Collections

Question 1

What classes extend List?

Answer 1

ArrayList, LinkedList

Question 2

What classes extend Queue?

Answer 2

LinkedList, Deque

Question 3

What classes extend Set?

Answer 3

HashSet, TreeSet

Question 4

What classes extend Map?

Answer 4

HashMap, TreeMap

Question 5

What is characteristic for ArrayList?

Answer 5

+ quick retrieval of items
- slow add and remove of items

Question 6

What is characteristic for LinkedList?

Answer 6

+ quick add items to start and end
- slow retrieval via index

Question 7

What is characteristic for Deque?

Answer 7

+ quick add items to start and end
- slow retrieval via index

Question 8

What is characteristic for HashSet?

Answer 8

+ quick add of items and duplicate check
- sort is removed

Question 9

What is characteristic for TreeSet?

Answer 9

+ sorting
- slow add of items

Question 10

Objects added to TreeSet need to implement?

Answer 10

Comparable interface

Question 11

What is characteristic for HashMap?

Answer 11

+ quick adding and retrieval of items
- no sort

Question 12

What is characteristic for TreeMap?

Answer 12

+ sorted
- slow add and retrieval of items

Question 13

What does equal method in Map and Set do?

Answer 13

It checks if key-value pairs in Maps are the same, or values in Set are the same between to collections. Order is not checked, except in some collections like LinkedHashMap

Question 14

How can collections be initialized?

Answer 14

Trough construction initialization and immutable factory methods - of(), asList() and copyOf().

Question 15

What happens if you try and change immutable collections?

Answer 15

RuntimeException is thrown

Question 16

When initializing new ArrayList, can diamond operator be used with var type?

Answer 16

Yes, compiler implicitly adds Object to diamond operator

Question 17

What is the rule about collections that have auto. sort?

Answer 17

They cannot contain null values

Question 18

MAP: Map.of()

Answer 18

Creates an immutable map containing zero or a specified number of entries

Question 19

How do we create an immutable map containing zero or a specified number of entries?

Answer 19

Map.of()

Question 20

MAP: Map.ofEntries()

Answer 20

Creates an immutable map with multiple entries

Question 21

How do we create an immutable map with multiple entries?

Answer 21

Map.ofEntries()

Question 22

MAP: Map.entry()

Answer 22

Creates a map entry (key-value pair) for use in map creation methods

Question 23

How do we create a map entry (key-value pair) for use in map creation methods?

Answer 23

Map.entry()

Question 24

MAP: containsKey()

Answer 24

Returns true if the map contains the specified key

Question 25

How do we check if a map contains a specific key?

Answer 25

containsKey()

Question 26

MAP: containsValue()

Answer 26

Returns true if the map contains the specified value

Question 27

How do we check if a map contains a specific value?

Answer 27

containsValue()

Question 28

MAP: merge()

Answer 28

Combines two maps or adds a key-value pair if the key doesn't exist

Question 29

How do we combine two maps or add a key-value pair if the key doesn't exist?

Answer 29

merge()

Question 30

MAP: putIfAbsent()

Answer 30

Adds a key-value pair to the map if the key is not already present

Question 31

How do we add a key-value pair to a map if the key is not already present?

Answer 31

putIfAbsent()

Question 32

MAP: forEach()

Answer 32

Iterates through each entry in the map and performs a specified action

Question 33

MAP: replace()

Answer 33

Replaces the entry for a key only if it's currently mapped to a specific value

Question 33

QUEUE: element()

Answer 33

Retrieves, but does not remove, the head of the queue

Question 33

How do we iterate through each entry in a map and perform a specified action?

Answer 33

forEach()

Question 34

MAP: getOrDefault()

Answer 34

Returns the value for a given key, or a default value if the key isn't present

Question 35

QUEUE: add()

Answer 35

Inserts an element at the end of the queue

Question 35

How do we get a value for a given key, or a default value if the key isn't present?

Answer 35

getOrDefault()

Question 36

How do we replace the entry for a key only if it's currently mapped to a specific value?

Answer 36

replace()

Question 36

MAP: replaceAll()

Answer 36

Replaces all values in the map by applying a function to each entry

Question 37

How do we replace all values in a map by applying a function to each entry?

Answer 37

replaceAll()

Question 38

How do we insert an element at the end of the queue?

