• The String Class:
  • Encapsulates an ordered set of characters
• String Literals:
  • Enclosed in double quotes

• An Important Property of String Objects:
  • String objects are immutable (i.e., they can't be changed)
• An Observation:
  • This surprises people when they first hear it because they think they have changed them (e.g., using concatenation or toLowerCase()) but the result, which is another String, is always assigned to a variable or passed as an actual parameter

• The Methods:
  • String.toLowerCase()
  • String.toUpperCase()
• Implications of Immutability:
  • A String object is created (the owning object is not changed)

• An Improper Use:

  •   String       course;
    
      course = new String("CS149");
      course.toLowerCase();
      

• A Proper Use:

  •   String       course;
    
      course = new String("CS149");
      course = course.toLowerCase();
      

• The Operator:
  • +
• Implications of Immutability:
  • A String object is created

• An Improper Use:

  •   String       course, number;
    
      course = new String("CS");
      number = new String("149");
    
      // This line won't compile because it isn't a statement
      course + number;
      

• A Proper Use:

  •   String       course, number;
    
      course = new String("CS");
      number = new String("149");
    
      course += number;         // course = course + number;
      

• Right-Side Operands:
  • The right-side operand of the + operator need not be a String if it can be converted into one
• An Example of a Common Use:

  •   int          number;
      String       course, dept;
    
      dept   = new String("CS");
      number = 149;
    
      course = dept + number;
      

• A Potential Pitfall:
  • Since the + is also used as the "positive" operator and the addition operator, it is easy to make mistakes
• An Example:

  •   String s;
    
      s = "Value: ";
      s += 'a' + 'b';
      JMUConsole.println(s);
      

    The output will be Value: 195 because 'a' + 'b' is evaluated first and evalutes to the sum of the ASCII values

• A Better Approach:
  • Use String.format() which behaves just like printf() but returns a String rather than printing it
• An Example:

  •   String s;
    
      s = String.format("Value: %c%c", 'a', 'b');
      JMUConsole.println(s);
      

    The output will be Value: ab

• Remember:
  • The new operator creates (i.e., allocates memory for an initializes) an object
  • We've assigned literals to String variables before without using new
• What's Going On?
  • As a convenience, the compiler creates a String object for each String literal
  • String s = "CS"; assigns the reference to the literal to the variable s
  • String t = new String("CS"); creates a String object that contains the characters 'C' and 'S' and assigns the reference to that object to the variable t

• What To Do?
  • Now that you know about the new operator you should use it (even though it's a little inconvenient) because it will keep you from making some subtle mistakes
• What To Remember?
  • Other people will assign String literals because it is convenient
  • At some point you too will know enough to do so, but shouldn't now

• Length:
  • String.length()
• Examining Individual Characters:
  • String.charAt(int)
• Comparing String Objects:
  • String.compareTo(java.lang.String)
  • String.equals(java.lang.String)
  • Never use the == operator (it compares references not the characters)

• Searching/Containment:
  • String.indexOf(java.lang.String)
  • String.indexOf(java.lang.String, int)
  • String.endsWith(java.lang.String)
  • String.startsWith(java.lang.String, int)
• Extracting Substrings:
  • String.substring(int)
  • String.substring(int, int)

Counting Vowels

javaexamples/strings/VowelCounter.java

/**
 * Count the number of vowels in a String
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class VowelCounter
{
    /**
     * The entry point
     *
     * @param args   The command-line arguments
     */
    public static void main(String[] args)
    {
       char               letter;       
       int                n, vowels;
       String             line;

       JMUConsole.open();

       // Prompt for and read the text
       JMUConsole.printf("Enter the text: ");
       line = JMUConsole.readLine();

       // Get the length of the String (in characters)
       n = line.length();
       
       // Initialize the accumulator
       vowels = 0;
       
       // Loop through the String
       for (int i=0; i<n; i++)
       {
          // Get the next letter
          letter = line.charAt(i);           

          // Check if it's a vowel (this can be done more efficiently)
          if ((letter == 'a') || (letter == 'e') || (letter == 'i') ||
              (letter == 'o') || (letter == 'u') ||
              (letter == 'A') || (letter == 'E') || (letter == 'I') ||
              (letter == 'O') || (letter == 'U')                      )
          {
             vowels++;             
          }
       }
       
       // Print the answer
       JMUConsole.printf("Vowels: %d\n", vowels);

       JMUConsole.close();
    }
}
        

A Hex Converter

javaexamples/strings/HexConverter.java

/**
 * Convert a "number" in hexadecimal to a number in base 10
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class HexConverter
{
    /**
     * The entry point
     *
     * @param args   The command-line arguments
     */
    public static void main(String[] args)
    {
       char               digit;
       int                i, length, number, power, value;
       String             hex;

       JMUConsole.open();

       // Read the hexadecimal number
       JMUConsole.printf("Enter the number in base 16: ");
       hex = JMUConsole.readLine();

       // Clean-up the hex number (Note that these methods
       // cannot be void since String objects are immutable.)
       hex = hex.trim();
       hex = hex.toUpperCase();
        

       // Initialization
       length = hex.length();
       number = 0;
       power  = 1;


       // Process each character (This can be done more efficiently)
       for (i=length-1; i>=0; i--) 
       {
          digit  = hex.charAt(i);
          value  = Character.digit(digit, 16);

          number = number + (value * power);

          power  = power * 16;
       }

       JMUConsole.printf("Base 10: %d\n", number);
       JMUConsole.close();
    }
}