在什么行可以将此对象视为垃圾回收。 爪哇

Question

So my understanding is at line 1, a new object is created and it is referenced by obj. At line 2, obj is referencing to another new object. so the object which we created at line 1 is eligible for garbage collection because it is not referenced by any instance. But the answer from test question is at line 3. Reason? Is the answer correct?

Object obj = new Object();  //line 1
obj = new Object();  //line 2
obj = null;  //3

Edit: it is asking when the object created at line 1 can be eligible for garbage collection.

Answer 1

In the real world this question (as posed by the title) has no clear answer.

• aggressive dead code elimination could eliminate the first object allocation since it never becomes visible
• a modified Object Class (eg via bytecode instrumentation or bootstrap classloaders) could resurrect itself on finalization or hold onto additional references indefinitely
• garbage collection is asynchronous and happens at an indefinite point in the future, so while an object may be inaccessible from java code it cannot be "considered" garbage collected until it actually has been garbage collected.
  It might show up in a heap dump after all, which can be relevant if it contained security-sensitive data

I know it's supposed to be an academic question with a simple answer, but there are so many assumptions to make that I could rule-lawyer you any answer you want into existence.

Even if we don't pull any rabbits out of our hat the answer would still depend on whether "at line X" means before or after the execution of that

Answer 2

The garbage collector is checking for live objects and discarding anything that is not. When you create obj , you define it's type because that allows the compiler to know how much space to allot for that address, because something of Type Object will exist there. When the second line executes, new is creating an entirely new instance of the object and you are assigning its address to obj . So after reassigning obj to reference a new object, the old one still exists, but is no longer referenced. It would potentially be marked for collection as it occupies space in the heap, but is no longer active. The same happens with line 3, except, as mentioned previously, you are assigning it an address of null , and no new object is being created. So what used to be referenced by obj are now dead objects and the garbage man takes them away.