What is a garbage collector and how does it work?

Garbage Collector (GC) is an automatic memory management mechanism that allocates and frees memory no longer used by the program.

Working principles:

1. Memory allocation: When the program creates objects, the garbage collector allocates space for them in the heap.
2. Reachability determination: The GC determines which objects are accessible from the 'roots' of the program. Roots can be local variables, static variables, references from threads, etc.
3. Garbage collection: Objects that are unreachable from the roots are considered 'garbage' and their memory can be freed.

Various algorithms include:

• Mark-and-Sweep: The collector marks reachable objects and then sweeps the heap to free unmarked objects.
• Copying: The heap is divided into two parts; reachable objects are copied from one to the other, and the old part is cleared.
• Generational: Objects are divided into generations; new objects go to the young generation, old to the mature. Collection occurs more frequently in the young generation.

Example in Java (simplified):

// Program creates objects in heap
Object obj1 = new Object();
Object obj2 = new Object();

// obj1 becomes unreachable
obj1 = null;

// GC may free memory occupied by obj1