OCJP Preparation - Garbage Collection

Given the program

When the doSomething method is called, after which line does the Object created in line 9 become available for garbage collection?

• A. Line 9
• B. Line 10
• C. Line 11
• D. Line 12
• E. Line 13
• F. Line 14

Answer: D

Given the code

Which statement should be placed at line 14 to suggest that the virtual machine expend effort toward recycling the memory used by the object rbo?

• A. System.gc();
• B. Runtime.gc();
• C. System.freeMemory();
• D. Runtime.getRuntime().growHeap();
• E. Runtime.getRuntime().freeMemory();

Answer: A

Which two are true? (Choose two.)

• A. A finalizer may NOT be invoked explicitly.
• B. The finalize method declared in class Object takes no action.
• C. super.finalize() is called implicitly by any overriding finalize method.
• D. The finalize method for a given object will be called no more than once by the garbage collector.
• E. The order in which finalize will be called on two objects is based on the order in which the two objects became finalizable.

Answer: BD

Given the program

When // doStuff is reached, how many objects are eligible for GC?

• A. 0
• B. 1
• C. 2
• D. Compilation fails.
• E. It is not possible to know.
• F. An exception is thrown at runtime.

Answer:
-> C is correct. Only one CardBoard object (c1) is eligible, but it has an associated Short wrapper object that is also eligible.
->A, B, D, E, and F are incorrect based on the above.

Which is true? (Choose all that apply.)

• A. The invocation of an object’s finalize() method is always the last thing that happens before an object is garbage collected (GCed).
• B. When a stack variable goes out of scope it is eligible for GC.
• C. Some reference variables live on the stack, and some live on the heap.
• D. Only objects that have no reference variables referring to them can be eligible for GC.
• E. It’s possible to request the GC via methods in either java.lang.Runtime or java.lang.System classes.

Answer:
-> C and E are correct. When an object has a reference variable, the reference variable lives inside the object, on the heap.
->A is incorrect, because if, the first time an object’s finalize() method runs, the object is saved from the GC, then the second time that object is about to be GCed, finalize() will not run. B is incorrect—stack variables are not dealt with by the GC. D is incorrect because objects can live in "islands of isolation" and be GC eligible.

Given the program

At what point is only a single object eligible for GC?

• A. After line 8 runs.
• B. After line 9 runs.
• C. After line 10 runs.
• D. After line 11 runs.
• E. Compilation fails.
• F. Never in this program.
• G. An exception is thrown at runtime.

Answer:
-> G is correct. An error at line 10 causes a NullPointerException to be thrown because e2 was set to null in line 8. If line 10 was moved between lines 7 and 8, then F would be correct, because until the last reference is nulled none of the objects is eligible, and once the last reference is nulled, all three are eligible.
-> A, B, C, D, E, and F are incorrect based on the above.