Tune String's hashCode() + equals()

[Ulf Zibis]

Am 04.03.2010 23:38, schrieb Marek Kozieł:
> 2010/3/4 Ulf Zibis<Ulf.Zibis at gmx.de>:
>
>    
>>> 5.
>>> Intern string do not need hash codes co comparing cos they have same
>>> address, so first loop would return true if they are equal, after this
>>> we need only to check if they are not equal:
>>>
>>>        
>>>> if (isIntern()&&    anotherString.isIntern()) return false;
>>>>
>>>>          
>> You are right, but
>>     if (h1 != 0&&  h2 != 0&&  h1 != h2) return false;
>> would perform same (if already computed internal hash would be
>> back-propagated to the Java object).
>>
>>      
> Could you explain what do you mean ?
>    

h1 = this.hash;
h2 = otherString.hash;

See:
In hotspot/src/share/vm/prims/jvm.cpp :
JVM_ENTRY(jstring, JVM_InternString(JNIEnv *env, jstring str))
   JVMWrapper("JVM_InternString");
   JvmtiVMObjectAllocEventCollector oam;
   if (str == NULL) return NULL;
   oop string = JNIHandles::resolve_non_null(str);
   oop result = StringTable::intern(string, CHECK_NULL);
   return (jstring) JNIHandles::make_local(env, result);
JVM_END

In hotspot/src/share/vm/classfile/symbolTable.cpp :

oop StringTable::intern(Handle string_or_null, jchar* name,
                         int len, TRAPS) {
   unsigned int hashValue = hash_string(name, len);
   int index = the_table()->hash_to_index(hashValue);
   oop string = the_table()->lookup(index, name, len, hashValue);

   // Found
   if (string != NULL) return string;

   // Otherwise, add to symbol to table
   return the_table()->basic_add(index, string_or_null, name, len,
                                 hashValue, CHECK_NULL);
}

int StringTable::hash_string(jchar* s, int len) {
   unsigned h = 0;
   for (len = s + len*sizeof(jchar); s < len; s++)
     h = 31*h + (unsigned) *s;
   return h;
}