Ch14 Generics and Collections

Question 1

What is a method reference?

Answer 1

An operation that looks like this:

System.out::println;

Question 2

Given the following statement, write an equivalent statement using method reference.

Consumer> lambda = x -> Collections.sort(x);

Answer 2

Consumer> methodRef = Collections::sort;

Question 3

Given the following lamda expression, write an equivalent method reference.

var str = “abc”;
s -> str.startsWith(s);

Answer 3

var str = “abc”;
str::startsWith;

Question 4

What is a constructor reference?

Answer 4

A constructor reference is a special type of method reference that uses new instead of a method, and it instantiates an object. It is common to use a Supplier.

example
Supplier> methodRef = ArrayList::new;

Question 5

Which functional interface takes one parameter and returns a result? (not a boolean).

Answer 5

Function

Question 6

How do you convert a boolean to its wrapper class Boolean?

Answer 6

using Boolean.valueOf();

Question 7

How do you convert a byte with variable name x to its wrapper class Byte?

Answer 7

using Byte.valueOf((byte)x);

Question 8

How do you convert a short with variable name x to its wrapper class Short?

Answer 8

using Short.valueOf((short)x);

Question 9

What does the following code output?

List numbers = new ArrayList<>();
numbers.add(1);
numbers.add(Integer.valueOf(3));
numbers.add(Integer.valueOf(5));
numbers.remove(1);
numbers.remove(Integer.valueOf(5));
System.out.println(numbers);

Answer 9

It prints [1] because the remove() method is overloaded.

The first remove() call removes the object at index 1.
The second remove() call removes the object that should be removed.

Question 10

How can we use the diamond operator to make the following code shorter?

List numbers = new ArrayList();

Answer 10

List numbers = new ArrayList<>();

Question 11

How can we use the diamond operator to make the following code shorter?

List< Object> numbers = new ArrayList< Object>();

Answer 11

List< Object> numbers = new ArrayList<>();

Question 12

What are the four main interfaces in the Java Collections Framework?

Answer 12

• List
• Set
• Queue
• Map

Question 13

What is a List?

Answer 13

A list is an ordered collection of elements that allows duplicate entries. Elements in a list can be accessed by an int index.

Question 14

What is a Set?

Answer 14

A set is a collection that does not allow duplicate entries.

Question 15

What is a Queue?

Answer 15

A queue is a collection that orders its elements in a specific order for processing. A typical queue processes its elements in a first-in, first-out orderm but other orderings are possible.

Question 16

What is a Map?

Answer 16

A map is a collection that maps keys to values, with no duplicate keys allowed. The elements in a map are key/value pairs.

Question 17

What interfaces/data structures extend the Collection interface? And which one does not?

Answer 17

Interfaces that extend Collection:

• List
• Queue
• Set

Interface that does not extend Collection:
* Map

Question 18

How do we insert a new element into a Collection and what does it return?

Answer 18

Using the add() method.

It returns true, if the operation was succesful.

Question 19

How do we remove a new element into a Collection and what does it return?

Answer 19

Using the remove() method.

It returns true, if the operation was succesful.

Question 20

Is the following code valid?

Collection birds = new ArrayList<>();

birds. add(“hawk”);
birds. add(“hawk”);
birds. add(“hawk”);

for (String bird : birds) {
birds.remove(bird);
}

Answer 20

No. Java does not allow removing elements from a list while using the enhanced for loop.

It will throw a ConcurrentModificationException

Question 21

How do we discard all elements of a Collection?

Answer 21

using the clear() method.

Question 22

How do we remove an element using a condition?

Answer 22

With the removeIf() method, which takes a Predicate and returns true/false.

Question 23

How do we loop through a Collection (without using a for/while loop)?

Answer 23

Using the method forEach() which takes a Consumer.

Question 24

What is the main benefit of using an ArrayList?

Answer 24

• It resizes automatically
• You can look up any element in constant time.

Question 25

What data structure is best if you want to use one as a queue?

Answer 25

Linkedlist

Question 26

What factory methods exist to create a List?

Answer 26

• Arrays.asList(varags)
• List.of(varargs)
• List.copyOf(collection)

Question 27

Which factory methods for creating a list return an immutable list?

