1Z0-829 Working with Arrays and Collections Practice Test 5 — 15 Questions

Question 1

Which TWO are true about HashSet and TreeSet?

Accepted Answer

HashSet uses equals() and hashCode() for uniqueness, TreeSet uses compareTo() or Comparator.. Option B is correct because HashSet determines element uniqueness through the equals() and hashCode() contract (hashing into buckets and comparing with equals()), whereas TreeSet is a sorted set that relies on natural ordering via Comparable.compareTo() or an explicit Comparator to detect duplicates and order elements. Option E is correct because TreeSet's red-black tree implementation must compare elements to place them, so elements must implement Comparable (mutually comparable) unless a Comparator is supplied to the constructor. Option A is wrong because HashSet makes no ordering guarantee at all—insertion order is preserved by LinkedHashSet, not HashSet. Option C is wrong because both HashSet and TreeSet implement the Set interface (and NavigableSet/SortedSet for TreeSet), not List. Option D is wrong because TreeSet's basic operations run in O(log n) due to its balanced tree, and even HashSet only offers constant-time average performance, not a guarantee, since collisions can degrade operations toward O(n).

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).

Question 2

Which THREE are methods of the Map.Entry interface? (Java 17)

Accepted Answer

getValue(). The java.util.Map.Entry interface defines getKey() (option C), which returns the key corresponding to the entry, and getValue() (option A), which returns the value corresponding to the entry; both are core accessors used when iterating a map's entrySet(). It also defines setValue(V value) (option D), which replaces the value for this entry and returns the previous value, enabling in-place modification during iteration. Option B, getHash(), does not exist on Map.Entry (hashing is handled by hashCode() inherited from Object), and option E, getValueOrDefault(), is not a Map.Entry method — it is a default method on the Map interface (getOrDefault(Object key, V defaultValue)), so neither belongs here.

Answer

getHash()

Answer

getValueOrDefault()

Question 3

Given the code snippet: List list = new ArrayList<>(List.of("A","B","C")); list.add("D"); System.out.println(list.size()); What is the result?

Accepted Answer

4. The code creates an ArrayList from a fixed-size list returned by List.of() using the ArrayList constructor, which creates a new mutable list containing the elements "A", "B", "C". The subsequent call to list.add("D") successfully appends the element, resulting in a list of size 4. The ArrayList constructor copies the elements, so the original immutable list is not modified.

Answer

5

Answer

UnsupportedOperationException

Answer

Compilation fails

Answer

3

Question 4

Given: Map map = new HashMap<>(); map.put("x", 10); map.put("y", 20); map.computeIfAbsent("x", k -> 30); map.computeIfPresent("z", (k,v) -> 40); System.out.println(map); What is the output?

Accepted Answer

{x=10, y=20}. `computeIfAbsent` only inserts a mapping if the key is absent, and since "x" already exists with value 10, the lambda is not executed and the value remains 10. `computeIfPresent` only modifies a mapping if the key is present, and since "z" is not in the map, the lambda is not executed and no entry is added. Thus the map remains {x=10, y=20}.

Answer

Compilation fails

Answer

{x=30, y=20}

Answer

{x=30, y=20, z=40}

Answer

{x=10, y=20, z=40}

Question 5

Which TWO of the following statements about the Collections framework are true?

Accepted Answer

Collections.unmodifiableList returns an unmodifiable view of the list.. Option B is correct because Collections.unmodifiableList returns a read-only view backed by the original list, so any attempt to modify it throws UnsupportedOperationException while changes to the underlying list remain visible. Option D is correct because Collections.synchronizedList wraps the given list so that each method is synchronized on the wrapper, making individual operations thread-safe (though iteration still requires external synchronization). Option A is false because Collections.checkedList performs runtime type checking on inserted elements, not compile-time checking. Option C is false because List.of returns an immutable list whose mutator methods throw UnsupportedOperationException. Option E is false because Arrays.asList returns a fixed-size list backed by the array, so elements can be set but add/remove operations are unsupported.

Answer

Collections.checkedList performs type checking at compile time.

Answer

List.of returns a mutable list.

Answer

Arrays.asList returns a mutable list of variable size.

Question 6

Which THREE of the following variable declarations are valid in Java 17?

Accepted Answer

List list = new ArrayList();. Option A is valid because List is a wildcard type that can reference an ArrayList; the wildcard makes the assignment compatible with any parameterized List. Option C is valid because List accepts a List, since Number is a supertype of Integer and satisfies the lower-bounded wildcard constraint. Option D is valid because List accepts a List, since Integer is a subtype of Number and satisfies the upper-bounded wildcard constraint. Option B is invalid because generics are invariant: List

Answer

List<Object> list = new ArrayList<String>();

Answer

List list = new ArrayList();

Question 7

Which TWO of the following will sort a List<String> in natural (ascending) order?

