Computer Science 15-110, Spring 2010
Class Notes:  String and Character Methods

1. String Methods
   1. charAt
   2. compareTo
   3. compareToIgnoreCase
   4. concat
   5. contains
   6. endsWith
   7. equals
   8. equals versus "=="
   9. equalsIgnoreCase
   10. indexOf
   11. lastIndexOf
   12. length
   13. replace
   14. startsWith
   15. substring
   16. toLowerCase
   17. toUpperCase
   18. trim
   19. Online String API
2. Character Methods
   1. isDigit
   2. isLetter
   3. isLetterOrDigit
   4. isLowerCase
   5. isUpperCase
   6. isWhitespace
   7. toLowerCase
   8. toUpperCase
   9. Online Character API

String and Character Methods

1. String Methods
    
   1. charAt

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".charAt(0));
          System.out.println("abc".charAt(1));
          System.out.println("abc".charAt(2));
          System.out.println("abc".charAt(3)); // run-time error!
        }
      }

      Another Example:

      class MyCode  {
        public static void main(String[] args) {
          // The canonical loop-through-a-string
          String s = "Carpe diem!";
          for (int i=0; i<s.length(); i++)
            System.out.println(s.charAt(i));
        }
      }

   2. compareTo

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".compareTo("def"));
          System.out.println("def".compareTo("abc"));
          System.out.println("abc".compareTo("ABC"));
          System.out.println("ABC".compareTo("abc"));
          System.out.println("ab".compareTo("abc"));
          System.out.println("abc".compareTo("ab"));
          System.out.println("abc".compareTo("abc"));
        }
      }

      Another Example:

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc" < "def"); // will not compile!
        }
      }

      Fixed:

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".compareTo("def") < 0);
        }
      }

   3. compareToIgnoreCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".compareToIgnoreCase("def"));
          System.out.println("def".compareToIgnoreCase("abc"));
          System.out.println("abc".compareToIgnoreCase("ABC"));
          System.out.println("ABC".compareToIgnoreCase("abc"));
          System.out.println("ab".compareToIgnoreCase("abc"));
          System.out.println("abc".compareToIgnoreCase("ab"));
          System.out.println("abc".compareToIgnoreCase("abc"));
        }
      }

   4. concat

      class MyCode  {
        public static void main(String[] args) {
          String s = "abc";
          System.out.println(s.concat("def"));
          System.out.println(s + "def");
        }
      }

   5. contains

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".contains("abc"));
          System.out.println("abc".contains("ABC"));
          System.out.println("abc".contains("abcd"));
          System.out.println("abc".contains(""));
        }
      }

   6. endsWith

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".endsWith("bc"));
          System.out.println("abc".endsWith("c"));
          System.out.println("abc".endsWith(""));
          System.out.println("abc".endsWith("b"));
          System.out.println("abc".endsWith("C"));
        }
      }

   7. equals

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".equals("abc"));
          System.out.println("abc".equals("def"));
          System.out.println("abc".equals("ABC"));
          System.out.println("abc".equals("ab"));
          System.out.println("abc".equals(null));
        }
      }

   8. equals versus "=="

      class MyCode  {
        public static void main(String[] args) {
          // Do not use "==" with Strings -- the results are unpredictable!
          String ab = "ab";
          String cd = "cd";
          String s1 = "abcd";
          String s2 = ab + cd;
          String s3 = "ab" + "cd";
          String s4 = "ab".concat("cd");
          // Note that s1, s2, s3, and s4 all hold the characters "abcd"
          System.out.println("See how equals works as expected for ALL of them:");
          System.out.println(s1.equals(s2));
          System.out.println(s1.equals(s3));
          System.out.println(s1.equals(s4));
          System.out.println("But == only works as expected for SOME of them:");
          System.out.println(s1 == s2);
          System.out.println(s1 == s3);
          System.out.println(s1 == s4);
        }
      }

   9. equalsIgnoreCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println("abc".equalsIgnoreCase("abc"));
          System.out.println("abc".equalsIgnoreCase("def"));
          System.out.println("abc".equalsIgnoreCase("ABC"));
          System.out.println("abc".equalsIgnoreCase("ab"));
          System.out.println("abc".equalsIgnoreCase(null));
        }
      }

   10. indexOf

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("indexOf a char:");
           System.out.println("ABC".indexOf('A'));
           System.out.println("ABC".indexOf('B'));
           System.out.println("ABC".indexOf('D'));
           System.out.println();
           System.out.println("indexOf a String:");
           System.out.println("ABC".indexOf("BC"));
           System.out.println("ABC".indexOf("CD"));
           System.out.println();
           System.out.println("indexOf, starting from a given index:");
           System.out.println("ABCDBC".indexOf("BC",1));
           System.out.println("ABCDBC".indexOf("BC",2));
           System.out.println("ABCDBC".indexOf("BC",5));
         }
       }