Answer 27

• List.of(varargs)
• List.copyOf(collection)

Question 28

Is it possible to add an element to a list created by Arrays.asList(varargs)?

Answer 28

No.

Question 29

What is the result of the following code?

String[] array = new String[] {“a”, “b”};
List asList = Arrays.asList(array);

asList.set(0, “x”);

System.out.println(Arrays.toString(array));

Answer 29

[x, b]

Question 30

What is the result of the following code?

String[] array = new String[] {“a”, “b”};
List asList = Arrays.asList(array);

asList.add(0, “x”);

System.out.println(asList);

Answer 30

This code throws an UnsupportedOperationException because Arrays.asList returns a fixed sized list.

Question 31

Given the following list, what List method can we use to multiply all numbers by 2?

List numbers = Arrays.asList(1, 2, 3);

Answer 31

numbers.replaceAll(x -> x * 2);

Question 32

What (often-used) implementations of Set exist?

Answer 32

• HashSet
• TreeSet

Question 33

How does a HashSet store elements?

Answer 33

A HashSet stores its elements in a hash table, which means the keys are a hash and the values are an Object. This means that it uses the hashCode() method of the objects to retrieve them more efficiently.

Question 34

What is the main benefit of a Set?

Answer 34

Adding elements and checking wether an element is in the set both have constant time.

Question 35

What is the disadvantage of a Set?

Answer 35

You lose the order in which you inserted the elements.

Question 36

What is the main benefit of a TreeSet?

Answer 36

It is always in sorted order.

Question 37

What is the disadvantage of a TreeSet?

Answer 37

Adding and checking wether an element exists takes longer than with a HashSet, especially as the tree grows larger.

Question 38

What factory methods exist to create a Set?

Answer 38

• Set.of(varargs) // immutable
• Set.copyOf(Set) //immutable

Question 39

What does the second add method return?

Set set = new HashSet<>();

set. add(66);
set. add(66);

Answer 39

False.

Question 40

What does the print statement print?

Set set = new TreeSet<>();

set. add(66);
set. add(8);
set. add(10);

set.forEach(System.out::println);

Answer 40

8
10
66

(retains natural order)

Question 41

When do you want to use a queue?

Answer 41

When elements are added and removed in a specific order.

Question 42

Which Queue implementation is more efficient than a LinkedList?

Answer 42

ArrayDeque

Question 43

What does the queue method element() do?

Answer 43

Returns next element or throws an exception if empty queue.

Question 44

What does the queue method offer() do?

Answer 44

Adds an element to the back of the queue and returns wether succesful (boolean).

Question 45

What does the queue method remove() do?

Answer 45

Removes and returns next element or throws an exception if empty queue.

Question 46

What does the queue method add() do?

Answer 46

Adds an element to the back of the queue and returns true or throws an exception.

Question 47

What does the queue method poll() do?

Answer 47

Removes and returns next element or returns null if empty queue.

Question 48

What does the queue method peek() do?

Answer 48

Returns next element or returns null if empty queue. Does not remove the element from the queue.

Question 49

When do you want to use a Map?

Answer 49

When you want to identify values by a key. For example, when you use the contact list in your phone, you look up “George” rather than looking through ech phone number in turn.

Question 50

What Map interface method can we use to check if a key is inside a Map?

Answer 50

boolean containsKey(Object key)

Question 51

What Map interface method can we use to check if a value is inside a Map?

Answer 51

boolean containsValue(Object key)

Question 52

What Map interface method can we use to return a Set of key/value pairs?

Answer 52

Set> entrySet();

Question 53

What Map interface method can we use to loop through each key/value pair?

Answer 53

void forEach(BiConsumer(K key, V value))

Question 54

What Map interface method can we use to get a value, and if not exists, return a default value?

Answer 54

V getOrDefault(Object key, V defaultValue);

Question 55

What Map interface method can we use to get a Set of all keys?

Answer 55

Set keySet()

Question 56

What Map interface method can we use to add a value if key is not present?

Answer 56

V putIfAbsent(K key, V value)

Question 57

What Map interface method can we use to get a Collection of all values?

Answer 57

Collection values()

Question 58

What does the following code do?

