src/share/classes/java/lang/String.java

rev 5380 : 7126277: Alternative hashing implementation

*** 23,33 ****
   * questions.
   */
  
  package java.lang;
  
- import java.io.ObjectStreamClass;
  import java.io.ObjectStreamField;
  import java.io.UnsupportedEncodingException;
  import java.nio.charset.Charset;
  import java.util.ArrayList;
  import java.util.Arrays;
--- 23,32 ----
*** 35,44 ****
--- 34,44 ----
  import java.util.Formatter;
  import java.util.Locale;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  import java.util.regex.PatternSyntaxException;
+ import sun.misc.Hashing;
  
  /**
   * The {@code String} class represents character strings. All
   * string literals in Java programs, such as {@code "abc"}, are
   * implemented as instances of this class.
*** 106,116 ****
   * @see     java.nio.charset.Charset
   * @since   JDK1.0
   */
  
  public final class String
!     implements java.io.Serializable, Comparable<String>, CharSequence
  {
      /** The value is used for character storage. */
      private final char value[];
  
      /** The offset is the first index of the storage that is used. */
--- 106,116 ----
   * @see     java.nio.charset.Charset
   * @since   JDK1.0
   */
  
  public final class String
!     implements java.io.Serializable, Comparable<String>, CharSequence, Hashable32
  {
      /** The value is used for character storage. */
      private final char value[];
  
      /** The offset is the first index of the storage that is used. */
*** 3075,3080 ****
--- 3075,3119 ----
       * @return  a string that has the same contents as this string, but is
       *          guaranteed to be from a pool of unique strings.
       */
      public native String intern();
      
+     /**
+      * Seed value used for each alternative hash calculated.
+      */
+     private static final int HASHING_SEED;
+     
+     static {
+         // ??? Stronger random source? (Random/SecureRandom is not available)
+         HASHING_SEED = System.identityHashCode(String.class)
+                 ^ System.identityHashCode(System.class)
+                 ^ System.identityHashCode(Thread.currentThread())
+                 ^ (int) (System.currentTimeMillis() >>> 2) // resolution is poor
+                 ^ (int) (System.nanoTime() >>> 5); // resolution is poor
+     }
+ 
+     /**
+      * Cached value of the hashing algorithm result
+      */
+     private transient int hash32 = 0;
+ 
+     /**
+      * {@inheritDoc}
+      * 
+      * <p/>
+      * The hash value will never be zero.
+      */
+     public int hash32() {
+         int h = hash32;
+         if (0 == h) {
+            // harmless data race on hash32 here.            
+            h = Hashing.murmur3_32(HASHING_SEED, value, offset, count);
+            
+            // ensure result is not zero to avoid recalcing
+            h = (0 != h) ? h : 1;
+            
+            hash32 = h;
+         }
+ 
+         return h;
+     }
  }