The Switch Statement

Java's switch statement is designed to support decision making where there are more than two possibilities. You provide a laundry-list of possibilities. Which of them, if any, are selected depend on a single predicate.

Within the switch statmeent, there is a laundry list of "cases". Execution procedes down this laundry list. Nothing happens until one of the cases actually matches the value of the predicate. At that point, a "big switch in the sky" is thrown and everything thereafter is executed. And, everything means everything -- even if it is associated with a case that doesn't match. This is because the switch is already thrown.

To avoid this behavior, so that exactly one path can be selected, the code associated with each case is often followed by a "break statment". It works as it did within a loop. A "break" casues control to immediately leave the switch and pick up after it.

Additionally, the "default" case can be specified. This case matches anything. It alwasy cases the great switch in the sky to be turned on.

This is best illustrated with an example. Let's consider our program of last class, rewritten using a switch.

Detangled with Switch

Rememebr this program? Look how pretty it is:

  boolean quit = false;

  while (!quit) {

    // Display the menu
    System.out.println ("");
    System.out.println ("");
    System.out.println ("--- Menu ---");
    System.out.println ("F)irst name");
    System.out.println ("M)iddle name");
    System.out.println ("L)ast name");
    System.out.println ("Q)uit");
    System.out.println ("");
    System.out.println ("Enter your choice: ");

    // Get choice
    Scanner keyboard = new Scanner(System.in);
    char choice = keyboard.next().charAt(0);


    // Decide what to do -- and do it
    switch (choice) {

      case 'F':
        System.out.println ("Gregory");
        break;

      case 'M':
        System.out.println ("Michael");
        break;

      case 'L':
        System.out.println ("Kesden");
        break;

      case 'Q':
        // Notice: Can't just break here -- we'd leave the switch, not the loop
        quit = true; 
        break;

      default: 
        // Since this is the last case, not strictly necessary, but good form
        System.out.println ("Please select F, M, L, or Q.");
        break; 
    
    }

  }
  

...wasn't that nice?

A Closer Look at the Break (or Absence, Thereof)

Now, let's take a closer look at the break. Let's see how we can avoid the need to upperCase() the string, by accepting either lower- or upper-case, directly.

  boolean quit = false;

  while (!quit) {

    // Display the menu
    System.out.println ("");
    System.out.println ("");
    System.out.println ("--- Menu ---");
    System.out.println ("F)irst name");
    System.out.println ("M)iddle name");
    System.out.println ("L)ast name");
    System.out.println ("Q)uit");
    System.out.println ("");
    System.out.println ("Enter your choice: ");

    // Get choice
    Scanner keyboard = new Scanner(System.in);
    char choice = keyboard.next().charAt(0);


    // Decide what to do -- and do it
    switch (choice) {

      case 'f':
      case 'F':
        System.out.println ("Gregory");
        break;

      case 'm':
      case 'M':
        System.out.println ("Michael");
        break;

      case 'l':
      case 'L':
        System.out.println ("Kesden");
        break;

      case 'q':

      case 'Q':
        // Notice: Can't just break here -- we'd leave the switch, not the loop
        quit = true; 
        break;

      default: 
        // Since this is the last case, not strictly necessary, but good form
        System.out.println ("Please select F, M, L, or Q.");
        break; 
    
    }

  }
  

Take a look at the back-to-back use of upper- and lower-cases. This works because there is no break after the first case -- only the second. So, if the lower case "turns the switch on", it stays on until it hits the break.

Switch Statment, Important Notes

A few things to keep in mind:

• The "switch" must be based on an "ordinal" type: int, long, char, boolean. It can't be based on somethng that requires complicated matching, such as float, double, String (or any Object), &c.
  
  
• The "default" case is not requied. It is completely optional.
  
  
• You should always end a case with a break -- even if it is the last one. During maintenance, someone might just add a new case underneath without noticing the absence of the break.