Map map = new HashMap<>();
map.put(“koala”, “bamboo”);

System.out.println(map.contains(“koala”));

Answer 58

It will not compile because Map doesn’t have a contains method (Collection does!).

Map does have a containsKey() and containsValue() method.

Question 59

What method do we use for a Map object to get a value and return null if not present?

Answer 59

get()

Question 60

What method do we use for a Map object to update a value based on the condition of a function?

Answer 60

merge(K key, V value, Function( func))

Question 61

What is the result of the following code?

BiFunction mapper = (v1, v2) -> null;
Map favorites = new HashMap<>();

favorites. put(“Jenny”, “Bus Tour”);
favorites. put(“Tom”, “Bus Tour”);
favorites. merge(“Jenny”, “Skyride”, mapper);
favorites. merge(“Sam”, “Skyride”, mapper);

System.out.println(favorites);

Answer 61

{Tom=Bus Tour, Sam=Skyride}

Because the function returns null, existing keys will be removed. Therefore, Jenny was removed.

Sam was added because it was not present in the HashMap.

Question 62

What is the result of the following code?

BiFunction mapper = (v1, v2) -> v1.length() > v2.length ? v1 : v2;
Map favorites = new HashMap<>();
favorites.put(“Jenny”, null);
favorites.merge(“Jenny”, “Skyride”, mapper);

System.out.println(favorites);

Answer 62

{Jenny=Skyride}

If the value of a key is null and merge() is called on it, it will be updated. The mapper will not be called (which would produce a NullPointerException).

Question 63

Which data structures does not allow null keys or null values?

Answer 63

• TreeMap does not allow - Null Key
• TreeSet does not allow - Null Element

Question 64

Given the following characters: A, 8, b, when sorting a String, what is the result when sorting the characters?

Answer 64

1. 8
2. b
3. A
   numbers before letters.
   lowercase before uppercase letters.

Question 65

How do we use Collections.sort() (without parameters) on an object that we create ourself such that it sorts the object without issues?

Answer 65

By implementing the Comparable interface.

Question 66

Given the following code, how do we implement the compareTo method to be able to sort on name (ascending)?

public class Duck implements Comparable {
private String name;
// …
public int compareTo(Duck d) {
// code here
}
}

Answer 66

return name.compareTo(d.name);

Question 67

Given the following code, how do we implement the compareTo method to be able to sort on id (descending)?

public class Duck implements Comparable {
private int id;
// …
public int compareTo(Duck d) {
// code here
}
}

Answer 67

return d.id - id;

Question 68

How can we apply custom sorting without using the Comparable interface?

Answer 68

Using the Comparator interface

Question 69

When do you want to use a Comparator vs a Comparable?

Answer 69

A Comparator should be used when you want to sort objects in different ways at different times.

A Comparable should be used when you want to sort objects in one specific way.

Question 70

What package does Comparable reside in?

Answer 70

java.lang

Question 71

What package does Comparator reside in?

Answer 71

java.util

Question 72

What abstract method name does Comparator define?

Answer 72

compare()

Question 73

What abstract method name does Comparable define?

Answer 73

compareTo()

Question 74

Given a Squirrel class with a getName() method, how do we sort it in descending order using Comparator static helper methods?

Answer 74

Comparator.comparing(Squirrel::getName).reversed();

Question 75

Is the following code valid?

public class SortRabbits {
static class Rabbit { int id; }
public static void main(String[] args) {
List rabbits = new ArrayList<>();
rabbits.add(new Rabbit());
Collections.sort(rabbits);
}
}

Answer 75

It does not compile because Java knows that the Rabbit class is not Comparable.

Question 76

Is the following code valid?

public class SortRabbits {

static class Rabbit { int id; }

public static void main(String[] args) {
List rabbits = new ArrayList<>();
rabbits.add(new Rabbit());
Comparator c = (r1, r2) -> r1.id - r2.id;
Collections.sort(rabbits, c);
} }

Answer 76

This code is valid.

Collections.sort can take in a Comparator object when you don’t want to use natural order.

Question 77

Is the following code valid?

