Java JVM interview: reachability, publication and diagnostic evidence

Java 8+. This is a complete program using JDK classes.

Can a garbage-collected application leak memory?

Yes. A registry, queue, cache or listener list can retain objects that are no longer useful. They remain reachable and therefore are not garbage merely because business logic stopped needing them. Follow a retained object’s path to a root before choosing a cleanup change.

Are references and their objects the same thing?

A local reference can point to a shared heap object. Reassigning that reference does not replace another caller’s reference, and mutating the object can still be visible through other aliases. The program copies a byte payload at its ownership boundary so later source mutation cannot edit the retained bytes.

Does volatile repair a compound update?

No. Volatile visibility and ordering do not make read-modify-write increments indivisible. Use the operation’s actual synchronization contract, such as an atomic update or a lock that protects the whole invariant.

What evidence is needed for a performance claim?

Distinguish warmup, steady-state work, allocation rate, lock waiting and storage latency. A short run can include class loading and JIT work. A retained heap investigation and a CPU profile also answer different questions. Pick a repeatable workload and report the toolchain and measurement limits.

Working program

import java.util.Arrays;
public class PayloadOwnershipProbe {
    static final class StoredPayload {
        private final byte[] bytes;
        StoredPayload(byte[] input){bytes=input.clone();}
        byte[] snapshot(){return bytes.clone();}
    }
    public static void main(String[] args){
        byte[] incoming={1,2};StoredPayload stored=new StoredPayload(incoming);
        incoming[0]=9;
        byte[] first=stored.snapshot();first[1]=8;
        System.out.println(Arrays.toString(stored.snapshot()));
        System.out.println(Arrays.toString(incoming));
    }
}

Output

[1, 2]
[9, 2]

Costs and boundaries

Each payload copy uses O(n) byte work and O(n) storage. That ownership choice avoids alias mutation but can be costly for large payloads; immutable slices or bounded buffers need a different contract. Exact object sizes and collector behavior depend on the JVM configuration.

Common Mistakes

• Do not equate useful objects with reachable objects.
• A volatile counter increment can still lose updates.
• Avoid fixed object-size claims without identifying the runtime layout.

Read next

Runtime memory, Diagnostic recordings.