Package com.puppycrawl.tools.checkstyle.checks.coding

Class MissingSwitchDefaultCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable
    public class MissingSwitchDefaultCheck
extends AbstractCheck

    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.

    To configure the check: 

     <module name="MissingSwitchDefault"/>

    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;
        };
    }

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • missing.switch.default
    Since:
    3.1

    • Field Detail

      • MSG_KEY

        public static final java.lang.String MSG_KEY
        A key is pointing to the warning message text in "messages.properties" file.
        See Also:
        Constant Field Values

    • Method Detail

      • getAcceptableTokens

        public int[] getAcceptableTokens()
        Description copied from class: AbstractCheck
        The configurable token set. Used to protect Checks against malicious users who specify an unacceptable token set in the configuration file. The default implementation returns the check's default tokens.
        Specified by:
        getAcceptableTokens in class AbstractCheck
        Returns:
        the token set this check is designed for.
        See Also:
        TokenTypes

      • containsDefaultLabel

        private static boolean containsDefaultLabel​(DetailAST detailAst)
        Checks if the case group or its sibling contain the 'default' switch.
        Parameters:
        detailAst - first case group to check.
        Returns:
        true if 'default' switch found.

      • containsPatternCaseLabelElement

        private static boolean containsPatternCaseLabelElement​(DetailAST detailAst)
        Checks if a switch block contains a case label with a pattern variable definition. In this situation, the compiler enforces the given switch block to cover all possible inputs, and we do not need a default label.
        Parameters:
        detailAst - first case group to check.
        Returns:
        true if switch block contains a pattern case label element

      • containsDefaultCaseLabelElement

        private static boolean containsDefaultCaseLabelElement​(DetailAST detailAst)
        Checks if a switch block contains a default case label.
        Parameters:
        detailAst - first case group to check.
        Returns:
        true if switch block contains default case label

      • isSwitchExpression

        private static boolean isSwitchExpression​(DetailAST ast)
        Checks if this LITERAL_SWITCH token is part of a switch expression.
        Parameters:
        ast - the switch statement we are checking
        Returns:
        true if part of a switch expression