Branching with Switch Statements

Using Switch Statements to Control the Flow of Your Program

The switch statement is one of the five control flow statements available in the Java language. It allows for any number of execution path. A switch statement takes a selector variable as an argument and uses the value of this variable to choose the path that will be executed.

You must choose the type of your selector variable among the following types:

• byte, short, char, and int primitive data types
• Character, Byte, Short, and Integer wrapper types
• enumerated types
• the String type.

It is worth noting that the following primitive types cannot be used for the type of your selector variable: boolean, long, float, and double.

Let us see a first example of a switch statement.

int quarter = ...; // any value

String quarterLabel = null;
switch (quarter) {
    case 0: quarterLabel = "Q1 - Winter"; 
            break;
    case 1: quarterLabel = "Q2 - Spring"; 
            break;
    case 2: quarterLabel = "Q3 - Summer"; 
            break;
    case 3: quarterLabel = "Q3 - Summer"; 
            break;
    default: quarterLabel = "Unknown quarter";
};

The body of a switch statement is known as a switch block. A statement in the switch block can be labeled with one or more case or default labels. The switch statement evaluates its expression, then executes all statements that follow the matching case label.

You may have noticed the use of the break keyword. Each break statement terminates the enclosing switch statement. Control flow continues with the first statement following the switch block. The break statements are necessary because without them, statements in switch blocks fall through. All statements after the matching case label are executed in sequence, regardless of the expression of subsequent case labels, until a break statement is encountered.

The following code uses fall through to fill the futureMonths list.

int month = 8;
List<String> futureMonths = new ArrayList<>();

switch (month) {
    case 1:  futureMonths.add("January");
    case 2:  futureMonths.add("February");
    case 3:  futureMonths.add("March");
    case 4:  futureMonths.add("April");
    case 5:  futureMonths.add("May");
    case 6:  futureMonths.add("June");
    case 7:  futureMonths.add("July");
    case 8:  futureMonths.add("August");
    case 9:  futureMonths.add("September");
    case 10: futureMonths.add("October");
    case 11: futureMonths.add("November");
    case 12: futureMonths.add("December");
             break;
    default: break;
}

Technically, the final break is not required because flow falls out of the switch statement. Using a break is recommended so that modifying the code is easier and less error prone.

The default section handles all values that are not explicitly handled by one of the case sections.

The following code example, shows how a statement can have multiple case labels. The code example calculates the number of days in a particular month:

int month = 2;
int year = 2021;
int numDays = 0;

switch (month) {
    case 1: case 3: case 5:   // January March May
    case 7: case 8: case 10:  // July August October
    case 12:
        numDays = 31;
        break;
    case 4: case 6:   // April June
    case 9: case 11:  // September November
        numDays = 30;
        break;
    case 2: // February
        if (((year % 4 == 0) && 
             !(year % 100 == 0))
             || (year % 400 == 0))
            numDays = 29;
        else
            numDays = 28;
        break;
    default:
        System.out.println("Invalid month.");
        break;
}

This code has one statement for more than one case.

Choosing Between Switch Statements and If-then-else Statements

Deciding whether to use if-then-else statements or a switch statement is based on readability and the expression that the statement is testing. An if-then-else statement can test expressions based on ranges of values or conditions, whereas a switch statement tests expressions based only on a single integer, enumerated value, or String object.

For instance, the following code could be written with a switch statement.

int month = ...; // any month
if (month == 1) {
    System.out.println("January");
} else if (month == 2) {
    System.out.println("February");
} ... // and so on

On the other hand the following could not be written with a switch statement, because switch statements do not support labels of type boolean.

int temperature = ...; // any temperature
if (temperature < 0) {
    System.out.println("Water is ice");
} else if (temperature < 100){
    System.out.println("Water is liquid, known as water");
} else {
    System.out.println("Water is vapor");
}

Using String as a Type for the Case Labels

In Java SE 7 and later, you can use a String object in the switch statement's expression. The following code example displays the number of the month based on the value of the String named month.

String month = ...; // any month
int monthNumber = -1;

switch (month.toLowerCase()) {
    case "january":
        monthNumber = 1;
        break;
    case "february":
        monthNumber = 2;
        break;
    case "march":
        monthNumber = 3;
        break;
    case "april":
        monthNumber = 4;
        break;
    case "may":
        monthNumber = 5;
        break;
    case "june":
        monthNumber = 6;
        break;
    case "july":
        monthNumber = 7;
        break;
    case "august":
        monthNumber = 8;
        break;
    case "september":
        monthNumber = 9;
        break;
    case "october":
        monthNumber = 10;
        break;
    case "november":
        monthNumber = 11;
        break;
    case "december":
        monthNumber = 12;
        break;
    default: 
        monthNumber = 0;
        break;
}

The String in the switch expression is compared with the expressions associated with each case label as if the String.equals() method were being used. In order for this example to accept any month regardless of case, month is converted to lowercase (with the toLowerCase() method), and all the strings associated with the case labels are in lowercase.

Null Selector Variables

The selector variable of a switch statement can be an object, so this object can be null. You should protect your code from null selector variables, because in this case the switch statement will throw a NullPointerException.