1Z0-829 Practice Test 14 — 15 Questions

Question 1

A team uses a TreeSet with a custom Comparable that returns 0 for objects that are not logically equal (e.g., based on one field but objects differ in another). What is the likely outcome when adding such objects?

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

Accepted Answer

The second object is not added because TreeSet considers it a duplicate. TreeSet uses the compareTo() method of the Comparable interface (or Comparator) to determine ordering and equality. When compareTo() returns 0 for two objects that are not logically equal (e.g., they share the same field used in comparison but differ in other fields), TreeSet treats them as duplicates and does not add the second object. This is because TreeSet relies on the comparison result, not the equals() method, for uniqueness.

Answer

The second object replaces the first

Answer

A ClassCastException is thrown

Answer

Both objects are added and insertion order is preserved

Question 2

When should LinkedList be preferred over ArrayList?

Accepted Answer

Frequent insertions and deletions at arbitrary positions. LinkedList is preferred over ArrayList when frequent insertions and deletions occur at arbitrary positions because LinkedList uses a doubly-linked list structure, allowing O(1) time for insertions/deletions once the node is located, whereas ArrayList requires shifting elements (O(n)) due to its underlying array. This makes D correct for scenarios with heavy modification in the middle of the list.

Answer

Frequent random access by index

Answer

Fast iteration over all elements

Answer

Minimal memory overhead

Question 3

A developer needs to remove elements from an ArrayList<String> while iterating over it. Which approach is safest and avoids ConcurrentModificationException?

Accepted Answer

Iterator<String> it = list.iterator(); while (it.hasNext()) { if (it.next().equals("x")) it.remove(); }. The Iterator's remove() method is the only safe way to remove elements from a collection while iterating, as it updates both the iterator's internal cursor and the collection's modCount, preventing ConcurrentModificationException. The enhanced for-each loop (options A and D) uses an implicit iterator that does not expose a remove method, so calling list.remove() directly modifies the collection without updating the iterator's state, causing the exception. Option B avoids the exception by using an index-based loop, but it is unsafe because removing an element shifts subsequent elements left, causing the loop to skip the next element and potentially miss removals or throw an IndexOutOfBoundsException.

Answer

for (String s : list) { if (s.equals("x")) list.remove(s); break; }

Answer

for (int i = 0; i < list.size(); i++) { if (list.get(i).equals("x")) list.remove(i); }

Answer

for (String s : list) { if (s.equals("x")) list.remove(s); }

Question 4

Which three of the following are valid case values in a traditional switch statement (non-pattern)?

Accepted Answer

42. In a traditional switch statement (non-pattern), the case values must be compile-time constants of types that are compatible with the switch expression. Integer literals like 42 are valid case values, as they are constant expressions of type int, which is one of the allowed types (byte, short, char, int, their wrapper classes, String, and enum types).

Answer

true

Answer

3.14

Question 5

Which THREE statements are true about Java NIO.2 and its interaction with blocking I/O? (Choose three.)

Accepted Answer

A Selector can be used to manage multiple non-blocking channels.. A Selector in Java NIO.2 is specifically designed to manage multiple non-blocking channels, enabling a single thread to monitor I/O readiness across many channels. This is a core feature of the NIO.2 multiplexed I/O model, where channels must be configured in non-blocking mode to be registered with a Selector.

Answer

FileChannel operations are always non-blocking.

Answer

BufferedInputStream extends FilterInputStream.

Question 6

A development team wants to ensure that a Java 17 application runs with a specific set of modules. They want to minimize the footprint by including only necessary 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 custom runtime image containing only the modules explicitly required by an application. It analyzes module dependencies and produces a minimal JRE, reducing footprint by excluding unused modules, which aligns with the team's goal of minimizing size.

Answer

javac

Answer

jmod

Answer

jar

Question 7

Which TWO statements are true about the Java module system (JPMS) as of Java 17?

Accepted Answer

A named module is defined by a module-info.class file placed in the root of the module.. A named module in JPMS is defined by a module-info.class file (compiled from module-info.java) placed in the root directory of the module. This file declares the module's name, dependencies, and exported packages, making it a fundamental requirement for a named module.

Answer

An automatic module can contain a module-info.class file.

Answer

A jar file placed on the classpath that lacks module-info.class becomes an automatic module.

Answer

A named module can read an unnamed module by default.

Question 8

Which two statements about the enhanced for-each loop in Java are true? (Choose two.)

Accepted Answer

It requires the target to implement the Iterable interface, except for arrays.. The enhanced for-each loop requires the target to implement the Iterable interface, except for arrays, which are handled natively by the compiler using index-based iteration. This is specified in the Java Language Specification (JLS §14.14.2), where arrays are a special case that does not require Iterable.

Answer

It can safely modify the underlying collection during iteration.

Answer

It can be used to iterate directly over a Stream object.

Answer

It provides an implicit counter variable that can be accessed.

Question 9

Refer to the exhibit. What is the output?

Accepted Answer

true. The code uses `Boolean.valueOf(true)` which returns the `Boolean` constant `Boolean.TRUE`, and `Boolean.TRUE` is a singleton object that is `==` to itself. The `==` operator compares object references, not values, so the comparison `Boolean.valueOf(true) == Boolean.TRUE` evaluates to `true` because both refer to the same cached `Boolean` instance from the `Boolean` class's static final fields.

Answer

compile error

Answer

null

Answer

false

Question 10

Which TWO statements correctly use the DateTimeFormatter class? (Choose two.)

Accepted Answer

DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL);. `DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL)` is a valid static factory method that returns a formatter using the default locale's full date style. Option C is correct because `DateTimeFormatter.ofPattern("dd/MM/yyyy")` is the standard way to create a custom pattern-based formatter.

Answer

DateTimeFormatter formatter = DateTimeFormatter.ISO_LOCAL_DATE.ofPattern("yyyy-MM-dd");

Answer

DateTimeFormatter formatter = DateTimeFormatter.ofPattern(Locale.US, "yyyy-MM-dd");

Answer

DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedTime(Locale.US, FormatStyle.MEDIUM);

Question 11

A developer is writing a method that takes a LocalDate and a ZoneId and returns the current time in that time zone as an OffsetDateTime. Which approach correctly implements this?

Accepted Answer

ZonedDateTime.now(zone).toOffsetDateTime(). `ZonedDateTime.now(zone).toOffsetDateTime()` directly obtains the current date-time in the specified time zone and then converts it to an `OffsetDateTime` by extracting the zone offset. This approach correctly uses the provided `ZoneId` and returns the current time as an `OffsetDateTime` without any loss of precision or incorrect time manipulation.

Answer

OffsetDateTime.of(LocalDateTime.now(), ZoneOffset.from(ZonedDateTime.now(zone)))

Answer

LocalDate.now(zone).atStartOfDay(zone).toOffsetDateTime()

Answer

LocalDateTime.now().atZone(zone).toOffsetDateTime()

Question 12

A junior developer writes a method that uses a switch statement to handle different types of user input. The input is an integer representing an operation code. The developer uses a traditional switch statement with break statements. However, when operation code 2 is entered, the program also executes the code for operation 3. What is the most likely cause?

Accepted Answer

The break statement is missing after the operation 2 case.. In a traditional Java switch statement, execution continues into the next case block unless a break statement is encountered. If operation code 2 executes the code for operation 3, it indicates that the break statement is missing after the case for operation 2, causing fall-through to the next case.

Answer

The operation codes are not mutually exclusive.

Answer

The variable is being modified inside the switch.

Answer

The switch statement uses fall-through by design.

Question 13

Given a list of integers, a developer wants to compute the sum of squares of numbers greater than 10. The following code is written:
int sum = list.stream().filter(i -> i>10).mapToInt(i->i*i).sum();
But the sum is incorrect. What is the most likely reason?

Accepted Answer

The list might contain nulls causing NullPointerException.. If the list contains null elements, the lambda expression `i -> i > 10` will attempt to unbox a null `Integer` to an `int`, causing a `NullPointerException` at runtime. The stream pipeline itself is syntactically valid, but the presence of nulls in the source list is a common pitfall when using primitive-specialized operations like `mapToInt` after a filter that does not guard against nulls.

Answer

The mapToInt operation is not allowed after filter.

Answer

The stream is not sorted.

Answer

The filter should be before mapToInt, but it is after.

Answer

The sum() method returns an OptionalInt and must be orElse(0).

Question 14

Which method of Collection interface returns a primitive int?

Accepted Answer

size(). The `size()` method is the only one in the `Collection` interface that returns a primitive `int`, representing the number of elements in the collection. All other methods listed return either a `boolean` or an array of objects, making D the correct answer.

Answer

contains(Object o)

Answer

isEmpty()

Answer

toArray()

Question 15

Which TWO are true about HashSet and TreeSet?

Accepted Answer

HashSet uses equals() and hashCode() for uniqueness, TreeSet uses compareTo() or Comparator.. HashSet relies on the equals() and hashCode() methods to determine element uniqueness, while TreeSet uses the natural ordering (compareTo()) or a provided Comparator to maintain sorted order and check for duplicates. This distinction is fundamental to how these Set implementations enforce uniqueness.

Answer

HashSet maintains elements in insertion order.

Answer

Both implement the List interface.

Answer

Both guarantee constant-time performance for basic operations (add, remove, contains).