Let me try to give a complicated example for indra (this time I'll try not to make a mistake

)
Now lets try to build a memory map till line (1)
then at line (2), b1 is changed so the map becomes
after line (3), a2.b is set to null, so it becomes
at line (4), another reference variables starts pointing to a1's object
at 5, a1's (also a3's) b is set to null
And finally at (6), a3 is set to null
I hope this makes sense, the diagrams a little repetitive but that's how I find solution to such questions...