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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) stringBuilder4
D) stringBuilder1
E) stringBuilder2

2. How would you create a ConcurrentHashMap configured to allow a maximum of 10 concurrent writer threads and an initial capacity of 42?
Which of the following options meets this requirement?

A) var concurrentHashMap = new ConcurrentHashMap(42);
B) None of the suggestions.
C) var concurrentHashMap = new ConcurrentHashMap(42, 10);
D) var concurrentHashMap = new ConcurrentHashMap();
E) var concurrentHashMap = new ConcurrentHashMap(42, 0.88f, 10);

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

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

4. Given:
java
var array1 = new String[]{ "foo", "bar", "buz" };
var array2[] = { "foo", "bar", "buz" };
var array3 = new String[3] { "foo", "bar", "buz" };
var array4 = { "foo", "bar", "buz" };
String array5[] = new String[]{ "foo", "bar", "buz" };
Which arrays compile? (Select 2)

A) array1
B) array4
C) array2
D) array5
E) array3

5. Given:
java
CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("A");
list.add("B");
list.add("C");
// Writing in one thread
new Thread(() -> {
list.add("D");
System.out.println("Element added: D");
}).start();
// Reading in another thread
new Thread(() -> {
for (String element : list) {
System.out.println("Read element: " + element);
}
}).start();
What is printed?

A) Compilation fails.
B) It prints all elements, but changes made during iteration may not be visible.
C) It throws an exception.
D) It prints all elements, including changes made during iteration.

Solutions: