A generational garbage collector divides the heap into multiple generations; most JVMs use two generations, a young and an old generation. Objects are allocated in the young generation; if they survive past a certain number of garbage collections, they are considered long lived and get promoted into the old generation.
While HotSpot offers a choice of three young-generation collectors (serial copying, parallel copying, and parallel scavenge), they are all forms of copying collectors and have several important characteristics in common. A copying collector divides the memory space in half and only uses one half at a time. Initially, the half that is in use forms one big block of available memory; the allocator satisfies allocation requests by returning the first N bytes of the portion that is not in use, moving a pointer that separates the used part from the free part, as shown by the pseudocode in Listing 1. When the half in use fills up, the garbage collector copies all the live objects (those that are not garbage) to the bottom of the other half (compacting the heap), and starts allocating from the other half instead.

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