81.What is use of synchronized keyword?

synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.
public void synchronized method(){}
public void synchronized staticmethod(){}
public void myMethod(){
synchronized (this){
//synchronized keyword on block of code
}
}

82.What is the difference when the synchronized keyword is applied to a static method or to a non static method?

When a synch non static method is called a lock is obtained on the object. When a synch static method is called a lock is obtained on the class and not on the object. The lock on the object and the lock on the class don’t interfere with each other. It means, a thread accessing a synch non static method, then the other thread can access the synch static method at the same time but can’t access the synch non static method.

83.What is a volatile keyword?

In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased so the count variable is declared as volatile.
The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.

84.What is the difference between yield() and sleep()?

yield() method pauses the currently executing thread temporarily for giving a chance to the remaining waiting threads of the same priority to execute. If there is no waiting thread or all the waiting threads have a lower priority then the same thread will continue its execution. The yielded thread when it will get the chance for execution is decided by the thread scheduler whose behavior is vendor dependent. If doesn't release the lock on the objects acquired.
sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t releases the lock.

85.What is the difference between wait() and sleep()?

• wait() is a method of Object class. sleep() is a method of Object class.
• sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t release the lock. wait() allows thread to release the lock and goes to suspended state. The thread is only active when a notify() or notifAll() method is called for the same object.

86.What is difference between notify() and notfiyAll()?

notify() wakes up the first thread that called wait() on the same object. notifyAll() wakes up all the threads that called wait() on the same object. The highest priority thread will run first.

87.What happens if a start method is not invoked and the run method is directly invoked?

If a thread has been instantiated but not started its is said to be in new state. Unless until a start() method is invoked on the instance of the thread, it will not said to be alive. If you do not call a start() method on the newly created thread instance thread is not considered to be alive. If the start() method is not invoked and the run() method is directly called on the Thread instance, the code inside the run() method will not run in a separate new thread but it will start running in the existing thread.

98.) What happens when start() is called?

A new thread of execution with a new call stack starts. The state of thread changes from new to runnable. When the thread gets chance to execute its target run() method starts to run.

89.If code running is a thread creates a new thread what will be the initial priority of the newly created thread?

When a code running in a thread creates a new thread object, the priority of the new thread is set equal to the priority of the thread which has created it.

90.When jvm starts up, which thread will be started up first?

When jvm starts up the thread executing main method is started

91.What are the daemon threads?

Daemon thread are service provider threads run in the background,these not used to run the application code generally.When all user threads(non-daemon threads) complete their execution the jvm exit the application whatever may be the state of the daemon threads. Jvm does not wait for the daemon threads to complete their execution if all user threads have completed their execution.
To create Daemon thread set the daemon value of Thread using setDaemon(boolean value) method. By default all the threads created by user are user thread. To check whether a thread is a Daemon thread or a user thread use isDaemon() method.
Example of the Daemon thread is the Garbage Collector run by jvm to reclaim the unused memory by the application. The Garbage collector code runs in a Daemon thread which terminates as all the user threads are done with their execution.

92.What is Thread in Java?(

The thread is an independent path of execution. It's way to take advantage of multiple CPU available in a machine. By employing multiple threads you can speed up CPU bound task. For example, if one thread takes 100 milliseconds to do a job, you can use 10 thread to reduce that task into 10 milliseconds. Java provides excellent support for multithreading at the language level, and it's also one of the strong selling points.

93.What is the difference between Thread and Process in Java?

The thread is a subset of Process, in other words, one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space. Don't confuse this with stack memory, which is different for the different thread and used to store local data to that thread. For more detail see the answer.

94.How do you implement Thread in Java?

At the language level, there are two ways to implement Thread in Java. An instance of java.lang.Thread represent a thread but it needs a task to execute, which is an instance of interface java.lang.Runnable. Since Thread class itself implement Runnable, you can override run() method either by extending Thread class or just implementing Runnable interface. For detailed answer and discussion see this article

95.When to use Runnable vs Thread in Java?

This is a follow-up of previous multi-threading interview question. As we know we can implement thread either by extending Thread class or implementing Runnable interface, the question arise, which one is better and when to use one? This question will be easy to answer if you know that Java programming language doesn't support multiple inheritances of class, but it allows you to implement multiple interfaces. Which means, it's better to implement Runnable then extends Thread if you also want to extend another class e.g. Canvas or CommandListener. For more points and discussion you can also refer this post

96.What is the difference between start() and run() method of Thread class?

One of trick Java question from early days, but still good enough to differentiate between shallow understanding of Java threading model start() method is used to start newly created thread, while start() internally calls run() method, there is difference calling run() method directly. When you invoke run() as normal method, its called in the same thread, no new thread is started, which is the case when you call start() method. Read this answer for much more detailed discussion

97.What is the difference between Runnable and Callable in Java?

Both Runnable and Callable represent task which is intended to be executed in a separate thread. Runnable is there from JDK 1.0 while Callable was added on JDK 1.5. Main difference between these two is that Callable's call() method can return value and throw Exception, which was not possible with Runnable's run() method. Callable return Future object, which can hold the result of computation. See my blog post on the same topic for a more in-depth answer to this question.

98.What is the difference between CyclicBarrier and CountDownLatch in Java

Though both CyclicBarrier and CountDownLatch wait for number of threads on one or more events, the main difference between them is that you can not re-use CountDownLatch once count reaches to zero, but you can reuse same CyclicBarrier even after barrier is broken. See this answer for few more points and sample code example.

99.What is Java Memory model?

Java Memory model is set of rules and guidelines which allows Java programs to behave deterministically across multiple memory architecture, CPU, and operating system. It's particularly important in case of multi-threading. Java Memory Model provides some guarantee on which changes made by one thread should be visible to others, one of them is happens-before relationship. This relationship defines several rules which allows programmers to anticipate and reason behaviour of concurrent Java programs. For example, happens-before relationship guarantees :

• Each action in a thread happens-before every action in that thread that comes later in the program order, this is known as program order rule.
• An unlock on a monitor lock happens-before every subsequent lock on that same monitor lock, also known as Monitor lock rule.
• A write to a volatile field happens-before every subsequent read of that same field, known as Volatile variable rule.
• A call to Thread.start on a thread happens-before any other thread detects that thread has terminated, either by successfully return from Thread.join() or by Thread.isAlive() returning false, also known as Thread start rule.
• A thread calling interrupt on another thread happens-before the interrupted thread detects the interrupt( either by having InterruptedException thrown, or invoking isInterrupted or interrupted), popularly known as Thread Interruption rule.
• The end of a constructor for an object happens-before the start of the finalizer for that object, known as Finalizer rule.
• If A happens-before B, and B happens-before C, then A happens-before C, which means happens-before guarantees Transitivity.

100.What is volatile variable in Java?

volatile is a special modifier, which can only be used with instance variables. In concurrent Java programs, changes made by multiple threads on instance variables is not visible to other in absence of any synchronizers e.g. synchronized keyword or locks. Volatile variable guarantees that a write will happen before any subsequent read: as stated: "volatile variable rule" in previous question. Read this answer to learn more about volatile variable and when to use them.