code: // from gee.cs.oswego.edu/home/jsr166/jsr166
  
  package jsr166tests.jtreg.util.TreeMap;
  
  /*
   * Copyright 2005 Sun Microsystems, Inc.  All Rights Reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.
   *
   * This code 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 General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
   * CA 95054 USA or visit www.sun.com if you need additional information or
   * have any questions.
   */
  
  /*
   * @test
   * @bug     5018254
   * @summary Test null-allowing Comparators
   * @author  Martin Buchholz
   */
  
  import java.util.*;
  import java.util.concurrent.*;
  
  public class NullAtEnd {
      static volatile int passed = 0, failed = 0;
  
      static void fail(String msg) {
          failed++;
          new AssertionError(msg).printStackTrace();
      }
  
      static void pass() {
          passed++;
      }
  
      static void unexpected(Throwable t) {
          failed++;
          t.printStackTrace();
      }
  
      static void check(boolean condition, String msg) {
          if (condition)
              passed++;
          else
              fail(msg);
      }
  
      static void check(boolean condition) {
          check(condition, "Assertion failure");
      }
  
      private static boolean eq(Object x, Object y) {
          return x == null ? y == null : x.equals(y);
      }
  
      private static final Comparator<String> NULL_AT_END
          = new Comparator<String>() {
              /**
               * Allows for nulls.  Null is greater than anything non-null.
               */
              public int compare(String x, String y) {
                  if (x == null && y == null) return 0;
                  if (x == null && y != null) return 1;
                  if (x != null && y == null) return -1;
                  return x.compareTo(y);
              }
          };
  
      public static void main(String[] args) {
          try {
              SortedMap<String,String> m1
                  = new TreeMap<String,String>(NULL_AT_END);
              check(eq(m1.put("a", "a"), null));
              check(eq(m1.put("b", "b"), null));
              check(eq(m1.put("c", "c"), null));
              check(eq(m1.put(null, "d"), null));
  
              SortedMap<String,String> m2 = new TreeMap<String,String>(m1);
  
              check(eq(m1.lastKey(), null));
              check(eq(m1.get(m1.lastKey()), "d"));
              check(eq(m1.remove(m1.lastKey()), "d"));
              check(eq(m1.lastKey(), "c"));
  
              check(eq(m2.entrySet().toString(), "[a=a, b=b, c=c, null=d]"));
  
              SortedMap<String,String> m3 = m2.tailMap("b");
  
              check(eq(m3.lastKey(), null));
              check(eq(m3.get(m3.lastKey()), "d"));
              check(eq(m3.remove(m3.lastKey()), "d"));
              check(eq(m3.lastKey(), "c"));
  
          } catch (Throwable t) { unexpected(t); }
  
          System.out.printf("%nPassed = %d, failed = %d%n%n", passed, failed);
          if (failed > 0) throw new Error("Some tests failed");
      }
  }
  