Java Enhanced Switch Expressions

In Java, a switch statement generally allows the application to have multiple possible execution paths based on the value of a given expression in runtime. The evaluated expression is called the selector expression which must be of type char, byte, short, int, Character, Byte, Short, Integer, String, or an enum.

• Java 14 (JEP 361) adds a new form of switch label “case L ->” which allows multiple constants per case.
• New switch expressions can yield a value for the whole switch-case block that can then be assigned to a variable in same statement.

1. Switch Expressions

In Java 14, switch expressions are a standard feature. In Java 13 and Java 12, it was added as a preview feature.

• It has the support of multiple case labels and using keyword yield to return value in place of old return keyword.
• It also support returning value via label rules (arrow operator similar to lambda).
• If we use arraw (->) operator, we can skip yield keyword as shown in isWeekDayV1_1().
• If we use colon (:) operator, we need to use yield keyword as shown in isWeekDayV1_2().
• In case of multiple statements, use curly braces along with yield keyword as shown in isWeekDayV2().
• In case of enum, we can skip the default case. If there is any missing value not handled in cases, compiler will complain. In all other expression types (int, strings etc), we must provide default case as well.

public class SwitchExpressions
{
	public static void main(String[] argv)
	{
		System.out.println(isWeekDayV1_1(Day.MON));		//true
		System.out.println(isWeekDayV1_2(Day.MON));		//true
		System.out.println(isWeekDayV2(Day.MON));		//true
	}

	//1 - Return value directly

	enum Day {
		MON, TUE, WED, THUR, FRI, SAT, SUN
	};

	public static Boolean isWeekDayV1_1 (Day day)
	{
		Boolean result = switch(day) {
			case MON, TUE, WED, THUR, FRI -> true;
			case SAT, SUN -> false;
		};
		return result;
	}

	public static Boolean isWeekDayV1_2 (Day day)
	{
		Boolean result = switch(day) {
			case MON, TUE, WED, THUR, FRI : yield true;
			case SAT, SUN : yield false;
		};
		return result;
	}

	//2 - Multiple statements in case block

	public static Boolean isWeekDayV2 (Day day)
	{
		Boolean result = switch(day) {
			case MON, TUE, WED, THUR, FRI ->
			{
				System.out.println("It is WeekDay");
				yield true;
			}
			case SAT, SUN ->
			{
				System.out.println("It is Weekend");
				yield false;
			}
		};
		return result;
	}
}

2. Difference between yield and return

• A return statement returns control to the invoker of a method or constructor.
• A yield statement transfers control by causing an enclosing switch expression to produce a specified value.

SwitchExpression:
	YieldStatement:
    	       yield Expression;

In the above pseudo-code:

• SwitchExpression tries to find a correct YieldStatement to transfer control to innermost enclosing yield target.
• SwitchExpression terminates normally and the value of the Expression becomes the value of the SwitchExpression.
• If the evaluation of the Expression completes abruptly for some reason, then the yield statement completes abruptly for same reason.

Happy Learning !!

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