1Z0-829 Practice Exam 3 — 20 Questions

Question 1

You are designing a logging framework for a distributed application. The framework must support multiple output destinations (console, file, network socket) and allow clients to dynamically add new destinations at runtime without modifying existing code. Currently, the application uses a single static Logger class with methods like logToConsole(String msg) and logToFile(String msg). The team needs to refactor the code to adhere to the Open/Closed Principle and support extensibility. After reviewing the requirements, you propose using a combination of an interface and a strategy pattern. However, a senior developer argues that using a simple enum with abstract methods would be sufficient. Which course of action best adheres to object-oriented design principles and allows the most flexibility for future extensions?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Refactor Logger to be an interface, and implement concrete destination classes (ConsoleLogger, FileLogger, SocketLogger). Use dependency injection to pass the desired destination to the Logger client.. Refactors Logger to be an interface and uses dependency injection, allowing clients to pass any implementation. This adheres to the Open/Closed Principle because new destinations can be added by creating new classes implementing Logger without modifying existing code. Option C (using an enum with abstract methods) violates OCP because adding a new destination requires modifying the enum. Option A (keeping static methods and adding new ones) also requires modification of the Logger class. Option D (abstract class with subclasses and factory) still requires modifying the factory when adding new destinations, reducing flexibility.

Answer

Keep the existing static methods but add a new method for each destination as needed. Use conditional logic to select the destination at runtime.

Answer

Create an enum LogDestination with values CONSOLE, FILE, SOCKET, each overriding an abstract log(String) method. The Logger class uses a static method that accepts a LogDestination and calls its log method.

Answer

Refactor Logger to be an abstract class with an abstract log method, and create subclasses for each destination. Use a factory method to instantiate the correct subclass.

Question 2

Refer to the exhibit. What is the result?

Accepted Answer

Prints 10. The code accesses the private field `x` of the `Outer` class from within the `Inner` class, which is a member inner class. In Java, a non-static inner class has access to all members (including private fields) of its enclosing outer class. The `Inner` class's `printX()` method directly accesses `Outer.this.x`, which is initialized to 10 in the `Outer` constructor, so it prints 10.

Answer

Runtime exception because Inner class is not static.

Answer

Compilation error because x is private.

Answer

Prints 0 because x is not initialized in Inner.

Question 3

A developer needs to concatenate two Stream<String> into one. Which approach is most idiomatic?

Accepted Answer

Stream.concat(stream1, stream2). `Stream.concat(stream1, stream2)` is the idiomatic, built-in static method in the Java Stream API for concatenating two streams into a single sequential stream. It creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream, preserving encounter order.

Answer

stream1.flatMap(s -> stream2)

Answer

new StreamBuilder().add(stream1).add(stream2).build()

Answer

stream1.merge(stream2)

Question 4

Which class from the java.nio.file package is most appropriate for efficiently transferring data between two channels on the same machine?

Accepted Answer

FileChannel.transferTo(long, long, WritableByteChannel). `FileChannel.transferTo()` (and its counterpart `transferFrom()`) uses the operating system's zero-copy mechanism (e.g., `sendfile()` on Linux or `TransmitFile()` on Windows) to move data directly between file system caches and the target channel without intermediate buffers or user-space copies. This makes it the most efficient approach for transferring data between two channels on the same machine, as it minimizes CPU overhead and memory bandwidth usage.

Answer

Files.copy(Path, Path)

Answer

RandomAccessFile.getChannel().write(ByteBuffer)

Answer

InputStream.transferTo(OutputStream)

Question 5

A Java application running on a server reads configuration from a file 'config.properties' located in the same directory as the JAR. The application uses java.util.Properties.load(InputStream) to read the file. Recently, the file was modified by an unauthorized user, and the application started throwing runtime exceptions due to corrupted property values. The security team requires that the file be protected from unauthorized modifications while still being readable by the application. Which action should be taken to ensure the integrity of the configuration file?

Accepted Answer

Digitally sign the file and verify the signature before loading properties.. Digital signing provides both integrity and authenticity. By signing the file with a private key and verifying the signature with a public key before calling Properties.load(InputStream), the application can detect any unauthorized modification. This directly addresses the security requirement without relying on encryption (which protects confidentiality, not integrity) or file permissions (which can be bypassed by a privileged attacker).

Answer

Encrypt the file using AES and decrypt it in the application.

