(handle);
        mul = new JRadioButton("Multiply");
        mul.addActionListener(handle);
        div = new JRadioButton("Divide");
        div.addActionListener(handle);

        ButtonGroup operation = new ButtonGroup();
        operation.add(add);
        operation.add(sub);
        operation.add(div);
        operation.add(mul);

        panel.add(add);
        panel.add(sub);
        panel.add(mul);
        panel.add(div);




    }

    private class thehandler implements ActionListener{

        @Override
        public void actionPerformed(ActionEvent e) {


            if(e.getSource() == add){
                sum = x + y;
                x = Integer.parseInt(num1.getText());
                y = Integer.parseInt(num2.getText());           
            }
            if(e.getSource() == sub){
                sum = x - y;
                x = Integer.parseInt(num1.getText());
                y = Integer.parseInt(num2.getText());
            }
            if(e.getSource() == mul){
                sum = x * y;
                x = Integer.parseInt(num1.getText());
                y = Integer.parseInt(num2.getText());
            }
            if(e.getSource() == div){
                sum = x/y;
                x = Integer.parseInt(num1.getText());
                y = Integer.parseInt(num2.getText());
            }
            if(e.getSource() == button1){

                JOptionPane.showMessageDialog(null, "The sum of the desired calculation is... " + sum);
            }

        }

    }



}

Answer 1

It depends what you mean by integer, but if you just want to check if string contains only digits and optionally - at start you can check it using regex like

yourString.matches("-?\\d+");

Note that this will not check range of number.

Answer 2

Simply do:

String text = num1.getText();
try {
   Integer x = Integer.parseInt(text);
} catch (NumberFormatException) {
   System.out.println(text + " cannot be converted to integer");
}

If the String can't be parsed to Integer , a NumberFormatException will be thrown.

Answer 3

You should check, if the TextField is empty, and you should add some error handling to the integer parsing, like:

try {
    x = Integer.parseInt(num1.getText())>
catch (NumberFormatException ex) {
    //error handling
}

Answer 4

Convert to char[] using toCharArray()

Then loop through and check if each character isDigit()

edit* As others have said, catching exceptions is a much better way of handling this issue

Answer 5

When Integer.parseInt(...) is given something it cannot parse, it throws a NumberFormatException . You should wrap your parseInt(...) calls in a try/catch block and handle the situation appropriately:

try {
    x = Integer.parseInt(num1.getText());
} catch (NumberFormatException nfe) {
    // do something about broken data
}

It woul dbe convenient to use a seperate method to do all the parsing and handlng to reduce the amount of code duplication

Answer 6

You can use a function like this:

boolean isInt(String str) {
     for (int i = 0; i < str.length(); i++) {
         char c = str.charAt(i);
         if (!Character.isDigit(c)) {
            return false;
         }
     }
     return true;
}