The Java HotSpot JVM 1.3 uses generational garbage collection. While naive garbage collection examines every living object in the heap, generational garbage collection considers the lifetime of an object to avoid extra work.
The heap is divided into two general areas: New and Old. The New generation area is sub-divided further into Eden and two survivor spaces. Eden is the area where new objects are allocated. When garbage collection occurs, live objects in Eden are copied into the next survivor space. Objects are copied between survivor spaces in this way until they exceed a maximum threshold, and then they are moved out of the New area and into the Old.
Many objects become available for garbage shortly after being allocated. These objects are said to have infant mortality. The longer an object survives, the more garbage collection it goes through, and the slower garbage collection becomes. The rate at which your application creates and releases objects determines how often garbage collection occurs. Attempt to cache objects for re-use, whenever possible, rather than creating new objects
Knowing that a majority of objects die young allows you to tune for efficient garbage collection. When you manage memory in generations, you create memory pools to hold objects of different ages. Garbage collection can occur in each generation when it fills up. If you can arrange for most of your objects to survive less than one collection, garbage collection is very efficient. Poorly sized generations cause frequent garbage collection, impacting your performance.