Answer 38

add()

Question 39

How do we retrieve, but not remove, the head of the queue?

Answer 39

element()

Question 40

QUEUE: remove()

Answer 40

Retrieves and removes the head of the queue

Question 41

How do we retrieve and remove the head of the queue?

Answer 41

remove()

Question 42

DEQUE: addFirst()

Answer 42

Inserts an element at the front of the deque

Question 43

How do we insert an element at the front of the deque?

Answer 43

addFirst()

Question 44

DEQUE: removeFirst()

Answer 44

Retrieves and removes the first element of the deque

Question 45

How do we retrieve and remove the first element of the deque?

Answer 45

removeFirst()

Question 46

DEQUE: getFirst()

Answer 46

Retrieves, but does not remove, the first element of the deque

Question 46

DEQUE: addLast()

Answer 46

Inserts an element at the end of the deque

Question 47

How do we retrieve, but not remove, the first element of the deque?

Answer 47

getFirst()

Question 48

How do we insert an element at the end of the deque?

Answer 48

addLast()

Question 48

DEQUE: removeLast()

Answer 48

Retrieves and removes the last element of the deque

Question 49

How do we retrieve and remove the last element of the deque?

Answer 49

removeLast()

Question 50

DEQUE: getLast()

Answer 50

Retrieves, but does not remove, the last element of the deque

Question 51

What does deque's push() method do?

Answer 51

Adds an element to the front of the deque

Question 52

What does deque's pop() method do?

Answer 52

Removes and returns the first element

Question 53

What does deque's peek() method do?

Answer 53

Retrieves, but does not remove, the first element

Question 54

LIST: add()

Answer 54

Appends the specified element to the end of the list

Question 55

How do we append an element to the end of the list?

Answer 55

add()

Question 56

LIST: remove()

Answer 56

Removes the first occurrence of the specified element from the list

Question 57

How do we remove the first occurrence of a specified element from the list?

Answer 57

remove()

Question 58

LIST: get()

Answer 58

Returns the element at the specified position in the list

Question 59

How do we retrieve the element at a specific position in the list?

Answer 59

get()

Question 60

LIST: replaceAll()

Answer 60

Replaces each element of the list with the result of applying the operator to that element

Question 61

How do we replace all elements in the list by applying a function to each?

Answer 61

replaceAll()

Question 62

LIST: set()

Answer 62

Replaces the element at the specified position in the list with the specified element

Question 63

How do we replace an element at a specific position in the list?

Answer 63

set()

Question 64

LIST: sort()

Answer 64

Sorts the list according to the order induced by the specified Comparator

Question 65

How do we sort the elements in the list?

Answer 65

sort()

Question 66

What is the difference of remove(int) and remove(Integer) methods in List?

Answer 66

int one removes by index, Integer one removes object from List

Question 67

LIST: toArray(new Object[0])

Answer 67

Returns an array containing all of the elements in the list in proper sequence

Question 68

How do we convert a list to an array of Objects?

Answer 68

toArray(new Object[0])

Question 69

LIST: toArray()

Answer 69

Returns an array containing all of the elements in the list in proper sequence (element type depends on the list's type)

Question 70

How do we convert a list to an array of its elements?

Answer 70

toArray()

Question 71

What is the difference between Comparable and Comparator?

Answer 71

Comparable is interface that enables sorting, Comparator is used to create more complex sorting

Question 72

What method does Comparable use?

Answer 72

compareTo(Object o)

Question 73

What is the result of Comparable's compareTo method?

Answer 73

- Negative if instance objects is smaller than comparing object - 0 if instance object is equal to comparing object - Positive if instance object is bigger than comparing object

Question 74

How do we sort ASC and DESC using Comparable?

Answer 74

ASC - instance object - comparing object DESC - comparing object - instance object

Question 75

In what package is Comparable?

Answer 75

java.lang

Question 76

How is Comparable used?

Answer 76

It is implemented by a class

Question 77

What method does Comparator use?

Answer 77

compare(Object a, Object b)

Question 78

How is Comparator used?

Answer 78

It is instantiated using Comparator.comparing()

Question 79

What methods does Comparator also have?

Answer 79

comparingLong(), comparingInt(), comparingDouble(), thenComparingLong(), thenComparingInt(), thenComparingLong()

Question 80

Where can we use Comparator?

Answer 80

In Collections.sort() and Collections.binarySearch()