Re: Tune String's hashCode() + equals()

[Ulf Zibis]

Am 04.03.2010 23:38, schrieb Marek Kozieł:

2010/3/4 Ulf Zibis<ulf.zi...@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;
}