Answer

Set the file permissions to read-only for the application user.

Answer

Store the file in a ZIP archive with a checksum.

Question 6

A program reads user input as a string and needs to parse it into an integer, handling invalid input gracefully. Which code snippet correctly uses try-catch to parse and prints the result or 'Invalid number'?

Accepted Answer

try { int num = Integer.parseInt(input); System.out.println(num); } catch (NumberFormatException e) { System.out.println("Invalid number"); }. It uses `Integer.parseInt(input)` which throws a `NumberFormatException` for invalid input, and the catch block specifically catches that exception to print 'Invalid number'. This is the standard, idiomatic Java approach for parsing integers with error handling, as it directly addresses the requirement without unnecessary overhead or false positives.

Answer

try { int num = Integer.valueOf(input).intValue(); System.out.println(num); } catch (Exception e) { System.out.println("Invalid number"); }

Answer

int num = Integer.parseInt(input); System.out.println(num);

Answer

if (input.matches("\d+")) { int num = Integer.parseInt(input); System.out.println(num); } else { System.out.println("Invalid number"); }

Question 7

Which change fixes the exception?

Accepted Answer

Change line 4 to: for (int i = 0; i < arr.length; i++). It changes the loop to start at index 0 and continue while i < arr.length, which correctly iterates over all valid indices (0 to arr.length-1). The original code likely used i <= arr.length, causing an ArrayIndexOutOfBoundsException when accessing arr[i] at i = arr.length. By using i < arr.length, the loop stops before reaching the out-of-bounds index.

Answer

Change line 4 to: for (int i = 1; i <= arr.length; i++)

Answer

Change line 4 to: for (int i = 0; i <= arr.length-1; i++)

Answer

Change line 5 to: System.out.println(arr[i-1]);

Question 8

Refer to the exhibit. What is the output?

Accepted Answer

apple, banana, cherry. The code uses `Collectors.joining(", ")` which concatenates the stream elements into a single string separated by ", ". The stream contains three elements: "apple", "banana", and "cherry". The joining collector does not add a trailing delimiter, so the output is exactly "apple, banana, cherry".

Answer

banana, cherry

Answer

apple, banana, cherry,

Answer

apple, banana, cherry, date

Question 9

Match each functional interface to its abstract method signature.

Accepted Answer

Predicate: boolean test(T). The correct matches are Predicate -> boolean test(T), Consumer -> void accept(T), Supplier -> T get(). Common confusions involve swapping method signatures between interfaces.

Answer

Predicate: T get()

Answer

Consumer: boolean test(T)

Answer

Function: void accept(T)

Question 10

Refer to the exhibit. What is the output?

Accepted Answer

-1. The filter n > 100 yields an empty stream. findFirst returns an empty Optional. orElse(-1) returns -1.

Answer

10

Answer

null

Answer

Optional.empty

Question 11

A developer runs `java --list-modules` and sees the above output on a standard JDK installation. What does this output represent?

Accepted Answer

The set of modules available in the current JDK.. The `java --list-modules` command outputs the names of all observable modules that are part of the current JDK installation. This includes all platform modules (e.g., `java.base`, `java.sql`, `jdk.compiler`) that are available for use, regardless of whether they are currently required or loaded by an application. Option C correctly identifies this as the set of modules available in the current JDK.

Answer

The modules that are part of the java.base module.

Answer

The modules that are required by the current application.

Answer

The modules that are currently loaded by the JVM.

Question 12

Given: Object obj = null; String s = switch(obj) { case null -> "null"; case String str -> str; default -> "other"; }; What is the result?

Accepted Answer

"null". In Java 14+, switch expressions can handle null values directly using a 'case null' label. Since obj is null, the switch matches the 'case null' branch and returns the string "null". Option D is correct because the code compiles and executes without error, producing "null".

Answer

Compilation error

Answer

NullPointerException

Answer

"other"

Question 13

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

Accepted Answer

The code does not compile because methodA does not declare that it throws Exception.. The code does not compile because methodA explicitly throws an IOException, but the catch block in main catches Exception (a broader type). However, the compiler performs improved rethrow analysis only when the exception parameter is effectively final and the catch block is multi-catch or rethrows a checked exception that is declared in the method signature. Here, methodA does not declare that it throws Exception, and the catch block catches Exception, which is not a declared exception in methodA's throws clause, causing a compilation error.

