1Z0-829 Practice Test 19 — 15 Questions

Question 1

Which statement about the Stream API is true?

Accepted Answer

A stream can be consumed only once.. A Stream in Java represents a sequence of elements that can be traversed only once. Once a terminal operation (like collect() or forEach()) is invoked on a stream, the stream is consumed and cannot be reused; attempting to do so throws an IllegalStateException. This one-time consumption is a fundamental design principle of the Stream API, ensuring that operations are applied to a single, immutable pipeline.

Answer

Parallel streams always improve performance.

Answer

Streams are always finite.

Answer

Intermediate operations are executed eagerly.

Question 2

Which of the following is a best practice when designing custom exception classes?

Accepted Answer

Extend an existing exception class and provide constructors that call the superclass constructor.. The Java best practice for custom exceptions is to extend an existing exception class (e.g., Exception or RuntimeException) and provide constructors that call the superclass constructor. This ensures the custom exception inherits the standard exception mechanism, including proper chaining, stack trace handling, and compatibility with try-catch blocks, without reinventing the wheel.

Answer

Define a custom exception with a single constructor that takes an int error code.

Answer

Extend the Throwable class directly to create a new exception type.

Answer

Use a generic Exception class with a string message to indicate the type of error.

Question 3

Which of the following correctly uses DateTimeFormatter to parse the date "2023-01-15" into a LocalDate?

Accepted Answer

LocalDate.parse("2023-01-15", DateTimeFormatter.ofPattern("yyyy-MM-dd")). Options A and D are both correct. Option A uses DateTimeFormatter.ofPattern("yyyy-MM-dd") with the correct pattern, which parses the date successfully. Option D uses the predefined constant DateTimeFormatter.ISO_LOCAL_DATE, which also correctly parses the input "2023-01-15". Option B is wrong because lowercase "mm" represents minutes, not months. Option C is wrong because MonthDay.parse() expects a format like "--01-15" without a year.

Answer

LocalDate.parse("2023-01-15", DateTimeFormatter.ofPattern("yyyy-mm-dd"))

Answer

MonthDay.parse("2023-01-15")

Question 4

Which TWO statements are true about creating an unmodifiable List?

Accepted Answer

Collections.unmodifiableList() returns a view that is unmodifiable, but the backing list can still be changed.. A is correct because `Collections.unmodifiableList()` returns a view of the backing list that cannot be modified through the returned reference, but any changes made to the original backing list are reflected in the view. This means the view is unmodifiable but not immutable, as the underlying list can still be mutated.

Answer

List.copyOf() creates a list that, if the source list is changed, the copy also changes.

Answer

new ArrayList<>(Arrays.asList(...)) creates an unmodifiable list.

Answer

Arrays.asList() returns an unmodifiable list.

Question 5

Which THREE statements are true about the Java Platform editions? (Select three.)

Accepted Answer

Java ME provides a subset of Java SE APIs for embedded devices.. Java ME (Micro Edition) is designed for resource-constrained devices like sensors, mobile phones, and embedded systems. It provides a subset of the Java SE APIs, along with additional APIs tailored for small devices, such as the Connected Limited Device Configuration (CLDC) and the Mobile Information Device Profile (MIDP). This allows developers to write Java applications that run on devices with limited memory, processing power, and display capabilities.

Answer

Java ME (Micro Edition) is the same as Android SDK.

Answer

Java SE includes a full application server for enterprise applications.

Question 6

Which TWO are true about the PriorityQueue class?

Accepted Answer

It is not thread-safe.. 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 7

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. None of the listed exceptions are correct. When a required module is missing from the module path during startup in Java 17, the module system throws `java.lang.module.FindException`, not `ModuleNotFoundException`. `ExceptionInInitializerError` occurs during static initialization, `NoClassDefFoundError` when a class is missing at runtime, and `ClassNotFoundException` at class loading. `ModuleNotFoundException` is not a standard Java exception.

Answer

ExceptionInInitializerError

Answer

NoClassDefFoundError

Answer

ClassNotFoundException

Question 8

A developer adds .peek(System.out::println) to a stream pipeline to debug, but no output is printed. What is the most likely reason?

Accepted Answer

The pipeline lacks a terminal operation. Stream pipelines are lazy; intermediate operations like peek() are only executed when a terminal operation (e.g., forEach, collect, reduce) is invoked. Without a terminal operation, the pipeline never starts processing data, so peek() produces no output.

