SCJP Java Garbage Collection and Memory Management

Question 1

After the following code runs, how many objects are eligible for garbage collection?

String a = new String("hello");
String b = new String("world");
a = b;
b = null;

Accepted Answer

1

Answer

After `a = b`, variable `a` now points to the "world" object and the original "hello" object has no references — it is eligible for GC. After `b = null`, `b` no longer points to "world", but `a` still does, so "world" is NOT eligible. Only 1 object ("hello") is eligible.

Question 2

Which statement about the `finalize()` method is TRUE?

Accepted Answer

It is called by the garbage collector at most once per object

Answer

The JVM guarantees that finalize() is called at most once per object, even if the object becomes reachable again inside finalize() (resurrection). Its execution is NOT guaranteed at all — the GC may never call it, and if it does, the timing is undefined.

Question 3

What is the effect of calling `System.gc()`?

Accepted Answer

It is a request to the JVM to run garbage collection; the JVM may ignore it

Answer

`System.gc()` is only a hint to the JVM that now might be a good time to run garbage collection. The JVM is free to ignore this request. It does not guarantee immediate collection or finalization.

Question 4

Which of the following creates an 'island of isolation' (objects eligible for GC even though they reference each other)?

Accepted Answer

Two objects that reference each other but have no external references pointing to them

Answer

Java's GC uses reachability from GC roots, not reference counting. Two objects referencing each other but unreachable from any GC root form an 'island of isolation' and are both eligible for collection, despite referencing each other.

Question 5

Where are local primitive variables stored in a Java program?

Accepted Answer

The stack

Answer

Local variables (both primitive and object references) are stored on the thread's call stack. The objects that reference variables point to are stored on the heap. The method area/constant pool stores class-level data and string literals.

SCJP - Sun Certified Java Programmer Practice Test

SCJP - Sun Certified Java Programmer Practice Test

SCJP Java Garbage Collection and Memory Management