An ExecutorService once its tasks are done - which of the following is true?
Answer: C
Its worker threads are non-daemon by default and stay alive waiting for more work, so a program that never calls shutdown does not exit even after all its tasks are done. Forgetting the call is a common reason a JVM appears to hang at the end of a run.
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Which of these statements about how many threads a Java program starts with is correct?
Answer: D
The JVM creates a thread to run main, so single-threaded code is really one-thread code. That thread is not a daemon, which is why the program does not exit while it is still running, and it is the thread whose name Thread.currentThread().getName() reports as main.
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Which one of the following statements about the order in which two started threads execute is true?
Answer: A
Scheduling is left to the platform, so nothing about relative order is promised and a program that happens to print in a tidy order on one run may not on the next. Any required ordering has to be imposed explicitly, with join, a lock, a latch or a queue.
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Consider reusing a Thread object after run() has finished. Which statement is correct?
Answer: B
A finished thread is TERMINATED, and the javadoc states a thread may not be restarted once it has completed execution; a second start() throws IllegalThreadStateException. Reusing the object is impossible, which is one reason executors exist: they reuse threads while you submit fresh tasks.
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Where the synchronized keyword may be applied - which of the following is true?
Answer: C
The keyword marks a method or a block; there is no synchronized class and no synchronized field. Making a field's individual reads and writes atomic and visible is what volatile does, and neither keyword makes a compound update such as an increment safe on its own.
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Which of these statements about how data reaches a thread's run() method is correct?
Answer: D
start() takes no arguments and run() declares none, so anything the task needs is supplied when the Runnable or Thread is constructed, or captured by a lambda. Captured locals must be effectively final, which is one reason shared mutable state tends to live in a field instead.
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Which one of the following statements about two threads writing the same non-volatile int is true?
Answer: A
Atomicity of a single write is not the issue: each write lands whole, but without ordering or visibility guarantees one thread can overwrite the other's value or read a stale one, so an update disappears. The write being atomic is exactly why this looks safe and is not.
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Consider how much code a synchronized block should cover. Which statement is correct?
Answer: B
Every other thread needing the monitor waits for the whole region, so holding it across slow work such as I/O or a sleep serialises the program. The block still has to be long enough to cover the whole invariant, since splitting it lets another thread observe a half-finished update.
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When an interrupted thread actually stops - which of the following is true?
Answer: C
Interruption sets a flag and nothing more for a thread that is simply running; a loop that never consults it runs to completion regardless. That cooperative model is deliberate, because stopping a thread at an arbitrary point is what made stop() unsafe.
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