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

2. Given:
java
interface Calculable {
long calculate(int i);
}
public class Test {
public static void main(String[] args) {
Calculable c1 = i -> i + 1; // Line 1
Calculable c2 = i -> Long.valueOf(i); // Line 2
Calculable c3 = i -> { throw new ArithmeticException(); }; // Line 3
}
}
Which lines fail to compile?

A) Line 3 only
B) Line 1 and line 3
C) The program successfully compiles
D) Line 2 and line 3
E) Line 1 only
F) Line 1 and line 2
G) Line 2 only

3. What does the following code print?
java
import java.util.stream.Stream;
public class StreamReduce {
public static void main(String[] args) {
Stream<String> stream = Stream.of("J", "a", "v", "a");
System.out.print(stream.reduce(String::concat));
}
}

A) Optional[Java]
B) Compilation fails
C) Java
D) null

4. 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 does not allow null elements.
C) 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.
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.

5. Consider the following methods to load an implementation of MyService using ServiceLoader. Which of the methods are correct? (Choose all that apply)

A) MyService service = ServiceLoader.services(MyService.class).getFirstInstance();
B) MyService service = ServiceLoader.load(MyService.class).findFirst().get();
C) MyService service = ServiceLoader.load(MyService.class).iterator().next();
D) MyService service = ServiceLoader.getService(MyService.class);

Solutions: