|
Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types (discussed in
Enum Types), the
String class, and a few special classes that wrap certain primitive types:
Character,
Byte,
Short,
and
Integer (discussed in
Numbers and Strings).
The following code example,
SwitchDemo,
declares an int named month whose value represents a month. The code displays the name of the month, based on the value of month, using the switch statement.
public class SwitchDemo {
public static void main(String[] args) {
int month = 8;
String monthString;
switch (month) {
case 1: monthString = "January"; break;
case 2: monthString = "February"; break;
case 3: monthString = "March"; break;
case 4: monthString = "April"; break;
case 5: monthString = "May"; break;
case 6: monthString = "June"; break;
case 7: monthString = "July"; break;
case 8: monthString = "August"; break;
case 9: monthString = "September"; break;
case 10: monthString = "October"; break;
case 11: monthString = "November"; break;
case 12: monthString = "December"; break;
default: monthString = "Invalid month"; break;
}
System.out.println(monthString);
}
}
In this case, August is printed to standard output.
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 could also display the name of the month with if-then-else statements:
int month = 8;
if (month == 1) {
System.out.println("January");
} else if (month == 2) {
System.out.println("February");
}
. . . // and so on
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.
Another point of interest is the break statement. 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 program
SwitchDemoFallThrough
shows statements in a switch block that fall through. The program displays the month corresponding to the integer month and the months that follow in the year:
public class SwitchDemoFallThrough {
public static void main(String args[]) {
java.util.ArrayList<String> futureMonths = new java.util.ArrayList<String>();
int month = 8;
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;
}
if (futureMonths.isEmpty()) {
System.out.println("Invalid month number");
} else {
for (String monthName : futureMonths) {
System.out.println(monthName);
}
}
}
}
This is the output from the code:
August September October November December
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,
SwitchDemo2, shows how a statement can have multiple case labels. The code example calculates the number of days in a particular month:
class SwitchDemo2 {
public static void main(String[] args) {
int month = 2;
int year = 2000;
int numDays = 0;
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6:
case 9:
case 11:
numDays = 30;
break;
case 2:
if ( ((year % 4 == 0) && !(year % 100 == 0))
|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
default:
System.out.println("Invalid month.");
break;
}
System.out.println("Number of Days = " + numDays);
}
}
This is the output from the code:
Number of Days = 29
In Java SE 7 and later, you can use a String object in the switch statement's expression. The following code example,
StringSwitchDemo, displays the number of the month based on the value of the String named month:
public class StringSwitchDemo {
public static int getMonthNumber(String month) {
int monthNumber = 0;
if (month == null) { return monthNumber; }
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;
}
return monthNumber;
}
public static void main(String[] args) {
String month = "August";
int returnedMonthNumber =
StringSwitchDemo.getMonthNumber(month);
if (returnedMonthNumber == 0) {
System.out.println("Invalid month");
} else {
System.out.println(returnedMonthNumber);
}
}
}
The output from this code is 8.
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 the StringSwitchDemo 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.
Note: This example checks if the expression in the switch statement is null. Ensure that the expression in any switch statement is not null to prevent a NullPointerException from being thrown.
|
