Example 4.1 Fall-Through in a switch Statement with the Colon Notation
public class Advice {
private static final int LITTLE_ADVICE = 0;
private static final int MORE_ADVICE = 1;
private static final int LOTS_OF_ADVICE = 2;
public static void main(String[] args) {
dispenseAdvice(LOTS_OF_ADVICE);
}
public static void dispenseAdvice(int howMuchAdvice) {
switch (howMuchAdvice) { // (1)
case LOTS_OF_ADVICE: System.out.println(“See no evil.”); // (2)
case MORE_ADVICE: System.out.println(“Speak no evil.”); // (3)
case LITTLE_ADVICE: System.out.println(“Hear no evil.”); // (4)
break; // (5)
default: System.out.println(“No advice.”); // (6)
}
}
}
Output from the program:
See no evil.
Speak no evil.
Hear no evil.
Several case labels can prefix the same group of statements. This is the equivalent of specifying the same case constants in a single case label. The latter syntax is preferable as it is more concise than the former. Such case constants will result in the associated group of statements being executed. This behavior is illustrated in Example 4.2 for the switch statement at (1).
At (2) in Example 4.2, three case labels are defined that are associated with the same action. At (3), (4), and (5), a list of case constants is defined for some of the case labels. Note also the use of the break statement to stop fall-through in the switch block after the statements associated with a case label are executed.
The first statement in the switch block must always have a case or default label; otherwise, it will be unreachable. This statement will never be executed because control can never be transferred to it. The compiler will flag this case (no pun intended) as an error. An empty switch block is perfectly legal, but not of much use.
Since each group of statements associated with a case label can be any arbitrary statement, it can also be another switch statement. In other words, switch statements can be nested. Since a switch statement defines its own local block, the case labels in an inner block do not conflict with any case labels in an outer block. Labels can be redefined in nested blocks; in contrast, variables cannot be redeclared in nested blocks (§6.6, p. 354). In Example 4.2, an inner switch statement is defined at (6), which allows further refinement of the action to take on the value of the selector expression in cases where multiple case labels are used in the outer switch statement. A break statement terminates the innermost switch statement in which it is executed.
The print statement at (7) is always executed for the case constants 9, 10, and 11.
Note that the break statement is the last statement in the group of statements associated with each case label. It is easy to think that the break statement is a part of the switch statement syntax, but technically it is not.
Example 4.2 Nested switch Statements with the Colon Notation
public class Seasons {
public static void main(String[] args) {
int monthNumber = 11;
switch(monthNumber) { // (1) Outer
case 12: case 1: case 2: // (2)
System.out.println(“Snow in the winter.”);
break;
case 3, 4: case 5: // (3)
System.out.println(“Green grass in the spring.”);
break;
case 6, 7, 8: // (4)
System.out.println(“Sunshine in the summer.”);
break;
case 9, 10, 11: // (5)
switch(monthNumber) { // Nested switch (6) Inner
case 10:
System.out.println(“Halloween.”);
break;
case 11:
System.out.println(“Thanksgiving.”);
break;
} // End nested switch
// Always printed for case constant 9, 10, 11
System.out.println(“Yellow leaves in the fall.”); // (7)
break;
default:
System.out.println(monthNumber + ” is not a valid month.”);
}
}
}
Output from the program:
Thanksgiving.
Yellow leaves in the fall.
