Which of these constructs a thread that will run the code in an existing Runnable called task?
Answer: A
Thread has a constructor taking a Runnable, and that is the standard way to attach existing work to a thread. There is no static create factory on Thread, run takes no arguments, and Runnable is an interface with no constructor at all.
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Which of these are legal Thread constructors?
Answer: B
Thread offers a no-argument form, one taking a Runnable, one taking a name, and combinations of Runnable, name and ThreadGroup. Neither priority nor id is a constructor argument: priority is set afterwards with setPriority, and the id is assigned by the JVM and only readable.
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Which signature legally overrides Runnable.run?
Answer: C
The interface declares run with no parameters and a void return, so an override must match exactly. Adding a parameter makes it an unrelated overload rather than an override, changing the return type is illegal, and an interface method cannot be implemented with reduced visibility.
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Which of these Thread methods is static?
Answer: D
Correct answer: sleep
sleep is static because it acts on whichever thread is executing the call, not on the object it is written against. Writing t.sleep(100) compiles but puts the calling thread to sleep, which is misleading enough that compilers warn about it. join, interrupt and setName all act on a particular thread and are instance methods.
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Which one of these lists contains only static methods of Thread?
Answer: A
Correct answer: sleep, yield, currentThread
All three concern the thread that is running rather than any named one, which is what makes them static. The other lists mix in join, interrupt, start, setPriority and isAlive, each of which is addressed to a particular thread object and so cannot be static.
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Which of these methods declares InterruptedException?
Answer: B
sleep is declared to throw InterruptedException, so a caller must handle or declare it. yield, interrupt and isAlive declare no checked exception and can be called without a try block. join also declares it, for the same reason as sleep.
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What does Thread.currentThread() return?
Answer: C
Correct answer: A Thread
Thread.currentThread() returns the Thread object for the executing thread, from which the name, id, priority and state can then be obtained. It is the usual way of reaching the current thread from code that was not given a reference to it.
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Which of these correctly reads a thread's identifier?
Answer: D
Correct answer: long id = t.getId();
The accessor is getId() and it returns a long, since ids are not reused and could exhaust an int in a long-running process. Assigning it to an int would not compile without a cast, and there is no method simply called id().
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Which one of these lists contains only methods that Thread inherits from Object?
Answer: A
wait, notify and notifyAll belong to Object, so every object has them, not just threads. sleep, join and interrupt are declared on Thread itself. Beginners often expect the whole set to live on Thread, which is why they are so often confused.
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Which of these types is NOT declared in java.lang?
Answer: B
ExecutorService lives in java.util.concurrent and needs an import. Thread, Runnable and ThreadLocal are all in java.lang, which is imported implicitly, which is why thread code compiles with no import at all until the concurrency utilities are used.
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