Essential Java Concurrency Interview Topics – 12 Core Questions and Answers

In Java backend interviews, mastering concurrency basics is crucial; this guide lists twelve frequently asked questions and provides concise answers.

1. Ways to create a thread: extend Thread, implement Runnable, implement Callable (thread pool is optional).

2. Advantages and disadvantages of each method: extending Thread is simple but blocks single inheritance; Runnable avoids that but cannot return a result; Callable returns a result but is more complex.

3. Thread states: NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED (see java.lang.Thread.State).

4. Thread lifecycle: NEW → start() → RUNNABLE → BLOCKED/WAITING/TIMED_WAITING as needed → TERMINATED.

5. Starting a thread: invoke the start() method.

6. Difference between start() and run(): start() creates a new thread; run() executes like a normal method in the current thread.

7. Ways to terminate a running thread: use stop() (discouraged), interrupt the thread, or let the thread exit naturally via a flag.

8. Definition of thread safety: code produces the same result whether executed by a single thread or multiple threads concurrently.

9. Difference between thread and process: a process is an independent resource‑allocation unit; a thread is the smallest execution unit within a process, sharing the same address space.

10. Thread communication methods: shared memory (read/write shared variables) and message passing (e.g., wait() / notify()).

11. Purpose of yield(): hints to the scheduler that the current thread is willing to pause, allowing other threads of equal or higher priority to run.

12. Differences between yield() and sleep(): yield() does not change thread state and respects priority; sleep() puts the thread into TIMED_WAITING, ignores priority, throws InterruptedException, and is more portable.

Answering these topics demonstrates a solid foundation in Java multithreading, which interviewers often probe further.

Summary : If you can confidently address all twelve points, your concurrency basics are considered competent.