Accepted Answer

list.sort(Comparator.naturalOrder());. Option A is correct because List.sort(Comparator) accepts a Comparator, and Comparator.naturalOrder() returns a comparator that orders elements according to their natural ordering, which for String is lexicographic ascending order. Option D is correct because Collections.sort(List) sorts the specified list into ascending order according to the natural ordering of its elements, which for String means lexicographic ascending order. Option B is incorrect because the lambda (a,b) -> b.compareTo(a) reverses the comparison, producing descending order. Option C is incorrect because Collections.reverseOrder() supplies a comparator that imposes the reverse of natural ordering, yielding descending order. Option E is incorrect because Arrays.sort(list) does not apply to a List<String>; Arrays.sort accepts arrays, not List instances, so it would not compile for this argument type.

Answer

list.sort((a,b) -> b.compareTo(a));

Answer

Collections.sort(list, Collections.reverseOrder());

Answer

Arrays.sort(list);

Question 8

Which of the following collections maintains elements in the order they were inserted?

Accepted Answer

LinkedHashSet. LinkedHashSet maintains a doubly-linked list running through all of its entries, which defines the iteration ordering as the order in which elements were inserted into the set. This is specified in the Java documentation for LinkedHashSet, making it the only collection among the options that guarantees insertion order.

Answer

None of the above

Answer

PriorityQueue

Answer

HashSet

Answer

TreeSet

Question 9

Given: TreeSet set = new TreeSet<>(List.of(3,1,2)); set.add(2); System.out.println(set); What is the output?

Accepted Answer

[1,2,3]. E is correct because TreeSet maintains elements in their natural sorted order (ascending for integers). The list [3,1,2] is sorted to [1,2,3], and adding 2 again has no effect since TreeSet does not allow duplicates. The output is [1,2,3].

Answer

[1,2]

Answer

[1,3,2]

Answer

[3,1,2]

Answer

UnsupportedOperationException

Question 10

What does the following code print? List list = new ArrayList<>(List.of(1,2,3)); list.replaceAll(x -> x * 2); System.out.println(list);

Accepted Answer

[2,4,6]. The `replaceAll` method on a `List` applies the given `UnaryOperator` to each element, replacing it with the result. Here, `x -> x * 2` multiplies each integer by 2, so the list becomes `[2, 4, 6]`. Option B is correct because the code compiles and runs, producing that output.

Answer

[2,3,4]

Answer

Compilation fails

Answer

[1,4,9]

Answer

[1,2,3]

Question 11

Which of the following creates an immutable map with two entries?

Accepted Answer

Map.of("a",1,"b",2). `Map.of()` returns an immutable map containing exactly the key-value pairs provided as arguments. This factory method was introduced in Java 9 and creates a fixed-size, unmodifiable map with the two entries "a"→1 and "b"→2.

Answer

new HashMap<>(Map.of("a",1,"b",2))

Answer

Collections.emptyMap()

Answer

None of the above

Answer

new HashMap<>()

Question 12

Given: Set set = new HashSet<>(List.of(1,2,3)); List list = new ArrayList<>(set); Collections.sort(list); System.out.println(list); What is the output?

Accepted Answer

[1,2,3]. The code creates a HashSet from a List of 1, 2, 3, then copies it into an ArrayList. HashSet does not guarantee order, but Collections.sort() sorts the list in natural ascending order (1, 2, 3). The output is [1,2,3], making option C correct.

Answer

[3,2,1]

Answer

Compilation fails

Answer

[1,2]

Answer

[1,3,2]

Question 13

Which of the following correctly converts an array of strings to a List?

Accepted Answer

Arrays.asList(array) returns a fixed-size list backed by the array.. `Arrays.asList(array)` returns a fixed-size list backed by the original array, meaning changes to the list (like `set`) reflect in the array, but structural modifications (like `add` or `remove`) throw `UnsupportedOperationException`. This is the standard, exam-relevant way to convert an array to a List in Java.

Answer

Collections.toList(array) converts the array.

Answer

List.of(array) returns a mutable list.

Answer

Arrays.asList(array) returns a new ArrayList with a copy of the elements.

Question 14

Given ArrayList numbers = new ArrayList<>(List.of(1,2,3,4)); Which statement will insert element 10 at index 2?

Accepted Answer

numbers.add(2,10);. The `ArrayList.add(int index, E element)` method inserts the specified element at the given index, shifting existing elements to the right. Here, `numbers.add(2,10)` inserts 10 at index 2, resulting in the list [1,2,10,3,4].

Answer

numbers.add(10,2);

Answer

numbers.set(2,10);

Answer

numbers.insert(2,10);

Question 15

A TreeSet<String> is used to store a list of employee names. The set currently contains "Alice", "Bob", "Charlie". What is the output after calling set.add("Bob")?

Accepted Answer

false. A TreeSet implements the Set interface, which does not allow duplicate elements. When calling add("Bob") on a TreeSet that already contains "Bob", the add method returns false, indicating that the element was not added, and the set remains unchanged.

Answer

Exception

Answer

The set becomes ["Alice","Bob","Bob","Charlie"]

Answer

true