Java 12

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Switch Expressions (Preview) JEP 325

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More on this in Switch Expression

Java 13

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Switch Expressions (Second Preview) JEP 354

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More on this in Switch Expression

Java 14

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Switch Expression

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Below are the changes in the switch construct introduced in Java 14, formalized by JEP 361, JEP-354, and JEP-325. The new Switch Expression addresses three main issues with the traditional switch statement: verbosity, variable scope issues, and being limited to a statement.

1. Its unnecessarily verbose and error prone

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The traditional switch required break statements, leading to repetitive code and the potential for fall-through errors if a break was missed.

• Problem Example (Traditional Switch Statement):

  switch (day) {
      case MONDAY:
      case FRIDAY:
      case SUNDAY:
          System.out.println(6);
          break;
      case TUESDAY:
          System.out.println(7);
          break;
      case THURSDAY:
      case SATURDAY:
          System.out.println(8);
          break;
      case WEDNESDAY:
          System.out.println(9);
          break;
  }
  

• Solution (Switch Expression with ->):

  The new syntax uses case L ->, which eliminates the need for an explicit break and allows multiple labels to be grouped.

  switch (weekDay) {
      case MONDAY, FRIDAY, SUNDAY -> System.out.println(6);
      case TUESDAY                -> System.out.println(7);
      case THURSDAY, SATURDAY     -> System.out.println(8);
      case WEDNESDAY              -> System.out.println(9);
  }
  

2. Scope of variable within Switch

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A variable declared in one case block in the traditional switch is scoped to the entire switch block, causing errors if the same variable name is used in another arm.

• Problem Example (Variable Scope):

  switch (day) {
      case MONDAY:
      case TUESDAY:
          int temp = ...;     
          break;
      case WEDNESDAY:
      case THURSDAY:
          int temp = ...;    // Can't call this variable 'temp'
          break;
      default:
          int temp = ...;    // Can't call this variable 'temp'
  }
  

  Error: Variable ’temp’ is already defined in the scope

• Workaround: The common workaround was declaring a variable outside the switch and assigning values inside each case.

  int temp;
  switch (day) {
      case MONDAY:
      case FRIDAY:
      case SUNDAY:
          temp = 6;
          break;
      case TUESDAY:
          temp = 7;
          break;
      case THURSDAY:
      case SATURDAY:
          temp = 8;
          break;
      case WEDNESDAY:
          temp = 9;
          break;
      default:
          throw new IllegalStateException("Wat: " + day);
  }
  

• Solution: Using yield or Expression Form: The new switch can return a value, naturally handling scope.

  • Using yield (with :): The yield keyword is used to produce a value from the switch expression.

  Integer temp = switch (weekDay) {
              case MONDAY, FRIDAY, SUNDAY: 
                  yield 6;
              case TUESDAY:
                  yield 7;
              case THURSDAY, SATURDAY:
                  yield 8;
              case WEDNESDAY:
                  yield 9;
          };
  

  • Using -> (as expression): This concise syntax implicitly yields the result.

  Integer temp = switch (weekDay) {
              case MONDAY, FRIDAY, SUNDAY -> 6;
              case TUESDAY                -> 7;
              case THURSDAY, SATURDAY     -> 8;
              case WEDNESDAY              -> 9;
          };
  

3. Limited to a statement not expression

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The traditional switch was limited to a statement and could not compute and return a single value directly.

• As Statement (Original):

  int numLetters;
  switch (day) {
      case MONDAY:
      case FRIDAY:
      case SUNDAY:
          numLetters = 6;
          System.out.println(6);
          break;
      case TUESDAY:
          numLetters = 7;
          System.out.println(6);
          break;
      case THURSDAY:
      case SATURDAY:
          numLetters = 8;
          System.out.println(6);
          break;
      case WEDNESDAY:
          numLetters = 9;
          System.out.println(6);
          break;
      default:
          throw new IllegalStateException("Wat: " + day);
  }
  

• As Expression (New Feature): The switch can now be used as an expression, where its result can be directly used in a statement like System.out.println().

  System.out.println(
          switch (weekDay) {
              case MONDAY, FRIDAY, SUNDAY -> 6;
              case TUESDAY -> 7;
              case THURSDAY, SATURDAY -> 8;
              case WEDNESDAY -> 9;
          }
  );
  

Java 15

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Java 16

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Java 17

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Some important links

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