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A switch block comprises several case labels and an optional but highly recommended default label. Statements that follow each case label must end with a break statement, which is responsible for transferring the control to the end of the switch block. When omitted, the statements in the subsequent case label are executed. Because the break statement is optional, omitting it produces no compiler warnings. When this behavior is unintentional, it can cause unexpected control flow.

Noncompliant Code Example

In this noncompliant code example, the case where the card is 11 lacks a break statement. As a result, execution continues with the statements for card = 12.

int card = 11;

switch (card) {
  /* ... */
  case 11: 
    System.out.println("Jack");
  case 12: 
    System.out.println("Queen"); 
    break;
  case 13: 
    System.out.println("King"); 
    break;
  default: 
    System.out.println("Invalid Card"); 
    break;
}

Compliant Solution

This compliant solution terminates each case (including the default case) with a break statement:

int card = 11;

switch (card) {
  /* ... */
  case 11: 
    System.out.println("Jack");
    break;
  case 12: 
    System.out.println("Queen"); 
    break;
  case 13: 
    System.out.println("King"); 
    break;
  default: 
    System.out.println("Invalid Card"); 
    break;
}

Applicability

Failure to include break statements can cause unexpected control flow.

The break statement at the end of the final case in a switch statement may be omitted. By convention, this is the default label. The break statement serves to transfer control to the end of the switch block. Fall-through behavior also causes control to arrive at the end of the switch block. Consequently, control transfers to the statements following the switch block without regard to the presence or absence of the break statement. Nevertheless, the final case in a switch statement should end with a break statement in accordance with good programming style [Vermeulen 2000].

Exceptionally, when multiple cases require execution of identical code, break statements may be omitted from all cases except the last one. Similarly, when processing for one case is a proper prefix of processing for one or more other cases, the break statement may be omitted from the prefix case. This should be clearly indicated with a comment. For example:

int card = 11;
int value;

// Cases 11,12,13 fall through to the same case 
switch (card) {
  // Processing for this case requires a prefix
  // of the actions for the following three
  case 10:
    do_something(card);
    // Intentional fall-through
    // These three cases are treated identically 
  case 11:        // Break not required
  case 12:        // Break not required
  case 13: 
    value = 10; 
    break;        // Break required
  default: 
    // Handle error condition 
}

Also, when a case ends with a return or throw statement, the break statement may be omitted.

Bibliography