In Java, trash, or garbage, is heap memory that a program allocates for objects but no longer references; such memory serves no useful purpose.

In most cases, loitering objects arise from simple coding oversights or omissions, such as the failure to nullify a class-based or instance-based reference variable or neglecting to remove an object ...

Reference objects aren't just useful when memory is at a premium; they can also be used to provide slick caching abilities to any application.

Object references and memory The Java virtual machine (JVM) can take advantage of the way primitives are handled to optimize how it stores, retrieves, and operates on them.