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Oracle Java SE 21 Developer Professional Sample Questions:

1. Given:
java
Integer frenchRevolution = 1789;
Object o1 = new String("1789");
Object o2 = frenchRevolution;
frenchRevolution = null;
Object o3 = o2.toString();
System.out.println(o1.equals(o3));
What is printed?

A) Compilation fails.
B) false
C) true
D) A ClassCastException is thrown.
E) A NullPointerException is thrown.

2. Given:
java
var frenchCities = new TreeSet<String>();
frenchCities.add("Paris");
frenchCities.add("Marseille");
frenchCities.add("Lyon");
frenchCities.add("Lille");
frenchCities.add("Toulouse");
System.out.println(frenchCities.headSet("Marseille"));
What will be printed?

A) [Paris, Toulouse]
B) [Lille, Lyon]
C) [Paris]
D) Compilation fails
E) [Lyon, Lille, Toulouse]

3. Which of the following statements are correct?

A) You can use 'public' access modifier with all kinds of classes
B) You can use 'protected' access modifier with all kinds of classes
C) None
D) You can use 'final' modifier with all kinds of classes
E) You can use 'private' access modifier with all kinds of classes

4. 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) stringBuilder2
E) stringBuilder4

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

Solutions: