Threads, Runnable, synchronized, volatile, ExecutorService, CompletableFuture, locks, atomics, concurrent collections and virtual threads.
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- Why is implementing Runnable preferred to extending Thread?
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Java doesn't support multiple inheritance, so extending Thread locks you into that hierarchy. Runnable is an interface, allowing classes to extend another class too.- What does Thread.start() do?
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Starts a new thread and calls run() inside that thread. Does not execute run() in the current thread.- What happens if you call Thread.run() directly?
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The run() method executes in the current thread, not a new thread. No concurrent execution occurs.- How does the synchronized keyword protect code?
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It uses the object's monitor (intrinsic lock) to ensure only one thread can execute the synchronized block or method at a time.- Can a synchronized method call another synchronized method on the same object without deadlock?
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Yes, because ReentrantLock behavior is built into Java monitors. The same thread can reacquire the lock.- What does the volatile keyword guarantee?
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It ensures that changes to a volatile variable are immediately visible to all threads. It does not provide atomicity.- How does volatile differ from synchronized in terms of performance?
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Volatile is lighter-weight than synchronized. It only ensures visibility, not mutual exclusion, so multiple threads can access a volatile variable simultaneously.- What does ExecutorService abstract?
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It abstracts thread creation and management, allowing you to submit tasks without manually creating and managing Thread objects.- What does ExecutorService.newFixedThreadPool(10) create?
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A thread pool with 10 threads that reuse themselves for submitted tasks. The pool size is fixed.- How do execute() and submit() differ in ExecutorService?
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execute(Runnable) returns void and throws checked exceptions. submit() returns a Future and wraps checked exceptions in unchecked exceptions.- What is the difference between shutdown() and shutdownNow() on ExecutorService?
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shutdown() stops accepting new tasks but waits for existing tasks to complete. shutdownNow() stops accepting new tasks and attempts to interrupt running tasks.- What does CompletableFuture.thenApply() do?
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It takes a Function that transforms the result of the previous stage and returns a new CompletableFuture with the transformed result.- What does CompletableFuture.thenAccept() take as a parameter?
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A Consumer that accepts the result but returns void. It does not transform the value.- How does CompletableFuture.join() differ from get()?
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join() returns the result or throws an unchecked CompletionException. get() throws checked ExecutionException and InterruptedException.- What type of lambda does CompletableFuture.supplyAsync() accept?
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A Supplier with no parameters that returns a value. It is used to asynchronously compute and provide a result.
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