why does a letter return -1 jav code example

Example: islowercase java

// check lowercase or not in java..

public class Test {

   public static void main(String args[]) {
      System.out.println(Character.isLowerCase('c')); //true
      System.out.println(Character.isLowerCase('C')); //false
      System.out.println(Character.isLowerCase('\n')); //false
      System.out.println(Character.isLowerCase('\t')); //false
   }
}