In this post, Java Interview Questions and Answers are discussed, which are helpful for Java beginners and Experienced persons. When you go for Selenium Test Automation Interview, you can expect similar kind of questions.
101. What happens if we don’t override run method ?
If we don’t override run method .Then default implementation of Thread class run() method will be executed and hence the thread will never be in runnable state.
102. Can we overload run() method in java?
We can overload run method but Thread class start method will always cal run method with no arguments. But the overloaded method will not be called by start method we have to explicitly call this start() method.
103. What is a lock or purpose of locks in java?
Lock also called monitor is used to prevent access to a shared resource by multiple threads.
A lock is associated to shared resource. Whenever a thread wants to access a shared resource if must first acquire a lock . If already a lock has been acquired by other it can’t access that shared resource. At this
moment the thread has to wait until another thread releases the lock on shared resource. To lock an object we use synchronization in java.
A lock protects section of code allowing only one thread to execute at at a time.
104. In how many ways we can do synchronization in java?
There are two ways to do synchronization in java:
- Synchronized methods
- Synchronized blocks
To do synchronization we use synchronize keyword.
105. What are synchronized methods ?
If we want a method of object to be accessed by single thread at a time we declare that method with synchronized keyword.
Signature :
public synchronized void methodName(){}
To execute synchronized method first lock has to be acquired on that object. Once synchronized method is called lock will be automatically acquired on that method when no other thread has lock on that method. once lock has been acquired then synchronized method gets executed. Once synchronized method execution completes automatically lock will be released. The prerequisite to execute a synchronized method is to acquire lock before method execution. If there is a lock already acquired by any other thread it waits till the other thread completes.
106. When do we use synchronized methods in java?
If multiple threads tries to access a method where method can manipulate the state of object , in such scenario we can declare a method as synchronized.
107. When a thread is executing synchronized methods , then is it possible to execute other synchronized methods simultaneously by other threads?
No it is not possible to execute synchronized methods by other threads when a thread is inside a synchronized method.
108. When a thread is executing a synchronized method , then is it possible for the same thread to access other synchronized methods of an object ?
Yes it is possible for thread executing a synchronized method to execute another synchronized method of an object.
public synchronized void methodName()
{
}
To execute synchronized method first lock has to be acquired on that object. Once synchronized method is called lock will be automatically acquired on that method when no other thread has lock on that method. once lock has been acquired then synchronized method gets executed. Once synchronized method execution completes automatically lock will be released. The prerequisite to execute a synchronized method is to acquire lock before method execution. If there is a lock already acquired by any other thread it waits till the other thread completes.
109. What are synchronized blocks in java?
Synchronizing few lines of code rather than complete method with the help of synchronized keyword are called synchronized blocks.
Signature :
Synchronized (object reference){// code}
110. When do we use synchronized blocks and advantages of using synchronized blocks?
If very few lines of code requires synchronization then it is recommended to use synchronized blocks. The main advantage of synchronized blocks over synchronized methods is it reduces the waiting time of threads and improves performance of the system.
111. What is class level lock ?
Acquiring lock on the class instance rather than object of the class is called class level lock. The difference between class level lock and object level lock is in class level lock lock is acquired on class .class instance and in object level lock ,lock is acquired on object of class.
112. Can we synchronize static methods in java?
Every class in java has a unique lock associated with it. If a thread wants to execute static synchronize method it need to acquire first class level lock. When a thread was executing static synchronized method no other thread can execute static synchronized method of class since lock is acquired on class.
But it can execute the following methods simultaneously :
- Normal static methods
- Normal instance methods
- synchronize instance methods Signature : synchronized(Classname.class){}
113. Can we use synchronized block for primitives?
Synchronized blocks are applicable only for objects if we try to use synchronized blocks for primitives we get compile time error.
114. What are thread priorities and importance of thread priorities in java?
When there are several threads in waiting, thread priorities determine which thread to run. In java programming language every thread has a priority. A thread inherits priority of its parent thread. By default thread has normal priority of 5. Thread scheduler uses thread priorities to decide when each thread is allowed to run. Thread scheduler runs higher priority threads first.
115. Explain different types of thread priorities ?
Every thread in java has priorities in between 1 to 10. By default priority is 5 (Thread.NORM_PRIORITY). The maximum priority would be 10 and minimum would be 1.Thread class defines the following constants(static final variables) to define properties.
Thread. MIN_PRIORITY = 1; Thread.NORM_PRIORITY=5; Thread. MAX_PRIORITY=10;
116. How to change the priority of thread or how to set priority of thread?
Thread class has a set method to set the priority of thread and get method to get the priority of the thread.
Signature : final void setPriority(int value);
The setPriority() method is a request to jvm to set the priority. JVM may or may not oblige the request. We can get the priority of current thread by using getPriority() method of Thread class.
final int getPriority()
{
}
117. I f two threads have same priority which thread will be executed first ?
We are not guaranteed which thread will be executed first when there are threads with equal priorities in the pool. It depends on thread scheduler to which thread to execute. The scheduler can do any of the following things :
- It can pick any thread from the pool and run it till it completes.
- It can give equal opportunity for all the threads by time slicing.
118. What all methods are used to prevent thread execution ?
There are three methods in Thread class which prevents execution of thread.
- yield()
- join()
- sleep()