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Garbage collector work on Mark and sweep algorithem.It mark all the elements that have unreachable reference and garbage collect them in future. the objects that have un reachable reference goes to tenured generation from young generation survivor space

Garbage Collector uses mark and sweep algorithm, In the first phase(mark) only reachable objects are marked by GC as reachable and remaining objects are treated as unreachable, these will be reclaimed during second phase(sweep) of GC.

GCs are Sequential, Parellel and Concurrent

Read javarevisited.blogspot.com/2011/04/garbage-collection-in-java.html

GC marks only reachable objects and move/copy these objects to other area of heap based on the GC type - compacting or copying garbage collectors.

Refer:
en.wikipedia.org/wiki/File:Animation_of_the_Naive_Mark_and_Sweep_Garbage_Collector_Algorithm.gif
lambda.uta.edu/cse5317/notes/node47.html