Java Multithreading Essentials Quiz Quiz

Challenge your understanding of Java multithreading concepts, synchronization mechanisms, thread life cycles, and concurrent programming techniques crucial for backend development. This quiz covers core terms, methods, and safe practices essential for robust multithreaded Java applications.

1. Understanding Thread Creation

   Which of the following methods is used to create a new thread in Java by implementing an interface?

   1. Implementing the Runnable interface
   2. Using the Serializable interface
   3. Extending the Scanner class
   4. Extending the Math class

   Explanation: The correct way to create a thread is by implementing the Runnable interface and passing it to a Thread object. Serializable and Scanner are unrelated to threads; Serializable deals with object serialization, and Scanner is for input. Math is a utility class and cannot be extended.

2. Thread Start Method

   Which method should you call to begin execution of a thread in Java after creating it?

   1. start()
   2. run()
   3. init()
   4. launch()

   Explanation: Calling start() initiates a new thread and calls its run() method in a separate call stack. Calling run() directly executes the code in the current thread, not a new one. There are no init() or launch() methods for threads in Java.

3. Thread States

   In which state does a thread wait for another thread to finish executing using the join() method?

   1. Waiting
   2. New
   3. Terminated
   4. Blocked

   Explanation: A thread entering join() transitions to the Waiting state until the other thread completes. The New state is before start() is called, Terminated is when a thread is finished, and Blocked refers to waiting for a monitor lock.

4. Synchronization Basics

   Which keyword is used to prevent multiple threads from executing a block of code at the same time in Java?

   1. synchronized
   2. static
   3. transient
   4. volatile

   Explanation: The synchronized keyword ensures that only one thread can access a code block at a time. Static changes method or variable context, transient is for serialization, and volatile deals with memory visibility but not atomicity.

5. Race Condition Scenario

   What can happen if two threads update a shared variable without proper synchronization?

   1. A race condition
   2. Deadlock
   3. Thread starvation
   4. Infinite loop

   Explanation: Unsynchronized access to shared variables can cause race conditions, where outcomes depend on timing. Deadlock involves two threads waiting on each other. Thread starvation is about one thread never progressing, while an infinite loop is unrelated here.

6. Thread Sleep Usage

   What does the Thread.sleep(1000) statement achieve in Java?

   1. Pauses the current thread for 1 second
   2. Interrupts all running threads
   3. Starts a new thread
   4. Immediately terminates the JVM

   Explanation: Thread.sleep(1000) makes the executing thread sleep for 1 second (1000 milliseconds). It does not interrupt threads, does not start a new one, and cannot terminate the JVM. The pause only affects the current thread.

7. Volatile Keyword Purpose

   Why would you declare a shared variable as volatile in Java multithreading?

   1. To ensure changes are visible to all threads
   2. To make a variable immutable
   3. To synchronize code execution
   4. To set default value to zero

   Explanation: Volatile guarantees that updates to a variable are visible to all threads instantly. It does not make a variable immutable or synchronize execution, nor does it set a default value. It helps with visibility but not atomicity.

8. Thread Interruption

   What exception may be thrown when a thread is interrupted while sleeping in Java?

   1. InterruptedException
   2. IOException
   3. NullPointerException
   4. ClassNotFoundException

   Explanation: InterruptedException is thrown when a sleeping or waiting thread is interrupted. IOException is for input/output errors, NullPointerException is for null references, and ClassNotFoundException relates to class loading issues.

9. Daemon Threads

   Which statement accurately describes a daemon thread in Java?

   1. It runs in the background and does not prevent application exit
   2. It ends forcibly when started
   3. It has higher priority than all user threads
   4. It requires explicit join() calls to terminate

   Explanation: Daemon threads support other threads and do not keep the JVM running after user threads finish. They do not end forcibly on start, do not necessarily have higher priority, and join() is not required for their termination.

10. Thread Priority

    How can you set a Java thread's execution priority using code?

    1. By calling thread.setPriority(8)
    2. By writing thread.priority = 8
    3. By passing priority to the thread constructor
    4. By putting thread in a synchronized block

    Explanation: The setPriority method assigns a priority to a thread. Assigning directly with a property or via the constructor is not possible. Synchronized does not affect thread priority.

