Re: Unique Long for Map

Lasse Reichstein Nielsen <>
Tue, 11 Mar 2008 07:12:24 +0100
"" <> writes:

I have a common need to keep a small cache of values which are often
identified by two or more long values.

e.g. (1,2) ---- > "Abc"
        (1,9) -----> "Def"

For performance I like using a hashtable but to use it I do this:

public void addToCache(Long nbr1, Long nbr2, String value) {
   StringBuffer sb = new StringBuffer(32);
   hashtable.put (sb.toString(), value);

or something similar to that. The point is to make the key unique I
create a String.

That could be overkill. Also, comparing strings (of up to 41 chars) is
slower than comparing two longs.

I would imagine lookups would be more efficient if
the key was not a String but a number. But how to make a unique
number from these two. Perhaps if I knew both numbers would be less
than a billion then (nbr1 * 1,000,000,00 + nbr2) but that seems a
little clunky.

It is. It will break if the limit rises to 10 billion. Arbitrary limits
are ... well, arbitrary.

Is there a well defined method to create a single unique number based
upon two values?

Don't create a number, just create an object containing those longs:

public class TableKey {
 public final long l1; // or make it private and add a getLongValue1();
 public final long l2;
 public TableKey(long l1, long l2) {
   this.l1 = l1;
   this.l2 = l2;
 public boolean equals(Object o) {
   if (o == this) { return true; }
   if (!(o instanceof TableKey)) { return false; }
   TableKey other = (TableKey) o;
   return l1 == other.l1 && l2 == other.l2;
 public int hashCode() {
   return (int) (l1 ^ (l1 >> 32)) * 31 + (l2 ^ (l2 >> 32)); // or something

Not tested.

Good luck
Lasse Reichstein Nielsen -
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