A computer implemented method of storing a data object in a computer memory, wherein the data object is stored at a location in an object store as a portion of the memory, the location being determined by a hashing process based on a generated hash key, the memory further storing: a set of binary words in which each bit of each word identifies an occupancy state of a location in the object store such that an occupancy state of all locations is stored in the set of binary words; and a truncated hash key set storing, for each occupied location in the object store, a truncated hash key for a data object stored at the location, wherein the hashing process uses linear probing in which an occupancy of the location in the object store for storage of the data object is determined, wherein the occupancy is determined by reference to the set of binary words, and the linear probing further determines identity of the data object and an object stored in an occupied location, the identity being determined based on a comparison of the hash key for the data object and a truncated hash key for the occupied location.