MissingSwitchDefault

Since Checkstyle 3.1

Description

Checks that switch statement has a default clause.

Rationale: It's usually a good idea to introduce a default case in every switch statement. Even if the developer is sure that all currently possible cases are covered, this should be expressed in the default branch, e.g. by using an assertion. This way the code is protected against later changes, e.g. introduction of new types in an enumeration type.

This check does not validate any switch expressions. Rationale: The compiler requires switch expressions to be exhaustive. This means that all possible inputs must be covered.

This check does not validate switch statements that use pattern or null labels. Rationale: Switch statements that use pattern or null labels are checked by the compiler for exhaustiveness. This means that all possible inputs must be covered.

See the Java Language Specification for more information about switch statements and expressions.

See the Java Language Specification for more information about patterns.

Examples

To configure the check: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="MissingSwitchDefault"/>
  </module>
</module>

Example of violation: 

switch (i) {    // violation
  case 1:
    break;
  case 2:
    break;
}

Example of correct code: 

switch (i) {
  case 1:
    break;
  case 2:
    break;
  default: // OK
    break;
}
switch (o) {
    case String s: // type pattern
        System.out.println(s);
        break;
    case Integer i: // type pattern
        System.out.println("Integer");
        break;
    default:    // will not compile without default label, thanks to type pattern label usage
        break;
}

Example of correct code which does not require default labels: 

sealed interface S permits A, B, C {}
final class A implements S {}
final class B implements S {}
record C(int i) implements S {}  // Implicitly final

/**
 * The completeness of a switch statement can be
 * determined by the contents of the permits clause
 * of interface S. No default label or default case
 * label is allowed by the compiler in this situation, so
 * this check does not enforce a default label in such
 * statements.
 */
static void showSealedCompleteness(S s) {
    switch (s) {
        case A a: System.out.println("A"); break;
        case B b: System.out.println("B"); break;
        case C c: System.out.println("C"); break;
    }
}

/**
 * A total type pattern matches all possible inputs,
 * including null. A default label or
 * default case is not allowed by the compiler in this
 * situation. Accordingly, check does not enforce a
 * default label in this case.
 */
static void showTotality(String s) {
    switch (s) {
        case Object o: // total type pattern
            System.out.println("o!");
    }
}

enum Color { RED, GREEN, BLUE }

static int showSwitchExpressionExhaustiveness(Color color) {
    switch (color) {
        case RED: System.out.println("RED"); break;
        case BLUE: System.out.println("BLUE"); break;
        case GREEN: System.out.println("GREEN"); break;
        // Check will require default label below, compiler
        // does not enforce a switch statement with constants
        // to be complete.
        default: System.out.println("Something else");
    }

    // Check will not require default label in switch
    // expression below, because code will not compile
    // if all possible values are not handled. If the
    // 'Color' enum is extended, code will fail to compile.
    return switch (color) {
        case RED:
            yield 1;
        case GREEN:
            yield 2;
        case BLUE:
            yield 3;
    };
}

Example of Usage

Violation Messages

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker