1Z0-829 Hard Questions — 25 Difficult Questions

Question 1

A financial application uses Java SE 17 with a custom date format. The requirement is to parse strings like "2023-12-31T23:59:59.999Z" into an Instant. The existing code uses SimpleDateFormat with pattern "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" and then calls parse(). It works fine in single-threaded testing, but in production under load, intermittent parsing failures occur with DateTimeParseException or wrong values. The application is multi-threaded and reuses the same formatter instance. Which single change should be made to fix the issue while maintaining performance?

Accepted Answer

Replace SimpleDateFormat with DateTimeFormatter.ISO_INSTANT and use Instant.from(). Option C is correct because `SimpleDateFormat` is not thread-safe, and reusing a single instance across multiple threads causes race conditions leading to parsing failures. Replacing it with `DateTimeFormatter.ISO_INSTANT` (which is immutable and thread-safe) and using `Instant.from()` directly parses the ISO 8601 string without any synchronization overhead, maintaining performance while fixing the concurrency bug.

Answer

Wrap the parse call in a synchronized block

Answer

Use a ThreadLocal<SimpleDateFormat> to give each thread its own instance

Answer

Create a new SimpleDateFormat instance for each parse call

Question 2

Which THREE statements are true about the java.util.Collection and java.util.stream.Stream APIs? (Choose three.)

Accepted Answer

The Stream.forEach() operation processes elements in the encounter order of the stream only for sequential streams.. Option B is correct because the Stream.forEach() operation respects the encounter order of the stream only for sequential streams. For parallel streams, the order is not guaranteed unless the stream is explicitly forced to be sequential or uses forEachOrdered(). This behavior is defined in the Stream API specification.

Answer

The Collection.forEach() method is inherited from the Collection interface.

Answer

The Collection interface inherits the stream() method from the Iterable interface.

Question 3

What is the result of executing the code in the exhibit?

Accepted Answer

ConcurrentModificationException is thrown.. The code uses an ArrayList and iterates over it with an enhanced for-each loop while simultaneously removing an element via the list's remove() method. This structural modification directly from the list (not via the iterator's remove()) triggers a ConcurrentModificationException because the iterator's modCount check fails. The exception is thrown at the next iteration attempt after the removal.

Answer

The code compiles and runs without output.

Answer

[A, C]

Answer

[A, B]

Question 4

A Java 17 application is deployed on a server. The application uses modules but one required module is missing from the module path. Which exception will be thrown at startup?

Accepted Answer

ModuleNotFoundException. Option D is correct because when a required module is missing from the module path in Java 17, the module system throws `ModuleNotFoundException` during the resolution phase at startup. This exception is specific to the Java Platform Module System (JPMS) and indicates that a module declared in `requires` clauses cannot be located.

Answer

ExceptionInInitializerError

Answer

NoClassDefFoundError

Answer

ClassNotFoundException

Question 5

Which THREE are valid ways to package a Java 17 application for distribution? (Choose three.)

Accepted Answer

JMOD file. A is correct because a JMOD file is a native packaging format introduced in Java 9 (JEP 261) specifically for distributing Java modules. It can include compiled classes, native code, configuration files, and even other JMOD files, making it a valid distribution format for Java 17 applications, especially when using the Java Module System.

Answer

ZIP file containing compiled classes

Answer

Shell script that compiles and runs the application

Question 6

Which TWO are correct about parallel streams in Java? (Choose TWO.)

Accepted Answer

Parallel streams use the common ForkJoinPool by default.. Option D is correct because parallel streams in Java, by default, use the common ForkJoinPool (ForkJoinPool.commonPool()), which is a shared thread pool designed for parallel task execution. This pool is lazily initialized and has a default parallelism level equal to the number of available processors minus one, as per the ForkJoinPool implementation.

Answer

Parallel streams always improve performance over sequential streams.

Answer

Any lambda expression is safe to use in a parallel stream.

Answer

Using parallel streams guarantees the order of elements in the result.

Question 7

A developer needs to parse the string "2023-12-31T23:59:60Z" (a leap second) into a java.time.Instant. Which statement is true?

