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.imports;
  
  import java.util.HashSet;
  import java.util.Set;
  
  import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.FullIdent;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  
  /**
   * <p>
   * Checks for redundant import statements. An import statement is
   * considered redundant if:
   * </p>
   * <ul>
   *   <li>It is a duplicate of another import. This is, when a class is imported
   *   more than once.</li>
   *   <li>The class non-statically imported is from the {@code java.lang}
   *   package, e.g. importing {@code java.lang.String}.</li>
   *   <li>The class non-statically imported is from the same package as the
   *   current package.</li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="RedundantImport"/&gt;
   * </pre>
   * <p>
   * Example:
   * </p>
   * <pre>
   * package Test;
   * import static Test.MyClass.*; // OK, static import
   * import static java.lang.Integer.MAX_VALUE; // OK, static import
   *
   * import Test.MyClass; // violation, imported from the same package as the current package
   * import java.lang.String; // violation, the class imported is from the 'java.lang' package
   * import java.util.Scanner; // OK
   * import java.util.Scanner; // violation, it is a duplicate of another import
   * public class MyClass{ };
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code import.duplicate}
   * </li>
   * <li>
   * {@code import.lang}
   * </li>
   * <li>
   * {@code import.same}
   * </li>
   * </ul>
   *
   * @since 3.0
   */
  @FileStatefulCheck
  public class RedundantImportCheck
      extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_LANG = "import.lang";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_SAME = "import.same";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_DUPLICATE = "import.duplicate";
  
      /** Set of the imports. */
      private final Set<FullIdent> imports = new HashSet<>();
      /** Set of static imports. */
      private final Set<FullIdent> staticImports = new HashSet<>();
  
      /** Name of package in file. */
      private String pkgName;
  
      @Override
      public void beginTree(DetailAST aRootAST) {
          pkgName = null;
          imports.clear();
          staticImports.clear();
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {
              TokenTypes.IMPORT, TokenTypes.STATIC_IMPORT, TokenTypes.PACKAGE_DEF,
          };
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          if (ast.getType() == TokenTypes.PACKAGE_DEF) {
              pkgName = FullIdent.createFullIdent(
                      ast.getLastChild().getPreviousSibling()).getText();
          }
          else if (ast.getType() == TokenTypes.IMPORT) {
              final FullIdent imp = FullIdent.createFullIdentBelow(ast);
              final String importText = imp.getText();
              if (isFromPackage(importText, "java.lang")) {
                  log(ast, MSG_LANG, importText);
              }
              // imports from unnamed package are not allowed,
              // so we are checking SAME rule only for named packages
              else if (pkgName != null && isFromPackage(importText, pkgName)) {
                  log(ast, MSG_SAME, importText);
              }
              // Check for a duplicate import
              imports.stream().filter(full -> importText.equals(full.getText()))
                  .forEach(full -> log(ast, MSG_DUPLICATE, full.getLineNo(), importText));
  
              imports.add(imp);
          }
          else {
              // Check for a duplicate static import
              final FullIdent imp =
                  FullIdent.createFullIdent(
                      ast.getLastChild().getPreviousSibling());
              staticImports.stream().filter(full -> imp.getText().equals(full.getText()))
                  .forEach(full -> log(ast, MSG_DUPLICATE, full.getLineNo(), imp.getText()));
  
              staticImports.add(imp);
          }
      }
  
      /**
       * Determines if an import statement is for types from a specified package.
       *
       * @param importName the import name
       * @param pkg the package name
       * @return whether from the package
       */
      private static boolean isFromPackage(String importName, String pkg) {
          // imports from unnamed package are not allowed:
          // https://docs.oracle.com/javase/specs/jls/se7/html/jls-7.html#jls-7.5
          // So '.' must be present in member name and we are not checking for it
          final int index = importName.lastIndexOf('.');
          final String front = importName.substring(0, index);
          return pkg.equals(front);
      }
  
  }
  