11. Thread Safety Definition

    What does it mean for a class to be thread-safe in Java?

    1. Multiple threads can use instances safely without inconsistent results
    2. Only one instance can ever exist
    3. It can be executed only in the main thread
    4. The class consumes less memory

    Explanation: Thread-safe classes prevent data races and ensure correct results even with concurrent access. Only allowing one instance relates to the singleton pattern. Execution in main thread or memory consumption is unrelated to thread safety.

12. Runnable vs. Callable

    What is a key difference between the Runnable and Callable interfaces in Java?

    1. Callable can return a value and throw checked exceptions
    2. Runnable can return a value, but Callable cannot
    3. Callable can be run only once
    4. Runnable must declare checked exceptions

    Explanation: Callable allows returning a value and throwing checked exceptions, unlike Runnable, which cannot do either. Callable can be run multiple times and Runnable is not required to throw exceptions.

13. Atomic Operations

    Which Java class can be used to perform atomic operations on integer values?

    1. AtomicInteger
    2. SynchronizedString
    3. ThreadLocal
    4. HashMap

    Explanation: AtomicInteger provides methods for atomic integer operations suitable for concurrency. SynchronizedString is not a standard class, ThreadLocal manages thread-specific variables, and HashMap is not thread-safe by default.

14. Thread Pool Importance

    Why might you use a thread pool for handling multiple concurrent tasks in Java?

    1. To reuse threads efficiently and reduce resource overhead
    2. To ensure tasks execute serially
    3. To limit application memory to 512MB
    4. To automatically persist data

    Explanation: Thread pools manage threads efficiently, reducing creation overhead for each task. They do not guarantee serial execution, memory limitation, or data persistence automatically.

15. Synchronized Method Effect

    What is the effect of declaring a method as synchronized in Java?

    1. Only one thread can execute the method at a time on the same object
    2. Threads execute the method twice
    3. Method runs faster
    4. The method consumes no memory

    Explanation: Synchronized methods allow only one thread at a time per object instance. It has no effect on speed, execution count, or memory consumption directly.

16. ThreadGroup Usage

    What is a ThreadGroup used for in Java multithreading?

    1. Organizing and managing multiple threads as a single unit
    2. Making each thread inherit methods from another
    3. Automatically setting thread priorities
    4. Running threads on different JVMs

    Explanation: ThreadGroup helps structure and control collections of threads collectively. It does not control inheritance, does not directly set priorities, and is not related to running on different JVMs.

17. wait() vs. sleep()

    Which is true about the wait() method compared to sleep() in Java threading?

    1. wait() releases the object's monitor while sleep() does not
    2. wait() makes a thread run faster
    3. sleep() can only be used inside synchronized blocks
    4. wait() throws IOException

    Explanation: wait() pauses a thread and releases the object's lock, allowing others access, while sleep() keeps the lock. wait() does not affect speed, sleep() can be called anywhere, and neither throws IOException.

18. Thread Termination

    Which method signals a thread to stop, letting it finish its current task in Java?

    1. interrupt()
    2. suspend()
    3. pause()
    4. destroy()

    Explanation: interrupt() sets the thread's interrupted status, which can be checked for graceful termination. suspend() and destroy() are deprecated or nonexistent, and pause() is not a thread method.

19. Main Thread Name

    What is the default name of the main thread created by the JVM in Java?

    1. main
    2. MainThread
    3. primary
    4. core

    Explanation: The default name of the entry point thread is 'main'. The other options may seem plausible but are not used by the JVM.

20. Yield Method

    What is the effect of calling Thread.yield() in Java?

    1. Hints the scheduler to pause the current thread and let others run
    2. Automatically puts thread into waiting state
    3. Increases thread priority
    4. Causes immediate thread termination

    Explanation: Thread.yield() suggests the thread scheduler to move the current thread to runnable and let others execute. It does not force waiting, raise priority, or terminate the thread.

21. ThreadLocal Utility

    What is the primary purpose of the ThreadLocal class in Java?

    1. To provide each thread with its own copy of a variable
    2. To synchronize access to shared variables
    3. To create more threads automatically
    4. To pool multiple tasks into a single thread

    Explanation: ThreadLocal ensures each thread gets a separate instance of a variable. It does not synchronize access, create new threads, or pool tasks together.