compare biggest negative number

Hello, i have to write a program in java where i have to enter three
different numbers, and it will output the largest number. So for
example:

input: 2, 4 ,6 => output: 7
input: 5, 2 ,1 => output: 5
input: 7, 9 ,3 => output: 9

My program works well, but not with negative values, cause it always
returns a 0.
So if i would enter the following:

input: -2, -4 -,6

Then the output should be -2, but i get a '0' instead. Any idea why
this is happening and what i can do to fix it?
Here's my code:

import javax.swing.*;

public class test
{
    public static void main(String args[])
    {
        String input;
        int intNum;
        int prevNum=0;
        int output=0;

        for(int i=0; i<3; i++)
        {
            input = JOptionPane.showInputDialog(null,"Geef een
nummer","input",JOptionPane.INFORMATION_MESSAGE);
            intNum = Integer.parseInt(input);

            if(prevNum < intNum)
            {
                prevNum = intNum;
            }

        }
        JOptionPane.showMessageDialog(null,"Het grootste getal is:
"+prevNum,"getal",JOptionPane.INFORMATION_MESSAGE);

    }
}