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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) stringBuilder2
C) stringBuilder1
D) None of them
E) stringBuilder4

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

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

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

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

5. Given:
java
interface A {
default void ma() {
}
}
interface B extends A {
static void mb() {
}
}
interface C extends B {
void ma();
void mc();
}
interface D extends C {
void md();
}
interface E extends D {
default void ma() {
}
default void mb() {
}
default void mc() {
}
}
Which interface can be the target of a lambda expression?

A) A
B) D
C) E
D) None of the above
E) B
F) C

Solutions: