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Java Coffee Break

Section 3.6
The switch Statement

THE SECOND BRANCHING STATEMENT in Java is the switch statement, which is introduced in this section. The switch is used far less often than the if statement, but it is sometimes useful for expressing a certain type of multi-way branch. Since this section wraps up coverage of all of Java's control statements, I've included a complete list of Java's statement types at the end of the section.

A switch statement allows you to test the value of an expression and, depending on that value, to jump to some location within the switch statement. The expression must be either integer-valued or character-valued. It cannot be a String or a real number. The positions that you can jump to are marked with "case labels" that take the form: "case constant:". This marks the position the computer jumps to when the expression evaluates to the given constant. As the final case in a switch statement you can, optionally, use the label "default:", which provides a default jump point that is used when the value of the expression is not listed in any case label.

A switch statement has the form:

          switch (expression) {
             case constant-1:
                statements-1
                break;
             case constant-2:
                statements-2
                break;
                .
                .   // (more cases)
                .
             case constant-N:
                statements-N
                break;
             default:  // optional default case
                statements-(N+1)
          } // end of switch statement

The break statements are technically optional. The effect of a break is to make the computer jump to the end of the switch statement. If you leave out the break statement, the computer will just forge ahead after completing one case and will execute the statements associated with the next case label. This is rarely what you want, but it is legal. (I will note here -- although you won't understand it until you get to the next chapter -- that inside a subroutine, the break statement is sometimes replaced by a return statement.)

Note that you can leave out one of the groups of statements entirely (including the break). You then have two case labels in a row, containing two different constants. This just means that the computer will jump to the same place and perform the same action for each of the two constants.

Here is an example of a switch statement. This is not a useful example, but it should be easy for you to follow. Note, by the way, that the constants in the case labels don't have to be in any particular order, as long as they are all different:

          switch (N) {   // assume N is an integer variable
             case 1:
                System.out.println("The number is 1.");
                break;
             case 2:
             case 4:
             case 8:
                System.out.println("The number is 2, 4, or 8.");
                System.out.println("(That's a power of 2!)");
                break;
             case 3:
             case 6:
             case 9:
                System.out.println("The number is 3, 6, or 9.");
                System.out.println("(That's a multiple of 3!)");
                break;
             case 5:
                System.out.println("The number is 5.");
                break;
             default:
                System.out.println("The number is 7,");
                System.out.println("   or is outside the range 1 to 9.");
          }

The switch statement is pretty primitive as control structures go, and it's easy to make mistakes when you use it. Java takes all its control structures directly from the older programming languages C and C++. The switch statement is certainly one place where the designers of Java should have introduced some improvements.

One application of switch statements is in processing menus. A menu is a list of options. The user selects one of the options. The computer has to respond to each possible choice in a different way. If the options are numbered 1, 2, ..., then the number of the chosen option can be used in a switch statement to select the proper response.

In a TextIO-based program, the menu can be presented as a numbered list of options, and the user can choose an option by typing in its number. Here is an example that could be used in a variation of the LengthConverter example from the previous section:

          int optionNumber;   // Option number from menu, selected by user.
          double measurement; // A numerical measurement, input by the user.
                              //    The unit of measurement depends on which
                              //    option the user has selected.
          double inches;      // The same measurement, converted into inches.
          
          /* Display menu and get user's selected option number. */
          
          TextIO.putln("What unit of measurement does your input use?");
          TextIO.putln();
          TextIO.putln("         1.  inches");
          TextIO.putln("         2.  feet");
          TextIO.putln("         3.  yards");
          TextIO.putln("         4.  miles");
          TextIO.putln();
          TextIO.putln("Enter the number of your choice: ");
          optionNumber = TextIO.getlnInt();
          
          /* Read user's measurement and convert to inches. */
          
          switch ( optionNumber ) {
             case 1:
                 TextIO.putln("Enter the number of inches: );
                 measurement = TextIO.getlnDouble();
                 inches = measurement;
                 break;          
             case 2:
                 TextIO.putln("Enter the number of feet: );
                 measurement = TextIO.getlnDouble();
                 inches = measurement * 12;
                 break;          
             case 3:
                 TextIO.putln("Enter the number of yards: );
                 measurement = TextIO.getlnDouble();
                 inches = measurement * 36;
                 break;          
             case 4:
                 TextIO.putln("Enter the number of miles: );
                 measurement = TextIO.getlnDouble();
                 inches = measurement * 12 * 5280;
                 break;
             default:
                 TextIO.putln("Error!  Illegal option number!  I quit!");
                 System.exit(1);          
          } // end switch
          
          /* Now go on to convert inches to feet, yards, and miles... */

The Empty Statement

As a final note in this section, I will mention one more type of statement in Java: the empty statement. This is a statement that consists simply of a semicolon. The existence of the empty statement makes the following legal, even though you would not ordinarily see a semicolon after a }.

             if (x < 0) {
                 x = -x;
             };

The semicolon is legal after the }, but the computer considers it to be an empty statement, not part of the if statement. Occasionally, you might find yourself using the empty statement when what you mean is, in fact, "do nothing". I prefer, though, to use an empty block, consisting of { and } with nothing between, for such cases.

Occasionally, stray empty statements can cause annoying, hard-to-find errors in a program. For example, the following program segment prints out "Hello" just once, not ten times:

                 for (int i = 0; i < 10; i++);
                     System.out.println("Hello");

Why? Because the ";" at the end of the first line is a statement, and it is this statement that is executed ten times. The System.out.println statement is not really inside the for statement at all, so it is executed just once, after the for loop has completed.

A List of Java Statement Types

I mention the empty statement here mainly for completeness. You've now seen just about every type of Java statement. A complete list is given below for reference. The only new items in the list are the try..catch, throw, and synchronized statements, which are related to advanced aspects of Java known as exception-handling and multi-threading, and the return statement, which is used in subroutines. These will be covered in later sections.

Another possible surprise is what I've listed as "other expression statement," which reflects the fact that any expression followed by a semicolon can be used as a statement. To execute such a statement, the computer simply evaluates the expression, and then ignores the value. Of course, this only makes sense when the evaluation has a side effect that makes some change in the state of the computer. An example of this is the expression statement "x++;", which has the side effect of adding 1 to the value of x. Similarly, the function call "TextIO.getln()", which reads a line of input, can be used as a stand-alone statement if you want to read a line of input and discard it. Note that, technically, assignment statements and subroutine call statements are also considered to be expression statements.

Java statement types:

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