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Oracle Java SE 21 Developer Professional Sample Questions:

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

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

A) The CopyOnWriteArrayList class is not thread-safe and does not prevent interference amongconcurrent threads.
B) The CopyOnWriteArrayList class's iterator reflects all additions, removals, or changes to the list since the iterator was created.
C) Element-changing operations on iterators of CopyOnWriteArrayList, such as remove, set, and add, are supported and do not throw UnsupportedOperationException.
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.

3. Given:
java
Optional o1 = Optional.empty();
Optional o2 = Optional.of(1);
Optional o3 = Stream.of(o1, o2)
.filter(Optional::isPresent)
.findAny()
.flatMap(o -> o);
System.out.println(o3.orElse(2));
What is the given code fragment's output?

A) 1
B) 2
C) An exception is thrown
D) Optional[1]
E) Optional.empty
F) Compilation fails
G) 0

4. Given:
java
List<String> l1 = new ArrayList<>(List.of("a", "b"));
List<String> l2 = new ArrayList<>(Collections.singletonList("c"));
Collections.copy(l1, l2);
l2.set(0, "d");
System.out.println(l1);
What is the output of the given code fragment?

A) [a, b]
B) An IndexOutOfBoundsException is thrown
C) [d]
D) [d, b]
E) [c, b]
F) An UnsupportedOperationException is thrown

5. Which of the followingisn'ta correct way to write a string to a file?

A) java
try (FileOutputStream outputStream = new FileOutputStream("file.txt")) { byte[] strBytes = "Hello".getBytes(); outputStream.write(strBytes);
}
B) java
Path path = Paths.get("file.txt");
byte[] strBytes = "Hello".getBytes();
Files.write(path, strBytes);
C) java
try (FileWriter writer = new FileWriter("file.txt")) {
writer.write("Hello");
}
D) None of the suggestions
E) java
try (BufferedWriter writer = new BufferedWriter("file.txt")) {
writer.write("Hello");
}
F) java
try (PrintWriter printWriter = new PrintWriter("file.txt")) {
printWriter.printf("Hello %s", "James");
}

Solutions: