Java Threads Interview Questions – 1
1)What is threaded programming and when is it used?
Threaded programming is normally used when a program is required to do more than one task at the same time. Threading is often used in applications with graphical user interfaces; a new thread may be created to do
some processor-intensive work while the main thread keeps the interface responsive to human interaction.
The Java programming language has threaded programming facilities built in, so it is relatively easy to create threaded programs. However, multi-threaded programs introduce a degree of complexity that is not justified for most simple command line applications.
A: These methods are detailed on the Java Software Development Kit JavaDoc page for the
Object class, they are to implement threaded programming for all subclasses of
A: The static
Thread.sleep(long) method maintains control of thread execution but delays the next action until the sleep time expires. The
wait method gives up control over thread execution indefinitely so that other threads can run.
A: A Java
Thread controls the main path of execution in an application. When you invoke the Java Virtual Machine with the
java command, it creates an implicit thread in which to execute the
main method. The
Thread class provides a mechanism for the first thread to start-up other threads to run in parallel with it.
Runnable interface defines a type of class that can be run by a thread. The only method it requires is
run, which makes the interface very easy to to fulfil by extending existing classes. A runnable class may have custom constructors and any number of other methods for configuration and manipulation.
A: It may help to think of the
run method like the
main method in standard single threaded applications. The
run method is a standard entry point to run or execute a class. The
run method is normally only executed in the context of an independent
Thread, but is a normal method in all other respects.
run method normally invokes the
run method of the
Runnable type it is passed in its constructor. However, it is
possible to override the thread’s
run method with your own.
A: There is little difference in the work required to override the
Thread class compared with implementing the
Runnable interface, both require the body of the
run() method. However, it is much simpler to make an existing class hierarchy runnable because any class can be adapted to implement the
run() method. A subclass of
Thread cannot extend any other type, so application-specific code would have to be added to it rather than inherited.
Thread class from the
Runnable implementation also avoids potential
synchronization problems between the thread and the
run() method. A separate
Runnable generally gives greater flexibility in the way that runnable code is referenced and executed.
A: The separate
run() methods in the
Thread class provide two ways to create threaded programs. The
start() method starts
the execution of the new thread and calls the
run() method. The
returns immediately and the new thread normally continues until the
run() method returns.
run() method does nothing, so sub-classes should override the
method with code to execute in the second thread. If a
Thread is instantiated with a
Runnable argument, the thread’s
run() method executes the
run() method of
Runnable object in the new thread instead.
Depending on the nature of your threaded program, calling the
run() method directly can give the same output as calling via the
start() method, but in the latter case the code is actually executed in a new thread.
start() method is not marked
final, but should not be overridden. This method contains the code that creates a new executable thread and is very specialised. Your threaded application should either pass a
Runnable type to a new
Thread, or extend
Thread and override the
A: It depends whether the method affects method local variables, class static or instance variables. If only method local variables are changed, the value is said to be confined by the method and is not prone to threading issues.