Answer

The code compiles and prints 'Caught in main: IOException' because the compiler uses improved rethrow analysis.

Answer

Prints: Caught in main: Exception

Answer

Prints: Caught in main: IOException

Question 14

Which two statements are true about the java.util.Comparator and java.lang.Comparable interfaces? (Choose two.)

Accepted Answer

Comparator can be used to sort a list in multiple ways.. The Comparator interface allows you to define multiple different comparison strategies (e.g., by name, by age, by salary) for the same class, enabling sorting of a list in multiple ways. Option D is correct because the Comparable interface is used to define a natural ordering of objects (e.g., String's lexicographic order, Integer's numeric order) by implementing the compareTo method.

Answer

Comparator's compare method returns a boolean.

Answer

A class implementing Comparable must override equals().

Answer

Comparator is in java.lang package.

Question 15

A company runs a Java 17 microservice that processes real-time financial transactions. The application receives a large number of transactions per second, each with a timestamp, amount, and type. The current implementation uses a sequential stream to filter and aggregate transactions into a Map. The team observes high latency and CPU spikes during peak loads. They suspect the stream pipeline is inefficient. The pipeline code is: Map stats = transactions.stream() .filter(t -> t.getTimestamp().isAfter(Instant.now().minusSeconds(60))) .collect(Collectors.groupingBy(Transaction::getType, Collectors.summarizingDouble(Transaction::getAmount))); The transactions list is an ArrayList that is frequently modified by other threads (adding new transactions). The system has multiple CPU cores available. Which of the following changes is the MOST effective way to improve performance while maintaining correctness?

Accepted Answer

Use .parallelStream() on a snapshot (new ArrayList<>(transactions)) and keep the same collector.. Taking a snapshot via new ArrayList<>(transactions) provides a consistent, immutable view for the stream, reducing contention and allowing parallelStream to leverage multiple cores. However, the copy operation itself is not thread-safe and can throw ConcurrentModificationException if the list is being modified simultaneously. Despite this risk, option B is the most effective among the choices because it attempts to isolate the stream from concurrent modifications and uses a parallel-friendly collector. To guarantee correctness, additional synchronization would be required, but none of the other options offer a better combination of performance and safety.

Answer

Use .stream().parallel() with a custom thread pool and ensure the stream source is not modified during operation.

Answer

Replace ArrayList with CopyOnWriteArrayList and use .parallelStream() with the same collector.

Answer

Replace the collector with Collectors.groupingByConcurrent() and use .parallelStream() directly on the original list.

Question 16

The code fails to compile. What is the reason?

Accepted Answer

The flatMap method requires a function that returns a Stream, but String::chars returns an IntStream.. The `chars()` method on `String` returns an `IntStream`, not a `Stream`. The `flatMap` method on a `Stream` expects a function that returns a `Stream`, but `String::chars` returns an `IntStream`, which is a primitive stream and not a subtype of `Stream`. This type mismatch causes a compilation error.

Answer

The collect method is not applicable for a Stream<Character>.

Answer

The method reference String::chars is not a valid method reference.

Answer

The chars() method returns a Stream, but flatMap requires a function that returns a Stream.

Question 17

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(). `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 18

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.. 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 19

What is the output of the code in the exhibit?

Accepted Answer

1.234,57 €. The code uses `NumberFormat.getCurrencyInstance(Locale.GERMANY)`, which formats currency according to the German locale. In Germany, the currency symbol (€) is placed after the amount, a comma is used as the decimal separator, and a period is used as the thousands separator. Thus, 1234.5678 is rounded to two decimal places using half-even rounding and displayed as '1.234,57 €'.

Answer

1,234.57 €

Answer

€1,234.57

Answer

€ 1.234,57

Question 20

A lambda expression that takes two integers and returns a boolean indicating whether the first is greater than the second is best represented by which functional interface?

Accepted Answer

BiPredicate. BiPredicate is a functional interface that takes two arguments (both Integer) and returns a boolean, which exactly matches the lambda expression's signature (int, int) -> boolean. The lambda can be written as (a, b) -> a > b, and BiPredicate is the standard interface for such two-argument boolean-valued operations.

Answer

BiFunction<Integer, Integer, Boolean>

Answer

Predicate<Integer>

Answer

BinaryOperator<Boolean>