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Oracle Java SE 21 Developer Professional Sample Questions:

1. Given:
java
sealed class Vehicle permits Car, Bike {
}
non-sealed class Car extends Vehicle {
}
final class Bike extends Vehicle {
}
public class SealedClassTest {
public static void main(String[] args) {
Class<?> vehicleClass = Vehicle.class;
Class<?> carClass = Car.class;
Class<?> bikeClass = Bike.class;
System.out.print("Is Vehicle sealed? " + vehicleClass.isSealed() +
"; Is Car sealed? " + carClass.isSealed() +
"; Is Bike sealed? " + bikeClass.isSealed());
}
}
What is printed?

A) Is Vehicle sealed? true; Is Car sealed? false; Is Bike sealed? false
B) Is Vehicle sealed? true; Is Car sealed? true; Is Bike sealed? true
C) Is Vehicle sealed? false; Is Car sealed? false; Is Bike sealed? false
D) Is Vehicle sealed? false; Is Car sealed? true; Is Bike sealed? true

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

3. Given:
java
ExecutorService service = Executors.newFixedThreadPool(2);
Runnable task = () -> System.out.println("Task is complete");
service.submit(task);
service.shutdown();
service.submit(task);
What happens when executing the given code fragment?

A) It exits normally without printing anything to the console.
B) It prints "Task is complete" once and throws an exception.
C) It prints "Task is complete" twice and throws an exception.
D) It prints "Task is complete" once, then exits normally.
E) It prints "Task is complete" twice, then exits normally.

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

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

5. Given:
java
Map<String, Integer> map = Map.of("b", 1, "a", 3, "c", 2);
TreeMap<String, Integer> treeMap = new TreeMap<>(map);
System.out.println(treeMap);
What is the output of the given code fragment?

A) {a=1, b=2, c=3}
B) {a=3, b=1, c=2}
C) {b=1, a=3, c=2}
D) {b=1, c=2, a=3}
E) Compilation fails
F) {c=1, b=2, a=3}
G) {c=2, a=3, b=1}

Solutions: