Threads - Finding the output Questions and Answers

Q: Sample78 is run - what will be the output? public class Sample78 { public static void main(String[] args) throws InterruptedException { Thread t = new Thread(new Runnable() { public void run() { System.out.print("x"); } }); t.setName("printer"); t.start(); t.join(); t.setName("printer"); System.out.print(" " + t.getName()); } }

Correct answer: x printerThe thread runs and prints x, and join guarantees it has finished before main continues. setName may be called at any point in a thread’s life, including after it has terminated - it is setDaemon and setPriority before start that carry restrictions, and only start on an already-started thread throws IllegalThreadStateException. So the name is “printer” and the output is x printer.

71.

Sample71 is run - what will be the output?

import java.util.*;
import java.util.concurrent.*;

public class Sample71
{
    public static void main(String[] args)
    {
        List<String> names = new CopyOnWriteArrayList<>(Arrays.asList("a", "b"));
        for (String s : names)
        {
            names.add("c");
            System.out.print(s);
        }
        System.out.print(" " + names.size());
    }
}

A.A ConcurrentModificationException after the first element
B.ab 2
C.ab 4
D.abc 4

Answer: C

The iterator works from a snapshot taken when iteration began, so it visits only a and b however many elements are added, and no ConcurrentModificationException is possible. The two additions do take effect, so the final size is four.

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72.

Sample72 is run - what will be the output?

import java.util.*;

public class Sample72
{
    public static void main(String[] args)
    {
        List<String> names = new ArrayList<>(Arrays.asList("a", "b"));
        for (String s : names)
        {
            names.add("c");
            System.out.print(s);
        }
    }
}

A.ab
B.abc repeatedly
C.ab only
D.a, then a ConcurrentModificationException

Answer: D

An ArrayList's iterator is fail-fast: it notices the structural change on its next step, so the first element prints and the exception follows. Adding through the collection during a for-each is exactly the case the check exists to catch; the iterator's own methods are the supported route.

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73.

Sample73 is run - what will be the output?

public class Sample73
{
    static final InheritableThreadLocal<String> TAG = new InheritableThreadLocal<>();

    public static void main(String[] args) throws InterruptedException
    {
        TAG.set("parent");
        Thread child = new Thread(() -> System.out.println(TAG.get()));
        child.start();
        child.join();
    }
}

A.parent
B.null
C.An empty line
D.A NullPointerException from get()

Answer: A

An InheritableThreadLocal passes the creating thread's value to each thread it creates, so the child sees parent. A plain ThreadLocal would give null here, because each thread would have its own unset value. Inheritance is a copy taken at construction, not a live link.

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74.

Sample74 is run - what will be the output?

public class Sample74 extends Thread
{
    static final ThreadLocal<String> SCOPE = new ThreadLocal<>();

    public void run()
    {
        System.out.println(SCOPE.get());
    }

    public static void main(String[] args) throws InterruptedException
    {
        SCOPE.set("outer");
        Sample74 worker = new Sample74();
        worker.start();
        worker.join();
    }
}

A.A NullPointerException from get()
B.null
C.parent
D.An empty line

Answer: B

A ThreadLocal holds an independent value per thread and does not pass it on, so the worker has never set one and get returns null, which println renders as the four characters null. InheritableThreadLocal is the variant that copies the creating thread's value.

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75.

Sample75 is run - what will be the output?

public class Sample75
{
    public static void main(String[] args)
    {
        Thread t = new Thread();
        t.start();
        System.out.println("done");
    }
}

A.A NullPointerException
B.An IllegalThreadStateException from start()
C.done
D.An empty line then done

Answer: C

A Thread built with no Runnable inherits Thread's own run(), which the javadoc says does nothing and returns. So the thread starts, finds no work and dies quietly, leaving only main's output. Nothing is thrown, because a missing Runnable is a legitimate case.

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76.

Sample76 is run - what will be the output?

public class Sample76 extends Thread
{
    public void run()
    {
        System.out.print("body ");
    }

    public static void main(String[] args)
    {
        Sample76 t = new Sample76();
        t.run();
        t.run();
        System.out.print("end");
    }
}

A.body end
B.end
C.body body end, on three different threads
D.body body end

Answer: D

run() is being called as an ordinary method, so it may be called as often as you like and each call executes on main. No thread is ever started, which is why the restriction on calling start() twice does not apply here.

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77.

Sample77 is run - what will be the output?

public class Sample77
{
    public static void main(String[] args) throws InterruptedException
    {
        Thread first = new Thread(() -> System.out.print("1"));
        Thread second = new Thread(() -> System.out.print("2"));
        first.start();
        first.join();
        second.start();
        second.join();
        System.out.print("3");
    }
}

A.123
B.321
C.Either 123 or 213
D.132

Answer: A

Each thread is joined before the next is started, so only one is ever running and the order is fully determined. Starting both before joining either would leave the first two characters in any order. Sequencing threads this way gives up the concurrency, which is sometimes exactly what a test wants.

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78.

Sample78 is run - what will be the output?

public class Sample78
{
    public static void main(String[] args) throws InterruptedException
    {
        Thread t = new Thread(new Runnable()
        {
            public void run()
            {
                System.out.print("x");
            }
        });
        t.setName("printer");
        t.start();
        t.join();
        t.setName("printer");
        System.out.print(" " + t.getName());
    }
}

A.An IllegalThreadStateException from setName()
B.x printer
C.x Thread-0
D.x main

Answer: B

Correct answer: x printer

The thread runs and prints x, and join guarantees it has finished before main continues. setName may be called at any point in a thread’s life, including after it has terminated - it is setDaemon and setPriority before start that carry restrictions, and only start on an already-started thread throws IllegalThreadStateException. So the name is “printer” and the output is x printer.

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79.

Sample79 is run - what will be the output?

public class Sample79
{
    public static void main(String[] args) throws InterruptedException
    {
        Thread t = new Thread(() -> System.out.print(
                Thread.currentThread().getPriority()));
        t.start();
        t.join();
    }
}

A.10
B.1
C.5
D.0

Answer: C

A new thread takes its initial priority from the thread that created it, and the main thread runs at Thread.NORM_PRIORITY, which is 5. The constants are 1 for MIN_PRIORITY and 10 for MAX_PRIORITY, and the value is fixed by the class rather than the platform.

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80.

Sample80 is run - what will be the output?

public class Sample80
{
    public static void main(String[] args) throws InterruptedException
    {
        StringBuilder out = new StringBuilder();
        Thread t = new Thread(() -> out.append("worker"));
        t.start();
        t.join();
        System.out.println(out);
    }
}

A.An empty line
B.Either worker or an empty line
C.A compile error
D.worker

Answer: D

Only one thread touches the builder at a time here, so there is no race, and the join gives main a happens-before edge that makes the worker's write visible. The variable is effectively final, which is all a lambda requires.

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