A rarely used field is a field that is zero or null in >= 90% of the instances. These are not dead fields since <= 10% of the instances have a value in the field. Rarely Used Fields use 6.9% of the space in an average heap dump.

With JEP 159 Enhanced Class Redefinition, the JVM can use Shadow Structures to remove fields from the class and place them in the shadow class. This will save 4.0% of the heap.