Section 3.6
The switch Statement
The second branching statement in Java is the switch statement, which is introduced in this section. The switch statement is used far less often than the if statement, but it is sometimes useful for expressing a certain type of multiway branch.
3.6.1 The Basic switch Statement
A switch statement allows you to test the value of an expression and, depending on that value, to jump directly to some location within the switch statement. Only expressions of certain types can be used. The value of the expression can be one of the primitive integer types int, short, or byte. It can be the primitive char type. It can be String. Or it can be an enum type (see Subsection 2.3.4 for an introduction to enums). In particular, note that the expression cannot be a double or float value.
The positions within a switch statement to which it can jump are marked with case labels that take the form: "case constant:". The constant here is a literal of the same type as the expression in the switch. A case label marks the position the computer jumps to when the expression evaluates to the given constant value. 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, as it is most often used, 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
This has exactly the same effect as the following multiway if statement, but the switch statement can be more efficient because the computer can evaluate one expression and jump directly to the correct case, whereas in the if statement, the computer must evaluate up to N expressions before it knows which set of statements to execute:
if (expression == constant-1) { // but use .equals for String!! statements-1 } else if (expression == constant-2) { statements-2 } else . . . else if (expression == constant-N) { statements-N } else { statements-(N+1) }
The break statements in the switch are technically optional. The effect of a break is to make the computer jump past the end of the switch statement, skipping over all the remaining cases. 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, which terminates the subroutine as well as the switch.)
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, but they must all be 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 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.
3.6.2 Menus and switch Statements
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. */
System.out.println("What unit of measurement does your input use?");
System.out.println();
System.out.println(" 1. inches");
System.out.println(" 2. feet");
System.out.println(" 3. yards");
System.out.println(" 4. miles");
System.out.println();
System.out.println("Enter the number of your choice: ");
optionNumber = TextIO.getlnInt();
/* Read user's measurement and convert to inches. */
switch ( optionNumber ) {
case 1:
System.out.println("Enter the number of inches: ");
measurement = TextIO.getlnDouble();
inches = measurement;
break;
case 2:
System.out.println("Enter the number of feet: ");
measurement = TextIO.getlnDouble();
inches = measurement * 12;
break;
case 3:
System.out.println("Enter the number of yards: ");
measurement = TextIO.getlnDouble();
inches = measurement * 36;
break;
case 4:
System.out.println("Enter the number of miles: ");
measurement = TextIO.getlnDouble();
inches = measurement * 12 * 5280;
break;
default:
System.out.println("Error! Illegal option number! I quit!");
System.exit(1);
} // end switch
/* Now go on to convert inches to feet, yards, and miles... */
This example could instead be written using a String in the switch statement:
String units; // Unit of measurement, entered by user.
double measurement; // A numerical measurement, input by the user.
double inches; // The same measurement, converted into inches.
/* Read the user's unit of measurement. */
System.out.println("What unit of measurement does your input use?");
System.out.print("Legal responses: inches, feet, yards, or miles : ");
units = TextIO.getln().toLowerCase();
/* Read user's measurement and convert to inches. */
System.out.print("Enter the number of " + units + ": ");
measurement = TextIO.getlnDouble();
switch ( units ) {
case "inches":
inches = measurement;
break;
case "feet":
inches = measurement * 12;
break;
case "yards":
inches = measurement * 36;
break;
case "miles":
inches = measurement * 12 * 5280;
break;
default:
System.out.println("Wait a minute! Illegal unit of measure! I quit!");
System.exit(1);
} // end switch
3.6.3 Enums in switch Statements
The type of the expression in a switch can be an enumerated type. In that case, the constants in the case labels must be values from the enumerated type. For example, suppose that the type of the expression is the enumerated type Season defined by
enum Season { SPRING, SUMMER, FALL, WINTER }
and that the expression in a switch statement is an expression of type Season. The constants in the case label must be chosen from among the values Season.SPRING, Season.SUMMER, Season.FALL, or Season.WINTER. However, there is a quirk in the syntax: when an enum constant is used in a case label, only the simple name, such as "SPRING" is used, not the full name, such as "Season.SPRING". Of course, the computer already knows that the value in the case label must belong to the enumerated type, since it can tell that from the type of expression used, so there is really no need to specify the type name in the constant. For example, assuming that currentSeason is a variable of type Season, then we could have the switch statement:
switch ( currentSeason ) {
case WINTER: // ( NOT Season.WINTER ! )
System.out.println("December, January, February");
break;
case SPRING:
System.out.println("March, April, May");
break;
case SUMMER:
System.out.println("June, July, August");
break;
case FALL:
System.out.println("September, October, November");
break;
}
3.6.4 Definite Assignment and switch Statements
As a somewhat more realistic example, the following switch statement makes a random choice among three possible alternatives. Recall that the value of the expression (int)(3*Math.random()) is one of the integers 0, 1, or 2, selected at random with equal probability, so the switch statement below will assign one of the values "Rock", "Paper", "Scissors" to computerMove, with probability 1/3 for each case:
switch ( (int)(3*Math.random()) ) {
case 0:
computerMove = "Rock";
break;
case 1:
computerMove = "Paper";
break;
case 2:
computerMove = "Scissors";
break;
}
Now, this switch statement is perfectly OK, but suppose that we use it in the following code segment:
String computerMove; switch ( (int)(3*Math.random()) ) { case 0: computerMove = "Rock"; break; case 1: computerMove = "Paper"; break; case 2: computerMove = "Scissors"; break; } System.out.println("The computer's move is " + computerMove); // ERROR!
Now there is a subtle error on the last line! The problem here is due to definite assignment, the idea that the Java compiler must be able to determine that a variable has definitely been assigned a value before its value is used. Definite assignment was introduced in Subsection 3.1.4. In this example, it's true that the three cases in the switch cover all the possibilities, but the compiler is not smart enough to figure that out; it just sees that there is an integer-valued expression in the switch but not all possible integer values are covered by the given cases.
A simple solution is to replace the final case in the switch statement with default. With a default case, all possible values of the expression in the switch are certainly covered, and the compiler knows that computerMove is definitely assigned a value:
String computerMove;
switch ( (int)(3*Math.random()) ) {
case 0:
computerMove = "Rock";
break;
case 1:
computerMove = "Paper";
break;
default:
computerMove = "Scissors";
break;
}
System.out.println("The computer's move is " + computerMove); // OK!