What is aliasing in java?

http://www.ibm.com/developerworks/library/j-jtp06243/

Aliases, also known as ...

A primary contributor to the complexity of memory management is aliasing: having more than one copy of a pointer or reference to the same block of memory or object. Aliases occur naturally all the time. For example, in Listing 1 there are at least four references to the Something object created on the first line of makeSomething:

• The something reference
• At least one reference held within collection c1
• The temporary aSomething reference created when something is passed as an argument to registerSomething
• At least one reference held within collection c2

Listing 1. Aliases in typical code

    Collection c1, c2;
    
    public void makeSomething {
        Something something = new Something();
        c1.add(something);
        registerSomething(something);
    }

    private void registerSomething(Something aSomething) {
        c2.add(aSomething);
    }

There are two major memory-management hazards to avoid in non-garbage-collected languages: memory leaks and dangling pointers. To prevent memory leaks, you must ensure that each allocated object is eventually deleted. To avoid dangling pointers (the dangerous situation where a block of memory is freed but a pointer still references it), you must delete the object only after the last reference is released. The mental and digital bookkeeping required to satisfy these two constraints can be significant.

From which point GC in java starts collecting garbage?

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html

Automatic garbage collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects. An in use object, or a referenced object, means that some part of your program still maintains a pointer to that object. An unused object, or unreferenced object, is no longer referenced by any part of your program. So the memory used by an unreferenced object can be reclaimed.
In a programming language like C, allocating and deallocating memory is a manual process. In Java, process of deallocating memory is handled automatically by the garbage collector. The basic process can be described as follows.