Java Multithreading Interview Q&A
1. How do you create threads in Java? Explain the recommended approaches and their trade-offs.
Threads can be created in two main ways:
(a) Extending the
The preferred approach is using
Thread class and overriding its run() method.
(b) Implementing the Runnable or Callable interface and passing the object to a Thread instance.
The preferred approach is using
Runnable or Callable, as it allows inheritance from other classes and promotes better code organization.
2. What are the different states of a thread, and how do state transitions occur from creation to termination?
Thread states include New, Runnable, Blocked, Waiting, Timed Waiting, and Terminated.
Transitions occur as follows: A thread starts in the New state, becomes Runnable after
start(), may enter Blocked or Waiting when waiting for resources, and ends in Terminated once run() completes.
3. Why is synchronization necessary in a Java multithreaded environment?
Synchronization ensures controlled access to shared resources, preventing race conditions and inconsistent data. It guarantees that only one thread can execute a synchronized block or method at a time.
4. How do you assign thread priority in Java, and what effects (if any) does thread priority have on program behavior?
Thread priority can be set using
setPriority(int level) (values from 1 to 10).
While higher priority suggests preferred execution, actual scheduling depends on the JVM and underlying OS, so behavior may vary.
5. What is a deadlock in a multithreaded Java application, and what are common root causes?
A deadlock occurs when two or more threads are waiting indefinitely for each other’s locks.
Common causes include nested locks, inconsistent lock ordering, and poor synchronization design.
6. What are the consequences of a deadlock, and how can deadlocks be prevented or mitigated?
Deadlocks freeze program execution, leading to unresponsiveness.
Prevention techniques include acquiring locks in a consistent order, using try-lock mechanisms with timeouts, and minimizing shared resource usage.
7. What is a race condition, and under what circumstances do race conditions occur?
A race condition occurs when multiple threads access shared data simultaneously and the final outcome depends on the order of execution.
It typically occurs when synchronization is missing or improperly applied.
8. How can race conditions be prevented or controlled in Java?
Use synchronization constructs like
synchronized blocks, ReentrantLock, or atomic classes from java.util.concurrent to ensure thread-safe access to shared resources.
9. What is inter-thread communication?
Inter-thread communication allows threads to coordinate actions and share information efficiently, typically using methods like
wait(), notify(), and notifyAll() within synchronized contexts.
10. How can threads communicate with each other safely and gracefully? Explain the mechanisms and best practices in detail.
Threads communicate via shared objects using
wait() and notify() for coordination.
Modern best practices recommend higher-level concurrency utilities such as BlockingQueue, CountDownLatch, or Semaphore for safer, more maintainable designs.
11. What is the significance of Thread.sleep() and join() methods, write about Thread InterruptedException.
Thread.sleep() pauses the current thread for a specified duration, allowing other threads to execute.
Thread.join() waits for a thread to complete its execution before proceeding.
InterruptedException is thrown when a thread is interrupted while waiting or sleeping, allowing for responsive thread management. This is a checked exception and must be handled or declared.