Is there a separate stack for each thread in Java?

Yes. Every thread maintains its own separate stack, called Runtime Stack but they share the same memory. Elements of the stack are the method invocations,
called activation records or stack frame. The activation record contains pertinent information about a method like local variables.

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How would you implement a thread pool?

public class ThreadPool implements ThreadPoolInt

This class is an generic implementation of a thread pool, which takes the following input

a) Size of the pool to be constructed

b) Name of the class which implements Runnable and constructs a thread pool with active threads that are waiting for activation. Once the threads have finished processing they come back and wait once again in the pool.

This thread pool engine can be locked i.e. if some internal operation is performed on the pool then it is preferable that the thread engine be locked. Locking ensures that no new threads are issued by the engine. However, the currently executing threads are allowed to continue till they come back to the passivePool.

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What state does a thread enter when it terminates its processing?

When a thread terminates its processing, it enters the dead state.

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Can a lock be acquired on a class?

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

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What is an object's lock and which objects have locks?

Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

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Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

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When you will synchronize a piece of your code?

When you expect that your shared code will be accessed by different threads and these threads may change a particular data causing data corruption, then they are placed in a synchronized construct or a synchronized method.

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What is synchronization?

With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.
E.g. synchronizing a function:
public synchronized void Method1 () {
// method code.
}
E.g. synchronizing a block of code inside a function:
public Method2 (){
synchronized (this) {
// synchronized code here.
}
}

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What is daemon thread and which method is used to create the daemon thread?

Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()

When a standalone application is run then as long as any user threads are active the JVM cannot terminate, otherwise the JVM terminates along with any daemon threads which might be active. Thus a daemon thread is at the mercy of the runtime system. Daemon threads exist only to serve user threads.

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What is the difference between process and thread?

A thread is a separate path of execution in a program. A Process is a program in execution.

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What’s the difference between the methods sleep() and wait()?

The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.

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Can Java object be locked down for exclusive use by a given thread?

Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it. If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

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What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

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How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

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What is deadlock?

When two threads are waiting for each other and can’t proceed until the first thread obtains a lock on the other thread or vice versa, the program is said to be in a deadlock.

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What are the states associated in the thread?

A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.

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What is the purpose of the wait(), notify(), and notifyAll() methods?

The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication.

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What invokes a thread's run() method?

After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

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What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

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What is mutual exclusion? How can you take care of mutual exclusion using Java threads?

Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.

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