We're trying to make switches simpler to understand at a glance. Misunderstanding the control flow of a switch is a common source of bugs.

As part of this simplification, new-style arrow (->) switches are encouraged instead of old-style colon (:) switches. And where possible, neighboring cases are grouped together (e.g. case A, B, C).

Old-style colon (:) switches:

• Have a colon between the case and the case's code. For example, case HEARTS:
• Because of the potential for fall-through, it takes time and cognitive load to understand the control flow. When a switch block is large, just skimming each case can be toilsome. Fall-through can also be conditional (see example 5. below). In this scenario, one would potentially need to reason about all possible flows for each case. When conditionally falling-through multiple cases, the number of potential control flows can grow rapidly
• Lexical scopes overlap, which can lead to surprising behaviors: definitions of local variables from earlier cases are propagated down to later cases, however the values that initialize those local variables do not propagate in the same way

New-style arrow (->) switches:

• Have an arrow between the case and the case's code. For example, case HEARTS ->
• No cases fall through; no control flow analysis needed
• Safely and easily reorder cases (within a switch)
• Lexical scopes are isolated between different cases; if you define a local variable within a case, it can only be used within that specific case.

Examples

1. Eliminate fall through

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void foo(Suit suit) {
  switch(suit) {
    case HEARTS:
      System.out.println("Red hearts");
      break;
    case DIAMONDS:
      System.out.println("Red diamonds");
      break;
    case SPADES:
    // Fall through
    case CLUBS:
      bar();
      System.out.println("Black suit");
    }
}

Which can be simplified by grouping and using a new-style switch:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void foo(Suit suit) {
  switch(suit) {
    case HEARTS -> System.out.println("Red hearts");
    case DIAMONDS -> System.out.println("Red diamonds");
    case SPADES, CLUBS -> {
      bar();
      System.out.println("Black suit");
    }
  }
}

2. return switch ...

Sometimes switch is used with a return for each case, like this:

enum SideOfCoin {OBVERSE, REVERSE};

private String renderName(SideOfCoin sideOfCoin) {
  switch(sideOfCoin) {
    case OBVERSE:
      return "Heads";
    case REVERSE:
      return "Tails";
    }
    // This should never happen, but removing this will cause a compile-time error
    throw new RuntimeException("Unknown side of coin");
}

Note that even though a case is present for each possible value of the enum, a boilerplate “should never happen” clause is still needed. The transformed code is simpler and doesn't need a “should never happen” clause.

enum SideOfCoin {OBVERSE, REVERSE};

private String renderName(SideOfCoin sideOfCoin) {
  return switch(sideOfCoin) {
    case OBVERSE -> "Heads";
    case REVERSE -> "Tails";
  };
}

If you nevertheless wish to define an explicit “should never happen” clause, this can be accomplished by placing the logic inside a default case. For example:

enum SideOfCoin {OBVERSE, REVERSE};

private String foo(SideOfCoin sideOfCoin) {
  return switch(sideOfCoin) {
    case OBVERSE -> "Heads";
    case REVERSE -> "Tails";
    default -> throw new RuntimeException("Unknown side of coin"); // should never happen
  };
}

When the checker detects an existing default that appears to be redundant, it may suggest a secondary auto-fix which removes the redundant default and its code (if any).

3. Assignment switch

If every branch of a switch is making an assignment to the same variable, the code can be simplified into a combined assignment and switch:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

int score = 0;

private void updateScore(Suit suit) {
  switch(suit) {
    case HEARTS:
    // Fall thru
    case DIAMONDS:
      score += -1;
      break;
    case SPADES:
      score += 2;
      break;
    case CLUBS:
      score += 3;
    }
}

This can be simplified as follows:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

int score = 0;

private void updateScore(Suit suit) {
  score += switch(suit) {
    case HEARTS, DIAMONDS -> -1;
    case SPADES -> 2;
    case CLUBS -> 3;
  };
}

Taking this one step further: if a local variable is defined, and then immediately followed by a switch in which every case assigns to that same variable, then all three (the switch, the variable declaration, and the assignment) can be merged:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void updateStatus(Suit suit) {
  int score;

  switch(suit) {
    case HEARTS:
    // Fall thru
    case DIAMONDS:
      score = 1;
      break;
    case SPADES:
      score = 2;
      break;
    case CLUBS:
      score = 3;
    }
  ...

}

Becomes:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void updateStatus(Suit suit) {
  int score = switch(suit) {
    case HEARTS, DIAMONDS -> 1;
    case SPADES -> 2;
    case CLUBS -> 3;
  };
  ...
}

4. Just converting to new arrow switch

Even when the simplifications discussed above are not applicable, conversion to new arrow switches can be automated by this checker:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void processEvent(Suit suit) {
    switch (suit) {
      case CLUBS:
        String message = "hello";
        var anotherMessage = "salut";
        processMessages(message, anotherMessage);
        break;
      case DIAMONDS:
        anotherMessage = "bonjour";
        processMessage(anotherMessage);
    }
}

Note that the local variables referenced in multiple cases are hoisted up out of the switch statement, and var declarations are converted to explicit types, resulting in:

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private void processEvent(Suit suit) {
    String anotherMessage;
    switch (suit) {
      case CLUBS -> {
        String message = "hello";
        anotherMessage = "salut";
        processMessages(message, anotherMessage);
      }
      case DIAMONDS -> {
        anotherMessage = "bonjour";
        processMessage(anotherMessage);
      }
    }
}

5. Complex control flows

Here's an example of a complex statement switch with conditional fall-through and various control flows. Unfortunately, the checker does not currently have the ability to automatically convert such code to new-style arrow switches. Manually converting the code could be a good opportunity to improve its readability.

How many potential execution paths can you spot?

enum Suit {HEARTS, CLUBS, SPADES, DIAMONDS};

private int foo(Suit suit){
  switch(suit) {
    case HEARTS:
      if (bar()) {
        break;
      }
    // Fall through
    case CLUBS:
      if (baz()) {
        return 1;
      } else if (baz2()) {
        throw new AssertionError(...);
      }
    // Fall through
    case SPADES:
    // Fall through
    case DIAMONDS:
      return 0;
  }
  return -1;
}