Pass the Oracle Java SE 1z0-830 Questions and answers with CertsForce

In this code, the Test class has a static integer field count and a constructor that is declared with the synchronized modifier. In Java, the synchronized modifier can be applied to methods to control access to critical sections, but it cannot be applied directly to constructors. Attempting to declare a constructor as synchronized will result in a compilation error.

Compilation Error Details:

The Java Language Specification does not permit the use of the synchronized modifier on constructors. Therefore, the compiler will produce an error indicating that the synchronized modifier is not allowed in this context.

Correct Usage:

If you need to synchronize the initialization of instances, you can use a synchronized block within the constructor:

java

public class Test {

static int count;

Test() {

synchronized (Test.class) {

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);

}

}

In this corrected version, the synchronized block within the constructor ensures that the increment operation on count is thread-safe.

Conclusion:

The original program will fail to compile due to the illegal use of the synchronized modifier on the constructor. Therefore, the correct answer is E: Compilation fails.

In this code, the strip() method is used to remove leading and trailing whitespace from strings. The strip() method, introduced in Java 11, is Unicode-aware and removes all leading and trailing characters that are considered whitespace according to the Unicode standard.

docs.oracle.com

Analysis of Each Statement:

First Statement:

java

String s = " ";

System.out.print("[" + s.strip());

The string s contains four spaces.

Applying s.strip() removes all leading and trailing spaces, resulting in an empty string.

The output is "[" followed by the empty string, so the printed result is "[".

Second Statement:

java

s = " hello ";

System.out.print("," + s.strip());

The string s is now " hello ".

Applying s.strip() removes all leading and trailing spaces, resulting in "hello".

The output is "," followed by "hello", so the printed result is ",hello".

Third Statement:

java

s = "h i ";

System.out.print("," + s.strip() + "]");

The string s is now "h i ".

Applying s.strip() removes the trailing spaces, resulting in "h i".

The output is "," followed by "h i" and then "]", so the printed result is ",h i]".

Combined Output:

Combining all parts, the final output is:

css

[,hello,h i]

In this code, two Optional objects are created:

o1 is an empty Optional.

o2 is an Optional containing the integer 1.

A stream is created from o1 and o2. The filter method retains only the Optional instances that are present (i.e., non-empty). This results in a stream containing only o2.

The findAny method returns an Optional describing some element of the stream, or an empty Optional if the stream is empty. Since the stream contains o2, findAny returns Optional[Optional[1]].

The flatMap method is then used to flatten this nested Optional. It applies the provided mapping function (o -> o) to the value, resulting in Optional[1].

Finally, o3.orElse(2) returns the value contained in o3 if it is present; otherwise, it returns 2. Since o3 contains 1, the output is 1.

The ServiceLoader class in Java is used to load service providers implementing a given service interface. The following methods are evaluated for their correctness in loading an implementation of MyService:

A. MyService service = ServiceLoader.load(MyService.class).iterator().next();

This method uses the ServiceLoader.load(MyService.class) to create a ServiceLoader instance for MyService. Calling iterator().next() retrieves the next available service provider. If no providers are available, a NoSuchElementException will be thrown. This approach is correct but requires handling the potential exception if no providers are found.

B. MyService service = ServiceLoader.load(MyService.class).findFirst().get();

This method utilizes the findFirst() method introduced in Java 9, which returns an Optional describing the first available service provider. Calling get() on the Optional retrieves the service provider if present; otherwise, a NoSuchElementException is thrown. This approach is correct and provides a more concise way to obtain the first service provider.

C. MyService service = ServiceLoader.getService(MyService.class);

The ServiceLoader class does not have a method named getService. Therefore, this method is incorrect and will result in a compilation error.

D. MyService service = ServiceLoader.services(MyService.class).getFirstInstance();

The ServiceLoader class does not have a method named services or getFirstInstance. Therefore, this method is incorrect and will result in a compilation error.

In summary, options A and B are correct methods to load an implementation of MyService using ServiceLoader.

1. Valid Use Cases of var

var is alocal variable type inferencefeature.

The compilerinfers the type from the assigned value.

Example of valid use:

java

var a = 10; // Type inferred as int

var str = "Hello"; // Type inferred as String

2. Analyzing the Given Statements

Statement

Valid/Invalid

Reason

var a = 1;

Valid

Type inferred as int.

var b = 2, c = 3.0;

❌Invalid

var doesnot allow multiple declarationsin one statement.

var d[] = new int[4];

❌Invalid

Array brackets []are not allowedwith var.

var e;

❌Invalid

varrequires an initializer(cannot be declared without assignment).

var f = { 6 };

❌Invalid

{ 6 } is anarray initializer, which must have an explicit type.

var h = (g = 7);

Valid

g is assigned 7, and h gets its value.

Thus, the correct answers are:B, C, D, E

References:

Java SE 21 - Local Variable Type Inference (var)

Java SE 21 - var Restrictions

(BufferedWriter writer = new BufferedWriter("file.txt") is incorrect.)

