A technique for defragmenting data stored in memory in a computer system includes the step of creating a table in memory including the locations of all pointers associated with stored data. The pointers are stored in a separate block of memory from the table. Data is defragmented only at idle time. In response to the defragmentation, only the values of the pointers are updated, while the locations of the pointers and the corresponding table entries remain fixed. Pointers can be updated using pointer update methods that are specific to particular classes of objects or using a generic method for all classes in conjunction with a list of pointer offsets. Pointer fields may be locating using a bit mask having values which indicate whether a particular byte or group of bytes in memory associated with an object include a pointer field.