Switch statement

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The switch statement is Java's multi-way branch statement. It is used to take the place of long if-else if-else chains, and make them more readable. However, unlike if statements, one may not use inequalities; each value must be concretely defined.

There are three critical components to the switch statement:

  • case: This is the value that is evaluated for equivalence with the argument to the switch statement.
  • default: This is an optional, catch-all expression, should none of the case statements evaluate to true.
  • Abrupt completion of the case statement; usually break: This is required to prevent the undesired evaluation of further case statements.

With the exception of continue, it is possible to use any statement which would cause the abrupt completion of a statement. This includes:

  • break
  • return
  • throw

In the example below, a typical switch statement is written with four possible cases, including default.

Scanner scan = new Scanner(System.in);
int i = scan.nextInt();
switch (i) {
    case 0:
        System.out.println("i is zero");
        break;
    case 1:
        System.out.println("i is one");
        break;
    case 2:
        System.out.println("i is two");
        break;
    default:
        System.out.println("i is less than zero or greater than two");
}

By omitting break or any statement which would an abrupt completion, we can leverage what are known as "fall-through" cases, which evaluate against several values. This can be used to create ranges for a value to be successful against, but is still not as flexible as inequalities.

Scanner scan = new Scanner(System.in);
int foo = scan.nextInt();
switch(foo) {
    case 1:
        System.out.println("I'm equal or greater than one");
    case 2:
    case 3:    
        System.out.println("I'm one, two, or three");
        break;
    default:
        System.out.println("I'm not either one, two, or three");
}

In case of foo == 1 the output will be:

I'm equal or greater than one
I'm one, two, or three

In case of foo == 3 the output will be:

I'm one, two, or three
Java SE 5

The switch statement can also be used with enums.

enum Option {
    BLUE_PILL,
    RED_PILL
}

public void takeOne(Option option) {
    switch(option) {
        case BLUE_PILL:
            System.out.println("Story ends, wake up, believe whatever you want.");
            break;
        case RED_PILL:
            System.out.println("I show you how deep the rabbit hole goes.");
            break;
            
    }
}
Java SE 7

The switch statement can also be used with Strings.

public void rhymingGame(String phrase) {
    switch (phrase) {
        case "apples and pears":
            System.out.println("Stairs");
            break;
        case "lorry":
            System.out.println("truck");
            break;
        default:
            System.out.println("Don't know any more");
    }
}

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2016-07-29
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