Theincorrect statementisoption Bbecause BufferedWriterdoes nothave a constructor that accepts a String (file name) directly. The correct way to use BufferedWriter is to wrap it around a FileWriter, like this:

java

try (BufferedWriter writer = new BufferedWriter(new FileWriter("file.txt"))) {

writer.write("Hello");

}

Evaluation of Other Options:

Option A (Files.write)→ Correct

Uses Files.write() to write bytes to a file.

Efficient and concise method for writing small text files.

Option C (FileOutputStream)→ Correct

Uses a FileOutputStream to write raw bytes to a file.

Works for both text and binary data.

Option D (PrintWriter)→ Correct

Uses PrintWriter for formatted text output.

Option F (FileWriter)→ Correct

Uses FileWriter to write text data.

Option E (None of the suggestions)→ Incorrect becauseoption Bis incorrect.

Understanding LongStream.range(1, 1945).boxed().toList();

LongStream.range(1, 1945) generates a stream of numbersfrom 1 to 1944.

.boxed() converts the primitive long values to Long objects.

.toList() (introduced in Java 16)creates an immutable list.

Understanding Executors.newVirtualThreadPerTaskExecutor()

Java 21 introducedvirtual threadsto improve concurrency.

Executors.newVirtualThreadPerTaskExecutor()creates a new virtual thread per submitted task, allowing highly concurrent execution.

Execution Behavior

cannesFestivalfeatureFilms.stream().limit(25) → Limits the stream to thefirst 25 numbers(1 to 25).

.forEach(film -> executor.submit(() -> System.out.println(film)))

Each film is printed inside a virtual thread.

Virtual threads execute asynchronously, meaning numbers arenot guaranteed to print sequentially.

Output will contain numbers from 1 to 25, but their order is random due to concurrent execution.

Possible Output (Random Order)

python-repl

3

1

5

2

4

7

25

The ordermay differ in each rundue to concurrent execution.

Thus, the correct answer is:"Numbers from 1 to 25 randomly."

References:

Java SE 21 - Virtual Threads

Java SE 21 - Executors.newVirtualThreadPerTaskExecutor()

1. Why Does Compilation Fail?

The error is in this line:

java

final Stream strings = Files.readAllLines(Paths.get("orders.csv"));

Files.readAllLines(Paths.get("orders.csv")) returns a List<String>,not a Stream<String>.

A List cannot be assigned to a Stream.

2. Correcting the Code

The correct way to create a stream from the file:

java

Stream strings = Files.lines(Paths.get("orders.csv"));

This correctly creates a Stream from the file.

3. Expected Output After Fixing

java

Files.lines(Paths.get("orders.csv"))

skip(1) // Skips the header row

limit(2) // Limits to first two data rows

forEach(System.out::println);

Output:

arduino

1,Kylian Mbappé,Keyboard,2,25.50

2,Teddy Riner,Mouse,1,15.99

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - Files.readAllLines

Java SE 21 - Files.lines

In this code, an ArrayDeque named deque is created, and the integers 1 and 2 are added to it using the offer method. The offer method inserts the specified element at the end of the deque.

State of deque after offers:[1, 2]

The peek method retrieves, but does not remove, the head of the deque, returning 1. Therefore, i1 is assigned the value 1.

State of deque after peek:[1, 2]

Value of i1:1

The poll method retrieves and removes the head of the deque, returning 1. Therefore, i2 is assigned the value 1.

State of deque after poll:[2]

Value of i2:1

Another peek operation retrieves the current head of the deque, which is now 2, without removing it. Therefore, i3 is assigned the value 2.

State of deque after second peek:[2]

Value of i3:2

The System.out.println statement then outputs the values of i1, i2, and i3, resulting in 1 1 2.

In Java, FileOutputStream and ObjectOutputStream are used for writing data to files, but they have different purposes and methods. Let's analyze each statement:

fos.write("Today");

The FileOutputStream class is designed to write raw byte streams to files. The write method in FileOutputStream expects a parameter of type int or byte[]. Since "Today" is a String, passing it directly to fos.write("Today"); will cause a compilation error because there is no write method in FileOutputStream that accepts a String parameter.

fos.writeObject("Today");

The FileOutputStream class does not have a method named writeObject. The writeObject method is specific to ObjectOutputStream. Therefore, attempting to call fos.writeObject("Today"); will result in a compilation error.

oos.write("Today");

The ObjectOutputStream class is used to write objects to an output stream. However, it does not have a write method that accepts a String parameter. The available write methods in ObjectOutputStream are for writing primitive data types and objects. Therefore, oos.write("Today"); will cause a compilation error.

oos.writeObject("Today");

The ObjectOutputStream class provides the writeObject method, which is used to serialize objects and write them to the output stream. Since String implements the Serializable interface, "Today" can be serialized. Therefore, oos.writeObject("Today"); is valid and compiles successfully.

In summary, the only statement that compiles without errors is oos.writeObject("Today");.

References:

Java SE 21 & JDK 21 - ObjectOutputStream

Java SE 21 & JDK 21 - FileOutputStream