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Oracle 1z0-830 Exam Syllabus Topics:

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) stringBuilder4
B) stringBuilder2
C) None of them
D) stringBuilder3
E) stringBuilder1

2. What do the following print?
java
import java.time.Duration;
public class DividedDuration {
public static void main(String[] args) {
var day = Duration.ofDays(2);
System.out.print(day.dividedBy(8));
}
}

A) PT0H
B) It throws an exception
C) Compilation fails
D) PT0D
E) PT6H

3. Which three of the following are correct about the Java module system?

A) If a package is defined in both a named module and the unnamed module, then the package in the unnamed module is ignored.
B) The unnamed module can only access packages defined in the unnamed module.
C) If a request is made to load a type whose package is not defined in any known module, then the module system will attempt to load it from the classpath.
D) The unnamed module exports all of its packages.
E) Code in an explicitly named module can access types in the unnamed module.
F) We must add a module descriptor to make an application developed using a Java version prior to SE9 run on Java 11.

4. 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) Java
C) Compilation fails
D) null

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 prints all elements, including changes made during iteration.
D) It throws an exception.

Solutions: