SCJP Java Threads and Concurrency

Question 1

Which of the following is a correct way to create a new thread in Java?

Accepted Answer

Extend Thread class or implement Runnable interface

Answer

In Java, threads can be created by extending `Thread` or implementing `Runnable` and passing it to a `Thread` constructor.

Question 2

What does the `synchronized` keyword ensure in Java?

Accepted Answer

That only one thread executes the synchronized block/method at a time

Answer

`synchronized` acquires a monitor lock on an object, ensuring mutual exclusion so only one thread can execute the guarded code at a time.

Question 3

What is a deadlock in Java threading?

Accepted Answer

Two or more threads permanently blocked waiting for each other's locks

Answer

Deadlock occurs when two or more threads each hold a lock the other needs, causing them to wait indefinitely.

Question 4

What is the difference between `Thread.sleep()` and `Object.wait()` in Java?

Accepted Answer

wait() releases the object's lock; sleep() does not release any lock

Answer

`wait()` releases the object's monitor lock and suspends the thread until notified, while `sleep()` pauses the thread without releasing any locks.

Question 5

Which thread state indicates that a thread is eligible to run but waiting for CPU time?

Accepted Answer

RUNNABLE

Answer

A thread in the `RUNNABLE` state is ready to execute and waiting for the CPU scheduler to assign it processor time.

SCJP - Sun Certified Java Programmer Practice Test