Accepted Answer

It returns an Instant representing 2023-12-31T23:59:59Z with an added nanosecond.. Option B is correct because java.time.Instant.parse() handles leap seconds by converting them to the last valid second of the minute (23:59:59) and then adding a nanosecond to account for the extra second. This behavior is specified by the ISO-8601 standard and the Java Time API, which does not support a true 60th second but represents it as an Instant with a fractional second adjustment.

Answer

It returns an Instant representing 2023-12-31T23:59:59Z, ignoring the leap second.

Answer

It throws a DateTimeParseException because 60 seconds is invalid.

Answer

It returns null because the string is invalid.

Question 8

A method contains a try-with-resources statement that uses two resources: a FileInputStream and a BufferedInputStream. The FileInputStream constructor throws a FileNotFoundException. Which statement about resource closing is true?

Accepted Answer

No resources are closed because the FileInputStream constructor failed before any resource was opened.. In a try-with-resources statement, resources are closed only if they have been successfully initialized. If the FileInputStream constructor throws a FileNotFoundException, the resource is never created, so the try-with-resources statement does not attempt to close it. The BufferedInputStream is never constructed because the exception occurs before its declaration is reached, so no resources are opened and none need to be closed. Therefore, option C is correct.

Answer

The BufferedInputStream is closed if it was successfully created, but the FileInputStream is not.

Answer

Both resources are closed even if the FileInputStream constructor throws an exception.

Answer

The programmer must explicitly close the resources in a finally block.

Question 9

Which THREE statements are true about loops in Java?

Accepted Answer

The break statement can be used to exit a loop prematurely.. The break statement in Java is used to immediately terminate the execution of a loop (for, while, or do-while) and transfer control to the statement following the loop. This allows premature exit based on a condition, which is a fundamental control flow mechanism in Java.

Answer

A while loop always executes at least once.

Answer

A do-while loop may execute zero times if the condition is false.

Question 10

Which TWO statements about the try-with-resources statement are correct? (Choose two.)

Accepted Answer

The resource class must implement AutoCloseable.. Option A is correct because the try-with-resources statement requires that each resource implements the AutoCloseable interface (or its subinterface Closeable). This interface defines the close() method, which is automatically invoked by the JVM when the try block completes, ensuring proper resource management without explicit finally blocks.

Answer

The resource variable must be declared as final.

Answer

A try-with-resources statement cannot have a finally block.

Answer

Resources are closed before the try block executes.

Question 11

Which TWO are true about the PriorityQueue class?

Accepted Answer

It is not thread-safe.. Option C is correct because PriorityQueue is not thread-safe; it does not synchronize access, so concurrent modifications without external synchronization can lead to inconsistent behavior. Option D is correct because PriorityQueue can accept a custom Comparator at construction time to define the ordering of elements, overriding the natural order.

Answer

It allows null elements.

Answer

It orders elements based on insertion order.

Answer

It provides O(1) time for contains().

Question 12

What is the cause of the ClassCastException?

Accepted Answer

The value stored is Integer but retrieved as String.. The ClassCastException occurs because the code retrieves a value from the store map and attempts to cast it to String, but the actual stored object is an Integer. Since the map is not using generics (raw type), the compiler does not enforce type safety, allowing the mismatch to be caught only at runtime.

Answer

The store map is not type-safe.

Answer

The get method should use (T) cast instead of type.cast().

Answer

The type parameter T is not used correctly.

Question 13

Given: TreeSet ts = new TreeSet<>(Comparator.reverseOrder()); ts.add(10); ts.add(5); ts.add(20); ts.add(15); System.out.println(ts.first()); What is the result?

Accepted Answer

5. Option C is correct because `TreeSet` with `Comparator.reverseOrder()` sorts elements in descending order (20, 15, 10, 5). The `first()` method returns the smallest element according to the comparator, which in descending order is the last element in the natural order, i.e., 5.

Answer

20

Answer

15

Answer

10

Question 14

Which THREE statements about lambda expressions are true? (Choose three.)
A. A lambda expression can be assigned to a functional interface variable.
B. A lambda expression can access final or effectively final local variables.
C. A lambda expression can throw any checked exception.
D. A lambda expression can be used to create an instance of an abstract class.
E. A lambda expression can be used to implement multiple abstract methods.

Accepted Answer

A lambda expression can throw any checked exception.. Option A is correct because a lambda expression can be assigned to a variable of a functional interface type. The lambda provides the implementation for the single abstract method of that interface, and the assignment is valid as long as the lambda's parameter types and return type match the method signature.

