Garbage Collections SCJP / OCJP

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Question: 01 Given:
1. public class GC {
2. private Object o;
3. private void doSomethingElse(Object obj) { o = obj; }
4. public void doSomething() {
5. Object o = new Object();
6. doSomethingElse(o);
7. o = new Object();
8. doSomethingElse(null);
9. o = null;
10. }
11. }
When the doSomething method is called, after which line does the Object created in line 5 become
available for garbage collection?
A. Line 5
B. Line 6
C. Line 7
D. Line 8
E. Line 9
F. Line 10
Answer: D

Question: 02 Given:
11. public void genNumbers() {
12. ArrayList numbers = new ArrayList();
13. for (int i=0; i<10; i++) {
14. int value = i * ((int) Math.random());
15. Integer intObj = new Integer(value);
16. numbers.add(intObj);
17. }
18. System.out.println(numbers);
19. }
Which line of code marks the earliest point that an object referenced by intObj becomes a candidate for
garbage collection?
A. Line 16
B. Line 17
C. Line 18
D. Line 19
E. The object is NOT a candidate for garbage collection.
Answer: D

Question: 03 Given:
11. rbo = new ReallyBigObject();
12. // more code here
13. rbo = null;
14. /* insert code here */
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

Question: 05
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

06. Given:
class CardBoard {
Short story = 5;
CardBoard go(CardBoard cb) {
cb = null;
return cb;
public static void main(String[] args) {
CardBoard c1 = new CardBoard();
CardBoard c2 = new CardBoard();
CardBoard c3 = c1.go(c2);
c1 = null;
// do Stuff
} }
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.
-> 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 are incorrect based on the above.

07. 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.
-> and are correct. When an object has a reference variable, the reference variable lives inside the object, on the heap.
->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. is incorrect—stack variables are not dealt with by the GC. is incorrect because objects can live in "islands of isolation" and be GC eligible.

Question 08. Given:
1. class Eco {
2. public static void main(String[] args) {
3. Eco e1 = new Eco();
4. Eco e2 = new Eco();
5. Eco e3 = new Eco();
6. e3.e = e2;
7. e1.e = e3;
8. e2 = null;
9. e3 = null;
10. e2.e = e1;
11. e1 = null;
12. }
13. Eco e;
14. }
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.
-> 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 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 are incorrect based on the above.

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