   11. lastIndexOf

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("lastIndexOf a char:");
           System.out.println("ABCAB".lastIndexOf('A'));
           System.out.println("ABCAB".lastIndexOf('B'));
           System.out.println("ABCAB".lastIndexOf('D'));
           System.out.println();
           System.out.println("lastIndexOf a String:");
           System.out.println("ABCAB".lastIndexOf("AB"));
           System.out.println("ABCAB".lastIndexOf("CD"));
           System.out.println();
           System.out.println("lastIndexOf, starting from a given index:");
           System.out.println("ABCDBC".lastIndexOf("BC",4));
           System.out.println("ABCDBC".lastIndexOf("BC",3));
           System.out.println("ABCDBC".lastIndexOf("BC",0));
         }
       }

   12. length

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("abcd".length());
           System.out.println("a b c d".length());
           System.out.println("a\nb\nc\nd".length());
           System.out.println("".length());
         }
       }

   13. replace

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("Replace char-for-char:");
           System.out.println("abcabc".replace('b','z'));
           System.out.println();
           System.out.println("Replace string-for-string:");
           System.out.println("abcabc".replace("bc","xyz"));
           System.out.println("abcabc".replace("bc",""));
         }
       }

       Illegal uses of replace:

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("abcabc".replace("bc",'d')); // Error!
           System.out.println("abcabc".replace('b','')); // A different error!
         }
       }

   14. startsWith

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("abc".startsWith("ab"));
           System.out.println("abc".startsWith("a"));
           System.out.println("abc".startsWith(""));
           System.out.println("abc".startsWith("b"));
           System.out.println("abc".startsWith("A"));
         }
       }

   15. substring

       class MyCode  {
         public static void main(String[] args) {
           // Note: substring includes start index but excludes end index
           System.out.println("ABCD".substring(0,1));
           System.out.println("ABCD".substring(0,2));
           System.out.println("ABCD".substring(1,2));
           System.out.println("ABCD".substring(1,3));
           System.out.println();
           // Note: if omitted, the default end index is the string's length!
           String s = "ABCD";
           System.out.println(s.substring(2,s.length()));
           System.out.println(s.substring(2));
         }
       }

   16. toLowerCase

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("This Needs 4 Case Changes!".toLowerCase());
         }
       }

   17. toUpperCase

       class MyCode  {
         public static void main(String[] args) {
           System.out.println("Carpe Diem!".toUpperCase());
         }
       }

   18. trim

       class MyCode  {
         public static void main(String[] args) {
           String s1 = "   ab   cd   ";
           String s2 = s1.trim();
           System.out.println("[" + s1 + "]");
           System.out.println("[" + s2 + "]");
         }
       }

   19. Online String API
       See:  http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html
        
2. Character Methods
    
   1. isDigit

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isDigit('A'));
          System.out.println(Character.isDigit('3'));
          System.out.println(Character.isDigit(3));  // Surprise!
        }
      }

   2. isLetter

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isLetter('A'));
          System.out.println(Character.isLetter('a'));
          System.out.println(Character.isLetter(' '));
          System.out.println(Character.isLetter('\n'));
          System.out.println(Character.isLetter('3'));
          System.out.println(Character.isLetter(3));
        }
      }

   3. isLetterOrDigit

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isLetterOrDigit('A'));
          System.out.println(Character.isLetterOrDigit('a'));
          System.out.println(Character.isLetterOrDigit(' '));
          System.out.println(Character.isLetterOrDigit('\n'));
          System.out.println(Character.isLetterOrDigit('3'));
          System.out.println(Character.isLetterOrDigit(3));
        }
      }

   4. isLowerCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isLowerCase('A'));
          System.out.println(Character.isLowerCase('a'));
          System.out.println(Character.isLowerCase(' '));
          System.out.println(Character.isLowerCase('3'));
        }
      }

   5. isUpperCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isUpperCase('A'));
          System.out.println(Character.isUpperCase('a'));
          System.out.println(Character.isUpperCase(' '));
          System.out.println(Character.isUpperCase('3'));
        }
      }

   6. isWhitespace

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.isWhitespace('A'));
          System.out.println(Character.isWhitespace('a'));
          System.out.println(Character.isWhitespace(' '));
          System.out.println(Character.isWhitespace('3'));
          System.out.println(Character.isWhitespace('\n'));
          System.out.println(Character.isWhitespace('\t'));
        }
      }

   7. toLowerCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.toLowerCase('A'));
          System.out.println(Character.toLowerCase('a'));
          System.out.println(Character.toLowerCase('#'));
          System.out.println(Character.toLowerCase('3'));
        }
      }

   8. toUpperCase

      class MyCode  {
        public static void main(String[] args) {
          System.out.println(Character.toUpperCase('A'));
          System.out.println(Character.toUpperCase('a'));
          System.out.println(Character.toUpperCase('#'));
          System.out.println(Character.toUpperCase('3'));
        }
      }

   9. Online Character API
      See:  http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Character.html

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