What happens when you call thread.run() instead of thread.start()?
Only start() creates a new thread of execution; run() is an ordinary method call.
QuizzesModern Java
Concurrency & the JVM
Threads, executors, the memory model, locks, atomics, CompletableFuture, GC and class loading.
16 questions
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Question 1 of 16Score 0
What happens when you call
Eight threads each run counter++ 100,000 times on a shared volatile int. The final value is…
volatile gives visibility, not atomicity. ++ is read-modify-write, so updates are lost. Use AtomicInteger or a lock.
What does volatile guarantee?
A volatile write happens-before every later read of that variable. It does not make x++ or check-then-act atomic.
Why does double-checked locking need the instance field to be volatile?
Without volatile, the reference write can be reordered before the constructor writes complete.
A ThreadPoolExecutor has core 4, max 16 and an unbounded LinkedBlockingQueue. How many threads will it ever run?
Extra threads beyond the core size are only created when the queue is full — an unbounded queue is never full.
Which ReentrantLock feature does synchronized lack?
ReentrantLock adds tryLock, interruptible locking, fairness and multiple conditions; both are reentrant and give visibility.
Which is the classic way to prevent deadlock between two locks?
Consistent lock ordering breaks the circular-wait condition. tryLock with timeouts is another option.
You need N threads to wait for each other at the end of each phase, repeatedly. Which synchroniser fits?
CyclicBarrier is a reusable rendezvous. CountDownLatch is one-shot.
Your function returns a CompletableFuture<User>. Which method chains it without nesting futures?
thenCompose is flatMap; thenApply would produce CompletableFuture<CompletableFuture<User>>.
Where do supplyAsync(supplier) tasks run if you pass no executor?
The common pool is shared JVM-wide and sized for CPU work — pass a dedicated executor for blocking I/O.
Which statement about virtual threads (Java 21) is correct?
That unmounting is what makes millions of blocking virtual threads cheap. They improve I/O throughput, not CPU speed, and should not be pooled.
Where are objects created with new allocated?
Objects live on the heap; the stack holds local primitives and references (escape analysis may optimise some allocations away).
How does the GC decide an object is garbage?
Java GCs trace reachability from roots (stacks, statics, JNI), so unreachable cycles are collected too.
Which collector targets sub-millisecond pauses largely independent of heap size?
ZGC (generational since Java 21) relocates objects concurrently with the application.
What replaced PermGen in Java 8?
Class metadata moved to Metaspace, which grows dynamically (cap it with -XX:MaxMetaspaceSize).
What does String.class.getClassLoader() return?
Core classes are loaded by the bootstrap loader, which is represented as null in Java.
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