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Java Multithreading Interview Questions and Answers

Preemptive scheduling vs. time slicing.

In preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence.

In time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks.
The scheduler then determines which task should execute next, based on priority and other factors.

What is the difference between Process and Thread?

A process is a self contained execution environment and it can be seen as a program or application whereas Thread is a single task of execution within the process. Java runtime environment runs as a single process which contains different classes and programs as processes. Thread can be called lightweight process. Thread requires less resources to create and exists in the process, thread shares the process resources.

What is volatile?
  • When more than one threads access the same variable then each thread will maintain its own copy of that variable in the form of local cache.
  • When a thread is changing the value of variable, it is actually changing the local cache not the main variable memory.
  • Every thread which is using the same variable doesn’t know anything about the values changed by another thread.
  • To overcome this problem, if you declare a variable as volatile, then whenever a thread is changing the value of a volatile, it is actually changing to the main memory. So, all other threads which are accessing the same variable can get the changed value.
Which is more preferred – Synchronized method or Synchronized block?

Synchronized block is more preferred way because it doesn’t lock the Object, synchronized methods lock the Object and if there are multiple synchronization blocks in the class, even though they are not related, it will stop them from execution and put them in wait state to get the lock on Object.

Briefly explain daemon thread.

Daemon thread is a low priority thread which runs in the background performs garbage collection operation for the java runtime system.

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