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Oracle Java SE 21 Developer Professional Sample Questions:

1. Given:
java
public class Test {
static int count;
synchronized Test() {
count++;
}
public static void main(String[] args) throws InterruptedException {
Runnable task = Test::new;
Thread t1 = new Thread(task);
Thread t2 = new Thread(task);
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(count);
}
}
What is the given program's output?

A) Compilation fails
B) It's either 0 or 1
C) It's always 2
D) It's either 1 or 2
E) It's always 1

2. Given:
java
public class Test {
public static void main(String[] args) throws IOException {
Path p1 = Path.of("f1.txt");
Path p2 = Path.of("f2.txt");
Files.move(p1, p2);
Files.delete(p1);
}
}
In which case does the given program throw an exception?

A) Both files f1.txt and f2.txt exist
B) File f1.txt exists while file f2.txt doesn't
C) Neither files f1.txt nor f2.txt exist
D) An exception is always thrown
E) File f2.txt exists while file f1.txt doesn't

3. Given:
java
public class BoomBoom implements AutoCloseable {
public static void main(String[] args) {
try (BoomBoom boomBoom = new BoomBoom()) {
System.out.print("bim ");
throw new Exception();
} catch (Exception e) {
System.out.print("boom ");
}
}
@Override
public void close() throws Exception {
System.out.print("bam ");
throw new RuntimeException();
}
}
What is printed?

A) bim bam boom
B) Compilation fails.
C) bim boom bam
D) bim bam followed by an exception
E) bim boom

4. Given:
java
Stream<String> strings = Stream.of("United", "States");
BinaryOperator<String> operator = (s1, s2) -> s1.concat(s2.toUpperCase()); String result = strings.reduce("-", operator); System.out.println(result); What is the output of this code fragment?

A) United-States
B) United-STATES
C) UnitedStates
D) UNITED-STATES
E) -UnitedSTATES
F) -UNITEDSTATES
G) -UnitedStates

5. Given:
java
var sList = new CopyOnWriteArrayList<Customer>();
Which of the following statements is correct?

A) The CopyOnWriteArrayList class's iterator reflects all additions, removals, or changes to the list since the iterator was created.
B) Element-changing operations on iterators of CopyOnWriteArrayList, such as remove, set, and add, are supported and do not throw UnsupportedOperationException.
C) The CopyOnWriteArrayList class is not thread-safe and does not prevent interference amongconcurrent threads.
D) The CopyOnWriteArrayList class is a thread-safe variant of ArrayList where all mutative operations are implemented by making a fresh copy of the underlying array.
E) The CopyOnWriteArrayList class does not allow null elements.

Solutions: