[HELP] Converting letter from lowercase to uppercase

import javax.swing.JOptionPane;
 public class LowerCasetoUpperCase {
  public static void main(String[] args) {
   //Enter a lowercase letter
   String LowerCaseString=JOptionPane.showInputDialog("Enter a lowercase letter: ");
   
   //Convert lowercase to uppercase
    char UpperCase=String.toUpperCase(LowerCaseString);
   
    //Display results
    String output="The Uppercase letter is "+UpperCase+" ";
    JOptionPane.showMessageDialog(null, output);
 }
}