Java Threads Interview Questions And Answers for freshers and experienced

A thread is a path of execution that run on CPU, a process is a collection of threads that share the same virtual memory. A process has at least one thread of execution, and a thread always run in a process context. A process is a collection of virtual memory space, code, data, and system resources. A thread is code that is to be serially executed within a process. A processor executes threads, not processes. One more significant difference between process and thread is every process has its own data memory location but all related threads can share same data memory and have their own individual stacks. Thread is a light weighted process, collection of threads become process.

Thread safe is used to describe a method that can run safely in multithreaded environments allowing accessing to data in a safe and efficient way. Thread safety is achieved by synchronization. Synchronization assures that the object data is not accessed by multiple threads at the same time.

Semaphore is an object which helps two threads to communicate with one another in order to synchronize there operation.

Monitor is a body of code which can be executed by only one thread at a time. If any other thread tries to get access at the same time, it will be suspended until the current thread releases the Monitor.

There are two ways to create a thread.

extend the java.lang.Thread class.
Create a class which extends the thread class. When the class is instantiated, the thread and object are created together and the object is automatically bound to the thread. By calling the object's start() method, the thread is started and immediately calls the object's run() method. implement the java.lang.Runnable interface
Create the thread and supply it an object with a run() method. This object will be permanently associated with the thread. The object's run() method will be invoked when the thread is started. This method of thread creation is useful if you want many threads sharing an object. .

Main differences between threads using runnable interface and by extending the thread class are
Thread is a class and Runnable is an interface in java.
If a class is not extending another class we can use either extending Thread class or implementing Runnable interface. If our class is already extending another class we can't extend Thread class because java doesn't support multiple inheritance so we have left only one choice that is implementing Runnable interface.
When you extend a thread each of your threads has a unique object associate it, whereas with Runnable, many threads share the same object instance.

Thread.sleep() is a static method of the Thread class that causes the currently executing thread to delay for a specified number of milliseconds.

By use of thread.stop;

By using wait() and notify(), a thread can give up its hold on a lock at an arbitrary point, and then wait for another thread to give it back before continuing. By executing wait() from a synchronized block, a thread gives up its hold on the lock and goes to sleep.Later, when the necessary event happens, the thread that is running it calls notify() from a block synchronized on the same object. Now the first thread wakes up and begins trying to acquire the lock again.

It causes the currently executing thread object to temporarily pause and allow other threads to execute.

Daemon threads are designed run in background. One example of a daemon thread is the garbage collector thread. You can use setDaemon() to mark a thread as daemon.

Every thread has priority.If, at any time, a thread of a higher priority than the current thread becomes runnable, it preempts the lower priority thread and begins executing. By default, threads at the same priority are scheduled round robin, which means once a thread starts to run