Answer

The stream is parallel

Answer

The peek operation is placed after the terminal operation

Answer

The stream is empty

Question 9

A Java 17 application uses a HashMap> to group error messages by error code. The map may contain up to 5000 error codes, and each list may contain up to 1000 messages. The application frequently retrieves the list for a given error code and iterates over it. The lists are rarely modified after initial population. The current performance is acceptable, but the team wants to reduce memory footprint. The developer suggests replacing the inner List with an array (String[]), but then iteration would require conversion. Another developer suggests using a TreeMap instead of HashMap to save memory because TreeMap uses less memory per entry? Actually TreeMap has more overhead. Actually HashMap typically has lower overhead than TreeMap. The correct approach: Since lists are rarely modified, consider using List.of to create immutable lists that can be shared? Or use ArrayList with initial capacity to reduce resizing. But the stem says 'reduce memory footprint'. Option C: Use ArrayList with initial capacity to reduce internal array resizing overhead. Option A: Use TreeMap - wrong because TreeMap uses more memory per entry due to tree nodes. Option B: Use array of arrays - not type-safe. Option D: Use LinkedList - higher memory overhead per element. So best is to use ArrayList with proper sizing to minimize wasted space.

Accepted Answer

Use ArrayList<String> with initial capacity set to expected list size. Using an ArrayList with an initial capacity set to the expected list size eliminates the overhead of repeated internal array resizing (which wastes memory by leaving unused capacity). Since the lists are rarely modified after initial population, this approach minimizes wasted space without sacrificing iteration performance, unlike the other options.

Answer

Replace ArrayList with LinkedList

Answer

Replace HashMap with TreeMap

Answer

Replace List<String> with String[]

Question 10

Which of the following correctly uses a switch expression with multiple constants per case?

```java
int day = 2;
String result = switch (day) {
    case 1, 2, 3 -> "Weekday";
    case 6, 7 -> "Weekend";
    default -> "Invalid";
};
```

What is the value of result?

Accepted Answer

Weekday. The switch expression correctly uses multiple constants per case with the arrow syntax. Since day is 2, it matches case 1, 2, 3, so the expression evaluates to "Weekday". The switch expression is exhaustive (covers all possible int values via the default branch), so it compiles and runs without error.

Answer

The code does not compile because multiple constants are not allowed.

Answer

Invalid

Answer

Weekend

Question 11

A developer runs 'jdeps -s --module-path lib myapp.jar' and gets output: 'myapp.jar -> java.base, myapp.jar -> java.sql, myapp.jar -> notfound'. What does the 'notfound' entry indicate?

Accepted Answer

The application has a dependency on a package that is not provided by any module on the module path.. The 'notfound' entry in the jdeps -s output indicates that the application has a dependency on a package that is not provided by any module on the module path. The -s (summary) flag shows module-level dependencies, and when a required module cannot be located among the specified modules or the JDK's built-in modules, jdeps reports it as 'notfound'. This means the dependency cannot be resolved at analysis time, typically because the required module is missing from the module path.

Answer

The dependency is resolved but its module name is not printed due to a summary flag.

Answer

The jar file is missing or corrupt.

Answer

There is a cyclic dependency among modules.

Question 12

Which class provides a convenient means to read text from a file line by line?

Accepted Answer

BufferedReader. BufferedReader (option D) is the correct answer because it provides the `readLine()` method, which reads text from a character-input stream line by line, efficiently buffering characters to avoid frequent disk reads. When wrapped around a FileReader, it enables convenient and performant line-by-line reading of text files.

Answer

FileReader

Answer

Scanner

Answer

FileInputStream

Question 13

A developer wants to create an executable JAR file. Which jar command option is used to specify the main class?

Accepted Answer

--main-class. The `--main-class` option (or `-e` in older syntax) specifies the fully qualified name of the class containing the `public static void main(String[])` method when creating an executable JAR with the `jar` command. This entry is stored in the JAR's `META-INF/MANIFEST.MF` file under the `Main-Class` header, which the Java launcher reads to determine the entry point.

Answer

--class-path

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--module-version

Answer

--create

Question 14

Which method checks if a String is empty or contains only whitespace?

Accepted Answer

isBlank(). The `isBlank()` method, introduced in Java 11, returns `true` if the string is empty or contains only whitespace characters (as defined by `Character.isWhitespace()`). This makes it the correct and most direct way to check for blank strings, unlike older approaches that require combining multiple checks.

Answer

length() == 0

Answer

isEmpty()

Answer

trim().isEmpty()

Question 15

Match each concurrency utility to its purpose.

Accepted Answer

CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.. This question tests understanding of common concurrency utilities in java.util.concurrent such as CountDownLatch, CyclicBarrier, Semaphore, and Exchanger. A common mistake is confusing CountDownLatch (one-time use) with CyclicBarrier (reusable) and mixing up Semaphore's permit-based control with barrier synchronization.

Answer

CyclicBarrier: A synchronization aid that allows one or more threads to wait until a set of operations completes.

Answer

Semaphore: A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.