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- Question
Assume the following method is properly synchronized and called from a thread A on an object B:
wait(2000);
After calling this method, when will the thread A become a candidate to get another turn at the CPU?
Options- A. After thread A is notified, or after two seconds.
- B. After the lock on B is released, or after two seconds.
- C. Two seconds after thread A is notified.
- D. Two seconds after lock B is released.
- Correct Answer
- After thread A is notified, or after two seconds.
ExplanationOption A. Either of the two events (notification or wait time expiration) will make the thread become a candidate for running again.Option B is incorrect because a waiting thread will not return to runnable when the lock is released, unless a notification occurs.
Option C is incorrect because the thread will become a candidate immediately after notification, not two seconds afterwards.
Option D is also incorrect because a thread will not come out of a waiting pool just because a lock has been released.
Threads problems
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- 1. Which will contain the body of the thread?
Options- A. run();
- B. start();
- C. stop();
- D. main(); Discuss
Correct Answer: run();
Explanation:
Option A is Correct. The run() method to a thread is like the main() method to an application. Starting the thread causes the object's run method to be called in that separately executing thread.Option B is wrong. The start() method causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
Option C is wrong. The stop() method is deprecated. It forces the thread to stop executing.
Option D is wrong. Is the main entry point for an application.
- 2. Which class or interface defines the wait(), notify(),and notifyAll() methods?
Options- A. Object
- B. Thread
- C. Runnable
- D. Class Discuss
Correct Answer: Object
Explanation:
The Object class defines these thread-specific methods.Option B, C, and D are incorrect because they do not define these methods. And yes, the Java API does define a class called Class, though you do not need to know it for the exam.
- 3. Which of the following will not directly cause a thread to stop?
Options- A. notify()
- B. wait()
- C. InputStream access
- D. sleep() Discuss
Correct Answer: notify()
Explanation:
Option A is correct. notify() - wakes up a single thread that is waiting on this object's monitor.Option B is wrong. wait() causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.
Option C is wrong. Methods of the InputStream class block until input data is available, the end of the stream is detected, or an exception is thrown. Blocking means that a thread may stop until certain conditions are met.
Option D is wrong. sleep() - Causes the currently executing thread to sleep (temporarily cease execution) for a specified number of milliseconds. The thread does not lose ownership of any monitors.
- 4. Which method must be defined by a class implementing the java.lang.Runnable interface?
Options- A. void run()
- B. public void run()
- C. public void start()
- D. void run(int priority) Discuss
Correct Answer: public void run()
Explanation:
Option B is correct because in an interface all methods are abstract by default therefore they must be overridden by the implementing class. The Runnable interface only contains 1 method, the void run() method therefore it must be implemented.Option A and D are incorrect because they are narrowing the access privileges i.e. package(default) access is narrower than public access.
Option C is not method in the Runnable interface therefore it is incorrect.
- 5. Which method registers a thread in a thread scheduler?
Options- A. run();
- B. construct();
- C. start();
- D. register(); Discuss
Correct Answer: start();
Explanation:
Option C is correct. The start() method causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.Option A is wrong. The run() method of a thread is like the main() method to an application. Starting the thread causes the object's run method to be called in that separately executing thread.
Option B is wrong. There is no construct() method in the Thread class.
Option D is wrong. There is no register() method in the Thread class.
- 6. What is the name of the method used to start a thread execution?
Options- A. init();
- B. start();
- C. run();
- D. resume(); Discuss
Correct Answer: start();
Explanation:
Option B is Correct. The start() method causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.Option A is wrong. There is no init() method in the Thread class.
Option C is wrong. The run() method of a thread is like the main() method to an application. Starting the thread causes the object's run method to be called in that separately executing thread.
Option D is wrong. The resume() method is deprecated. It resumes a suspended thread.
- 7. Which of the following will directly stop the execution of a Thread?
Options- A. wait()
- B. notify()
- C. notifyall()
- D. exits synchronized code Discuss
Correct Answer: wait()
Explanation:
Option A is correct. wait() causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.Option B is wrong. notify() - wakes up a single thread that is waiting on this object's monitor.
Option C is wrong. notifyAll() - wakes up all threads that are waiting on this object's monitor.
Option D is wrong. Typically, releasing a lock means the thread holding the lock (in other words, the thread currently in the synchronized method) exits the synchronized method. At that point, the lock is free until some other thread enters a synchronized method on that object. Does entering/exiting synchronized code mean that the thread execution stops? Not necessarily because the thread can still run code that is not synchronized. I think the word directly in the question gives us a clue. Exiting synchronized code does not directly stop the execution of a thread.
- 8. Which of these will create and start this thread?
public class MyRunnable implements Runnable { public void run() { // some code here } }
Options- A. new Runnable(MyRunnable).start();
- B. new Thread(MyRunnable).run();
- C. new Thread(new MyRunnable()).start();
- D. new MyRunnable().start(); Discuss
Correct Answer: new Thread(new MyRunnable()).start();
Explanation:
Because the class implements Runnable, an instance of it has to be passed to the Thread constructor, and then the instance of the Thread has to be started.A is incorrect. There is no constructor like this for Runnable because Runnable is an interface, and it is illegal to pass a class or interface name to any constructor.
B is incorrect for the same reason; you can't pass a class or interface name to any constructor.
D is incorrect because MyRunnable doesn't have a start() method, and the only start() method that can start a thread of execution is the start() in the Thread class.
- 9. What will be the output of the program?
public class CommandArgsThree { public static void main(String [] args) { String [][] argCopy = new String[2][2]; int x; argCopy[0] = args; x = argCopy[0].length; for (int y = 0; y < x; y++) { System.out.print(" " + argCopy[0][y]); } } }and the command-line invocation is> java CommandArgsThree 1 2 3
Options- A. 0 0
- B. 1 2
- C. 0 0 0
- D. 1 2 3 Discuss
Correct Answer: 1 2 3
Explanation:
In argCopy[0] = args;, the reference variable argCopy[0], which was referring to an array with two elements, is reassigned to an array ( args) with three elements.- 10. What will be the output of the program?
public class X { public static void main(String [] args) { String names [] = new String[5]; for (int x=0; x < args.length; x++) names[x] = args[x]; System.out.println(names[2]); } }and the command line invocation is> java X a b
Options- A. names
- B. null
- C. Compilation fails
- D. An exception is thrown at runtime Discuss
Correct Answer: null
Explanation:
The names array is initialized with five null elements. Then elements 0 and 1 are assigned the String values "a" and "b" respectively (the command-line arguments passed to main). Elements of names array 2, 3, and 4 remain unassigned, so they have a value of null.
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