Answer

A lambda expression can be used to implement multiple abstract methods.

Answer

A lambda expression can be used to create an instance of an abstract class.

Question 15

Which TWO statements about the Stream API are correct? (Choose two.)
A. A stream can be traversed multiple times.
B. The peek() method is an intermediate operation.
C. The findFirst() method returns an Optional.
D. The collect() method is an intermediate operation.
E. The map() method returns a stream of the same type.

Accepted Answer

The peek() method is an intermediate operation.. Option A is correct because the `peek()` method is an intermediate operation in the Stream API. Intermediate operations return a new stream and are lazy; they are not executed until a terminal operation is invoked. `peek()` allows you to inspect elements as they flow through the pipeline without modifying them.

Answer

The collect() method is an intermediate operation.

Answer

A stream can be traversed multiple times.

Answer

The map() method returns a stream of the same type.

Question 16

A developer is designing a service that processes multiple files concurrently. To avoid resource leaks, which practice is essential?

Accepted Answer

Use try-with-resources for each AutoCloseable resource. Option B is correct because try-with-resources guarantees that each AutoCloseable resource is closed automatically at the end of the statement, even if an exception occurs. This is essential when processing multiple files concurrently, as it prevents resource leaks without requiring explicit close() calls in finally blocks.

Answer

Ensure each thread has its own copy of the file handle

Answer

Call close() on each stream in a separate try-catch block

Answer

Use FileLock to prevent concurrent access

Question 17

An application must read a configuration file that is updated frequently by another process. The developer wants to avoid stale data and minimize I/O operations. Which approach is best?

Accepted Answer

Use java.nio.file.WatchService to monitor the file for modifications and reload when notified. Option A is correct because java.nio.file.WatchService provides an event-driven mechanism to monitor file system changes, such as modifications to a configuration file. This approach avoids stale data by reloading the file only when a change is detected, and it minimizes I/O operations by eliminating the need for polling or repeated reads. The WatchService uses the underlying OS file system events (e.g., inotify on Linux, ReadDirectoryChanges on Windows) for efficient notification.

Answer

Read the file from disk every time it is accessed using Files.newInputStream()

Answer

Periodically check the file's lastModified timestamp using File.lastModified() and reload if changed

Answer

Read the file once at startup and cache the content in memory

Question 18

Which THREE are valid ways to read the contents of a text file into a String in Java?

Accepted Answer

String content = Files.readString(Path.of("file.txt"));. Option A is correct because `Files.readString()` (introduced in Java 11) reads the entire content of a text file into a `String` in one call, handling charset encoding (default UTF-8) and closing the file automatically. It is the simplest and most direct method for this task.

Answer

String content = new FileReader("file.txt").readString();

Answer

String content = Files.readAllLines(Path.of("file.txt"));

Question 19

What is the output of the program?

Accepted Answer

0 1 2 1 2 3 2 3. The program uses nested for loops where the outer loop runs from 0 to 2 and the inner loop runs from the outer loop's current value to that value plus 2. For each iteration, the inner loop variable is printed. This produces the sequence: outer=0 prints 0,1,2; outer=1 prints 1,2,3; outer=2 prints 2,3,4. However, after the inner loop completes for outer=2, the program prints a space (not shown) and then the loop ends, so the output is '0 1 2 1 2 3 2 3 4' — but note the last value 4 is printed, making option B correct.

Answer

0 1 2 1 2 3 2 3 4

Answer

0 1 2 1 2 3 2 3 4 5

Answer

0 1 2 1 2 3 2 3 3 4

Question 20

Which TWO statements about Java serialization are true?

Accepted Answer

During deserialization, the constructor of the serialized class is not invoked.. Option B is correct because during deserialization, Java does not invoke the constructor of the class being deserialized. Instead, the object is reconstructed from the serialized stream data using the no-arg constructor of the first non-serializable superclass (if any), and the serialized class's fields are populated via reflection or readObject() methods. This avoids re-executing initialization logic that may have side effects or depend on runtime state.

Answer

The serialVersionUID must be explicitly declared in every serializable class.

Answer

A class that implements Externalizable does not need to implement Serializable.

Answer

Fields marked as transient are serialized by default.