// …
static class Rabbit { int id; }
Set rabbits = new TreeSet<>();
rabbits.add(new Rabbit());
// …

Answer 77

It is invalid because TreeSet is not able to sort it and then throws a ClassCastException.

To fix this, Rabbit needs to implement Comparable.

Question 78

Is the following code valid?

// …
static class Rabbit { int id; }
Set rabbits = new TreeSet<>((r1, r2) -> r1.id - r2.id);
rabbits.add(new Rabbit());
// …

Answer 78

This is valid. You can pass a Comparator to a TreeSet to let Java know how you want to sort the Objects.

Question 79

What is type erasure?

Answer 79

Type erasure is the process of removing the generic syntax from your code, which allows your code to be compatible with older versions of Java that do not contain generics.

Question 80

How do you write a public generic method that take a generic parameter and returns a generic object?

Answer 80

public <T> T returnObject(T t) {
return t;
}

Question 81

is the following statement valid?

public class Lol {
public static T hello(T t) {}
}

Answer 81

No, because it is missing a formal parameter type.

correct:

public static <T> T hello(T t) {}

Question 82

What is an unbounded wildcard?

Answer 82

An unbouded wildcard represents any data type and uses the syntax ‘?’.

List< ?> a = new ArrayList();

Question 83

Are these statements equal?

List< Object> x1 = new ArrayList<>();
var x2 = new ArrayList<>();

Answer 83

No.

• x1 is of type List, x2 is of type ArrayList.
• x2 can only be assigned to List

Question 84

Is the following statement valid?

ArrayList< Number> list = new ArrayList< Integer>();

Answer 84

No. A generic type cannot use a subclass.

Question 85

How do we use an Upper-Bounded wildcard to fix the following code?

List list = new ArrayList< Number>();

Answer 85

List< ? extends Number> list = new ArrayList< Number>();

Question 86

Does the following code compile?

List< ? extends Bird> birds = new ArrayList();

birds. add(new Sparrow());
birds. add(new Bird());

Answer 86

It does not because when working with upper bounds or unbounded wildcards, the list becomes immutable. This is because Java does not know what type List< ? extends Bird> really is.

Question 87

What is a lower bound wildercard and how does it look like?

Answer 87

With lowerbound we tell Java the generic type has to be of a certain type or a superclass of that type.

example: List < ? super String>

Question 88

Why does the following code not work?

public static void addSound(List< ? extends Object> list) {
list.add(“quack”);
}

Answer 88

User upper-bounded wildcards will result in an immutable list.

Question 89

Why does the following code not work?

public static void addSound(List list) {
list.add(“quack”);
}

Answer 89

With generics, you must pass an exact match. String does not match Object.

Question 90

Given the following code, which of these statements compile and which don’t?

Class A {}
Class B extends A {}
Class C extends B {}

List< A> list1 = new ArrayList< A>();
List < ? extends B> list2 = new ArrayList< A>();
List < ? super A> list3 = new ArrayList();
List> list4 = new ArrayList< ? extends A>();
List < ? super B> list5 = new ArrayList();

Answer 90

1. compiles
2. does not compile
3. compiles
4. does not compile
5. compiles

Question 91

Does the following code compile?

< ? extends T> second(List< ? extends T> list) {
return list.get(0);
}

Answer 91

The return type isn’t actually a type.

Question 92

Why does the following code not compile?

< B> B third(List< B> list) {
return new B();
}

Answer 92

B is referred as a type parameter but also happens to be a class name (return new B()). Because B is seen as a type parameter, it cant be instantiated in the return statement.

Question 93

Which of the following statements are correct?

A. A List stores elements in a sorted order.
B. A Set keeps the elements sorted and a List keeps the elements in the order they were added.
C. A SortedSet keeps the elements in the order they were added.
D. An OrderedSet keeps the elements sorted.
E. An OrderedList keeps the elements ordered.
F. A NavigableSet keeps the elements sorted.

Answer 93

A NavigableSet is a SortedSet extended with navigation methods reporting closest matches for given search targets. Methods lower, floor, ceiling, and higher return elements respectively less than, less than or equal, greater than or equal, and greater than a given element, returning null if there is no such element. Since NavigableSet is a SortedSet, it keeps the elements sorted.

Answer = F