code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks.coding;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
  
  /**
   * <p>
   * Checks that switch statement has a {@code default} clause.
   * </p>
   * <p>
   * 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.
   * </p>
   * <p>
   * 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.
   * </p>
   * <p>
   * 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.
   * </p>
   * <p>
   * See the <a href="https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.28">
   *     Java Language Specification</a> for more information about switch statements
   *     and expressions.
   * </p>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;MissingSwitchDefault&quot;/&gt;
   * </pre>
   * <p>Example of violation:</p>
   * <pre>
   * switch (i) {    // violation
   *  case 1:
   *    break;
   *  case 2:
   *    break;
   * }
   * </pre>
   * <p>Example of correct code:</p>
   * <pre>
   * 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;
   * }
   * </pre>
   * <p>Example of correct code which does not require default labels:</p>
   * <pre>
   *    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.
   *     *&#47;
   *    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.
   *     *&#47;
   *    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;
   *        };
   *    }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code missing.switch.default}
   * </li>
   * </ul>
   *
   * @since 3.1
   */
  @StatelessCheck
  public class MissingSwitchDefaultCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "missing.switch.default";
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.LITERAL_SWITCH};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          if (!containsDefaultLabel(ast)
                  && !containsPatternCaseLabelElement(ast)
                  && !containsDefaultCaseLabelElement(ast)
                  && !isSwitchExpression(ast)) {
              log(ast, MSG_KEY);
          }
      }
  
      /**
       * Checks if the case group or its sibling contain the 'default' switch.
       *
       * @param detailAst first case group to check.
       * @return true if 'default' switch found.
       */
      private static boolean containsDefaultLabel(DetailAST detailAst) {
          return TokenUtil.findFirstTokenByPredicate(detailAst,
                  ast -> ast.findFirstToken(TokenTypes.LITERAL_DEFAULT) != null
          ).isPresent();
      }
  
      /**
       * 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.
       *
       * @param detailAst first case group to check.
       * @return true if switch block contains a pattern case label element
       */
      private static boolean containsPatternCaseLabelElement(DetailAST detailAst) {
          return TokenUtil.findFirstTokenByPredicate(detailAst, ast -> {
              return ast.getFirstChild() != null
                      && ast.getFirstChild().findFirstToken(TokenTypes.PATTERN_VARIABLE_DEF) != null;
          }).isPresent();
      }
  
      /**
       * Checks if a switch block contains a default case label.
       *
       * @param detailAst first case group to check.
       * @return true if switch block contains default case label
       */
      private static boolean containsDefaultCaseLabelElement(DetailAST detailAst) {
          return TokenUtil.findFirstTokenByPredicate(detailAst, ast -> {
              return ast.getFirstChild() != null
                      && ast.getFirstChild().findFirstToken(TokenTypes.LITERAL_DEFAULT) != null;
          }).isPresent();
      }
  
      /**
       * Checks if this LITERAL_SWITCH token is part of a switch expression.
       *
       * @param ast the switch statement we are checking
       * @return true if part of a switch expression
       */
      private static boolean isSwitchExpression(DetailAST ast) {
          return ast.getParent().getType() == TokenTypes.EXPR
                  || ast.getParent().getParent().getType() == TokenTypes.EXPR;
      }
  }
  