Java: Composite key in hashmaps

Question

I would like to store a group of objects in a hashmap , where the key shall be a composite of two string values. is there a way to achieve this?

i can simply concate

Answer 1

Why not create a (say) Pair object, which contains the two strings as members, and then use this as the key ?

e.g.

public class Pair {
   private final String str1;
   private final String str2;

   // this object should be immutable to reliably perform subsequent lookups
}

Don't forget about equals() and hashCode(). See this blog entry for more on HashMaps and keys, including a background on the immutability requirements. If your key isn't immutable, then you can change its components and a subsequent lookup will fail to locate it (this is why immutable objects such as String are good candidates for a key)

You're right that concatenation isn't ideal. For some circumstances it'll work, but it's often an unreliable and fragile solution (e.g. is AB/C a different key from A/BC ?).