Q: What is double-checked locking for Singleton and why is 'volatile' required?

volatile prevents partial object construction by disabling instruction reordering — without it, another thread may see a non-null but incompletely initialized instance. instance = new Singleton() is not atomic: (1) allocate memory, (2) call constructor, (3) assign pointer. Without volatile, step 3 can happen before step 2. Thread B sees non-null instance but accesses uninitialized fields. volatile prevents this reordering.

Question 1

What is the Java Memory Model (JMM) and why does it matter for concurrent code?

Accepted Answer

A specification defining how threads interact through memory — establishing rules for visibility and ordering of shared variable reads/writes. Without JMM guarantees, a write by Thread A may not be visible to Thread B. The JMM defines happens-before relationships. volatile, synchronized, and java.util.concurrent primitives all establish happens-before to ensure correct visibility.

Question 2

What is CompletableFuture and how does it improve on Future?

Accepted Answer

CompletableFuture supports chaining async operations, callbacks (thenApply, thenCompose), combining futures, and non-blocking completion; Future only supports blocking get(). Future.get() blocks the calling thread. CompletableFuture lets you chain: fetchUser().thenApply(user -> enrich(user)).thenAccept(this::display) — all non-blocking. You can also combine futures with allOf() and anyOf().

Question 3

What are virtual threads (Project Loom, Java 21+) and what problem do they solve?

Accepted Answer

Lightweight JVM-managed threads that do not block OS threads during I/O — enabling millions of concurrent tasks without thread pool tuning. Platform threads map 1:1 to OS threads — expensive. With virtual threads, a blocked I/O call parks the virtual thread and frees the OS thread for other work. You can create millions of virtual threads as cheaply as objects.

Question 4

What is the difference between G1GC and ZGC, and when would you choose ZGC?

Accepted Answer

ZGC is a low-latency GC with sub-millisecond pause times suitable for large heaps; G1 balances throughput and latency with configurable pause targets. G1GC: good general-purpose GC, pause targets via -XX:MaxGCPauseMillis. ZGC (Java 15+): concurrent GC with <1ms pauses regardless of heap size — ideal for latency-sensitive systems (trading, gaming, real-time APIs). ZGC trades some throughput for consistent low latency.

Question 5

What is double-checked locking for Singleton and why is 'volatile' required?

Accepted Answer

volatile prevents partial object construction by disabling instruction reordering — without it, another thread may see a non-null but incompletely initialized instance. instance = new Singleton() is not atomic: (1) allocate memory, (2) call constructor, (3) assign pointer. Without volatile, step 3 can happen before step 2. Thread B sees non-null instance but accesses uninitialized fields. volatile prevents this reordering.

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