1Z0-829 Practice Test 31 — 15 Questions

Question 1

A HashMap uses a mutable object as a key. After adding the key-value pair, the key's fields are changed such that its hashCode changes. Which statement is true?

Accepted Answer

The value may not be retrievable using the modified key. When a HashMap uses a mutable object as a key and the key's hashCode changes after insertion, the key is stored in the bucket corresponding to its original hash code. The modified key's new hash code will likely map to a different bucket during retrieval, so the value may not be found. Option B is correct because the value may not be retrievable using the modified key, depending on whether the new hash code leads to the same bucket and whether equals() still matches.

Answer

The value can still be retrieved using the modified key

Answer

The entry is automatically moved to the correct bucket

Answer

An IllegalArgumentException is thrown when modifying the key

Question 2

Which THREE conditions must be true for a method to override another method in a subclass? (Choose three.)

Accepted Answer

The method name must be the same.. Method overriding requires the subclass method to have exactly the same name as the superclass method. This is a fundamental rule of polymorphism in Java, ensuring that the JVM can correctly resolve the overridden method at runtime based on the method signature.

Answer

The method must be static in the superclass.

Answer

The access modifier must be the same or more restrictive.

Question 3

A method uses an enhanced for loop to iterate over a list of strings and prints each string. The code is:

```java
List<String> list = List.of("A", "B", "C");
for (String s : list) {
    if (s.equals("B")) {
        break;
    }
    System.out.print(s);
}
```

What is the result?

Accepted Answer

A. The enhanced for loop iterates over the list, and when it encounters the string "B", the break statement immediately exits the loop. Therefore, only "A" is printed before the loop terminates. Option C is correct because the output is just "A".

Answer

The code throws a ConcurrentModificationException.

Answer

ABC

Answer

AB

Question 4

Refer to the exhibit. The exhibit shows a stack trace from a Java application. Which line in the code caused the NullPointerException?

Accepted Answer

Line 8: int len = s.length();. The NullPointerException occurs when invoking the `length()` method on a null reference. In the stack trace, the exception is thrown at line 8, where `s.length()` is called, but `s` has not been assigned a non-null value (it is null by default if declared as a field, or uninitialized locally). The JVM throws NullPointerException when any instance method is called on a null object reference.

Answer

Line 10: System.out.println(len);

Answer

Line 12: String s = "hello";

Answer

Line 5: int x = 10;

Question 5

Which two of the following are valid ways to create a LocalDate representing the current date in the system's default timezone? (Choose two.)

Accepted Answer

LocalDate.now(). `LocalDate.now()` is the standard, straightforward method to obtain the current date in the system's default timezone, as defined in the Java Time API (JSR-310). It internally uses the system clock and the default timezone to return a `LocalDate` instance without requiring any additional parameters.

Answer

LocalDate.from(Instant.now())

Answer

new LocalDate()

Answer

LocalDate.ofEpochDay(System.currentTimeMillis() / (24*60*60*1000))

Question 6

A company runs a Java 17 microservice that reads stock market data from a WebSocket and processes it. The application is packaged as an executable JAR using Maven Shade Plugin. Recently, after a dependency update, the application started throwing 'javax.net.ssl.SSLHandshakeException: PKIX path building failed' when connecting to the WebSocket. The security team insists that no certificates should be imported into the default truststore. The application already includes a custom truststore file 'certs.jks' in the resources folder. The developer had been loading it programmatically but the new dependency uses a different SSL context. The application must trust the WebSocket server without modifying JVM defaults. Which action should be taken?

Accepted Answer

Set the system property 'javax.net.ssl.trustStore' to the path of 'certs.jks' and 'javax.net.ssl.trustStorePassword' to its password.. Setting the system properties 'javax.net.ssl.trustStore' and 'javax.net.ssl.trustStorePassword' overrides the default JVM truststore globally for all SSL contexts within the application. This allows the custom 'certs.jks' file to be used without modifying the JVM's default truststore, satisfying the security team's requirement. The new dependency causing the issue likely uses the default SSL context, which will now pick up the custom truststore via these system properties.

Answer

Add the certificate to the default truststore using keytool and restart the application.

Answer

Use the jlink tool to create a custom runtime image that includes the certificate.

Answer

Create a custom TrustManager that bypasses all certificate validation.

Question 7

Which of the following is a terminal operation of the Stream API?

Accepted Answer

forEach(). (forEach()) is correct because it is a terminal operation in the Stream API. Terminal operations produce a result or side effect and close the stream, meaning no further operations can be performed on the same stream. forEach() consumes each element of the stream and returns void, making it a terminal operation.

Answer

map()

Answer

filter()

Answer

peek()

Question 8

A class implements Serializable but the developer wants to completely prevent deserialization of its instances. Which approach accomplishes this?

Accepted Answer

Override readObject() to throw an InvalidClassException.. Overriding readObject() to throw an InvalidClassException (or any exception) prevents the deserialization mechanism from completing. When an ObjectInputStream calls readObject() on a class that defines its own readObject() method, that custom method is invoked instead of the default deserialization. Throwing an exception inside readObject() aborts the deserialization process, effectively blocking the reconstruction of the instance.

