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Oracle Java SE 21 Developer Professional Sample Questions:

1. 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) 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.
C) The CopyOnWriteArrayList class does not allow null elements.
D) Element-changing operations on iterators of CopyOnWriteArrayList, such as remove, set, and add, are supported and do not throw UnsupportedOperationException.
E) The CopyOnWriteArrayList class is not thread-safe and does not prevent interference amongconcurrent threads.

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

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

4. Given:
java
try (FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos)) {
fos.write("Today");
fos.writeObject("Today");
oos.write("Today");
oos.writeObject("Today");
} catch (Exception ex) {
// handle exception
}
Which statement compiles?

A) fos.write("Today");
B) oos.writeObject("Today");
C) fos.writeObject("Today");
D) oos.write("Today");

5. Which StringBuilder variable fails to compile?
java
public class StringBuilderInstantiations {
public static void main(String[] args) {
var stringBuilder1 = new StringBuilder();
var stringBuilder2 = new StringBuilder(10);
var stringBuilder3 = new StringBuilder("Java");
var stringBuilder4 = new StringBuilder(new char[]{'J', 'a', 'v', 'a'});
}
}

A) stringBuilder4
B) stringBuilder2
C) None of them
D) stringBuilder3
E) stringBuilder1

Solutions: