Is it possible to force immediate garbage collection in Java?

While System.gc() is a legal method that requests garbage collection, it is non-deterministic—the JVM is not required to honor the request or execute it immediately. The Java Language Specification explicitly states that System.gc() is only a hint to the garbage collector; the actual timing and execution are determined by the JVM implementation. Therefore, you cannot force immediate garbage collection in Java.

Step-by-step Derivation:
Analysis of each option:

• Option A (Correct): System.gc() exists and is legal, but the JVM may ignore it or defer collection. Garbage collection timing is non-deterministic and implementation-dependent.
• Option B (Incorrect): System.gc() is absolutely legal to call; there is no restriction on invoking it.
• Option C (Incorrect): While System.gc() exists, it does not force immediate collection. It is merely a request that the JVM can choose to ignore.
• Option D (Incorrect): Deleting/nullifying references does not trigger garbage collection; it only makes objects eligible for collection when the GC runs.

Key Concept: Java garbage collection is non-deterministic by design. The programmer cannot force immediate collection—only request it via System.gc(), which the JVM may or may not honor immediately.