Answer

Override readResolve() to return a null.

Answer

Override writeObject() to encrypt the object.

Answer

Override writeReplace() to throw an exception.

Question 9

A developer has created a modular Java application. They want to distribute a minimal runtime image containing only the required modules. Which tool should they use?

Accepted Answer

jlink. B is correct because `jlink` is the Java tool specifically designed to assemble and optimize a minimal runtime image containing only the modules explicitly required by the application and its transitive dependencies. It links the specified modules and their dependencies into a custom JRE that excludes unused modules, reducing footprint for distribution.

Answer

javac

Answer

jar

Answer

jpackage

Question 10

A method returns a List<Integer>. The caller wants to ensure the list cannot be modified. Which is the best approach?

Accepted Answer

return Collections.unmodifiableList(list);. `Collections.unmodifiableList()` returns a read-only view of the specified list. Any attempt to modify the returned list (e.g., via `add`, `remove`, `set`) will throw an `UnsupportedOperationException`. This is the standard, recommended approach in the Java Collections Framework to provide an unmodifiable wrapper without copying the underlying data.

Answer

return Arrays.asList(list.toArray());

Answer

return new ArrayList<>(list);

Answer

return (List<Integer>) list.clone();

Question 11

A team implements a stream pipeline that processes a large dataset in parallel. They use a stateful lambda expression inside the map operation to maintain a count. What is the most likely outcome?

Accepted Answer

The pipeline will produce non-deterministic results due to race conditions.. Using a stateful lambda (one that mutates shared mutable state) inside a parallel stream's map operation introduces race conditions. The map operation is intended to be stateless and non-interfering; when multiple threads concurrently update a shared counter without proper synchronization, the result becomes non-deterministic and unpredictable.

Answer

The pipeline will produce correct results if the stateful lambda is synchronized.

Answer

The pipeline will throw a ConcurrentModificationException.

Answer

The pipeline will always produce the same result regardless of parallelism.

Question 12

A developer wants to compute the product of all even numbers in a stream of integers. Which of the following correctly implements this using streams?

Accepted Answer

filter(n -> n % 2 == 0).reduce(1, (a, b) -> a * b). It first filters the stream to keep only even numbers (n % 2 == 0), then uses reduce with an identity of 1 and a multiplication lambda (a, b) -> a * b. The identity 1 is the neutral element for multiplication, ensuring that if no even numbers exist, the result is 1 rather than an error or incorrect value.

Answer

reduce(1, (a, b) -> (a % 2 == 0) ? a * b : a)

Answer

filter(n -> n % 2 == 0).reduce(0, (a, b) -> a * b)

Answer

reduce(1, (a, b) -> a * b)

Question 13

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.. The correct statements are B and C. Option B is correct because peek() is an intermediate operation that allows performing an action on each element without modifying the stream. Option C is correct because findFirst() returns an Optional describing the first element, or an empty Optional if the stream is empty. Option A is incorrect because a stream can only be consumed once; it cannot be traversed multiple times. Option D is incorrect because collect() is a terminal operation, not an intermediate operation. Option E is incorrect because map() can change the element type; for example, mapping a Stream to Stream is common.

Answer

A stream can be traversed multiple times.

Answer

The collect() method is an intermediate operation.

Answer

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

Question 14

Which THREE statements are true about HashSet in Java 17? (Choose three.)

Accepted Answer

Does not maintain insertion order. HashSet is backed by a HashMap, which uses hash codes to store elements in buckets. The iteration order depends on the hash values and bucket distribution, which can change when the set is resized, so insertion order is not preserved.

Answer

Allows duplicate elements

Answer

Is thread-safe

Question 15

You are developing a high-frequency trading application that processes a stream of market data ticks. Each tick is represented by a Tick object with fields: long timestamp, String symbol, double price, int volume. Ticks arrive in real-time and must be processed in order. A bug is reported: the application occasionally processes a tick out of order, causing incorrect trade decisions. The processing logic uses a while loop to read from a blocking queue and process each tick. The code is: BlockingQueue queue = new LinkedBlockingQueue<>(); while (true) { Tick tick = queue.take(); process(tick); } After investigation, you find that the queue is fed by multiple producer threads that sometimes reorder ticks due to network delays. Which course of action best ensures ticks are processed in the correct chronological order without sacrificing throughput?

Accepted Answer

Replace LinkedBlockingQueue with a PriorityBlockingQueue that orders by timestamp.. PriorityBlockingQueue maintains elements in natural order (or by a provided Comparator), so ticks are automatically ordered by timestamp when inserted. This ensures the consumer always processes the chronologically earliest tick first, even if producers add them out of order, without requiring additional sorting or blocking.

Answer

Use a SynchronousQueue and have producers wait for the consumer to acknowledge.

Answer

After taking from the queue, sort a batch of ticks before processing.

Answer

Add a delay before processing to allow out-of-order ticks to arrive.