Generics and Collections

Question 1

Declaring a class with generics

Answer 1

public class Crate  {
    private T contents;
    public T emptyCrate() {
         return contents;
    }
}

T can be used throughout the class and when the class is instantiated then the T’s become the class object type you specify

Question 2

How to call a method that uses generics

Answer 2

Two ways

methodName(“a string”) //Detects automatically

ClassName.methodName(“a string”) //Explicitly

Question 3

Raw Collections

Answer 3

Collections that do not use generics

Question 4

What goes wrong in this code?

class Dragon {}
class Unicorn { }
public class LegacyDragons {
    public static void main(String[] args) {
        List unicorns = new ArrayList();
        unicorns.add(new Unicorn());
        printDragons(unicorns);
     }
    private static void printDragons(List dragons) {
         for (Dragon dragon: dragons) { 
             System.out.println(dragon);
          }
     }
}

Answer 4

The List unicorns doesn’t use generics, so when it adds a Unicorn, that’s fine but it sees it as an Object. So basically, it compiles because that’s technically legal to pass to printDragons, but the for loop then tries top cast the Unicorn element to Dragon, which causes a ClassCastException.

It will also give you a compiler warning, but still compile

Question 5

What goes wrong with this code?

public class LegacyAutoboxing {
   public static void main(String[] args) { 
         java.util.List numbers = new java.util.ArrayList();
         numbers.add(5);
         int result = numbers.get(0); 
   }
}

Answer 5

When the number 5 is added to the List, it is added just as an Object since there isn’t a type specified by generics. So when you try to store that value in an int, it can’t be unboxed how an Integer usually would be able to.

Question 6

Unbounded Wildcard

Answer 6

Specified with a ?

public static void printList(List> list)

This indicates that the List could be of anything.

The difference here between just specifying List without generics, is that when we don’t use generics it will treat the list passed in as List, which could allow the wrong types of elements to be added.

Question 7

Upper-bounded Wildcard

Answer 7

Specified with a ? extends className

public static void printList(List extends Number> list)

This means the List can be of Number itself, or any class that is a subclass of Number

Question 8

Lower-bounded Wildcards

Answer 8

Specified with a ? super className

public static void printList(List super String> list)

This means that the List can be of String itself, or any class that is a superclass of String

Question 9

Mutability of Lists declared with wildcards

Answer 9

With unbounded or upper-bounded, the Lists are immutable, meaning you cannot add anything to them.

With lower-bounded, you can add anything that is the lower-bound specified or a subclass of that lower-bound

Question 10

Collection

Answer 10

A group of objects contained in a single object

Question 11

List

Answer 11

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

Question 12

Set

Answer 12

A collection that does not allow duplicate entries

Question 13

Queue

Answer 13

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

Question 14

Map

Answer 14

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

Question 15

add()

Answer 15

Inserts a new element into the Collection and returns whether it was
successful.

boolean add(E element)

Question 16

remove()

Answer 16

Removes a single matching value in the Collection and returns whether it was successful.

boolean remove(Object object)

Removes the element at the specified index

void remove(int index)

Question 17

clear()

Answer 17

Discards all elements in the Collection

void clear()

Question 18

contains()

Answer 18

Checks if a certain value is in the Collection

boolean contains(Object object)

Question 19

Benefit of ArrayList

Answer 19

You can look up any element in constant time.

Adding or removing an element is slower than accessing an element.

This makes an ArrayList a good choice when you are reading more often than writing

Question 20

Benefit of LinkedList

Answer 20

You can access, add, and remove from the
beginning and end of the list in constant time.

The tradeoff is that dealing with an arbitrary
index (not necessarily first or last) takes linear time.

This makes a LinkedList a good choice when you’ll be using it as Queue.

Question 21

What should you use instead of Stack?

Answer 21

ArrayDeque

Question 22

List method used to add an element at a specific index

Answer 22

void add(int index, E element)

Question 23

List method to grab a specific element in the list

Answer 23

E get(int index)

Question 24

List method to replace the element at a specific index

Answer 24

E set(int index, E e)

Question 25

List method to find the position in the List that matches a specified element

Answer 25

//For first match
int indexOf(Object o)

//For the last match
int lastIndexOf(Object o)

Both return -1 if there is no match found

Question 26

Benefit of HashSet

Answer 26

Adding elements and checking if an element is in the set are done in constant time

The tradeoff is that you lose the order in which you added the elements, but this is the case with sets usually, so if you care that much then don’t use a set ya dummy.

Question 27

Benefit of TreeSet

Answer 27

The set is always in sorted order.

The tradeoff is that adding and checking if an element is present are done in O(logn)

Question 28

TreeSet method

E lower(E e)

Answer 28

Returns the greatest element in the set that is

Question 29

TreeSet method

E floor(E e)

Answer 29

Returns the greatest element in the set that is <=e, or null if there is no such element

Question 30

TreeSet method

E ceiling(E e)

Answer 30

Returns the smallest element in the set that is >=e, or null if there is no such element

Question 31

TreeSet method

E higher(E e)

Answer 31

Returns the smallest element in the set that is >e, or null if there is no such element

Question 32

ArrayDeque

Answer 32

A double-ended queue that stores its elements in a resizable array

Like a LinkedList, but more efficient

Question 33

ArrayDeque method

boolean add(E e)

Answer 33

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

Question 34

ArrayDeque method

E element()

Answer 34

Returns next element or throws an exception if empty queue

Question 35

ArrayDeque method

boolean offer(E e)

Answer 35

Adds an element to the back of the queue and returns whether successful

Question 36

ArrayDeque method

E remove()

Answer 36

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

Question 37

ArrayDeque method

void push(E e)

Answer 37

Adds an element to the front of the queue

Question 38

ArrayDeque method

E poll()

Answer 38

Removes and returns next element or

returns null if empty queue

Question 39

ArrayDeque method

E peek()

Answer 39

Returns next element or returns null if

empty queue

Question 40

ArrayDeque method

E pop()

Answer 40

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

Question 41

HashMap

Answer 41

Stores keys in a hash table

Adding elements and retrieving the element by key both have constant time.

Question 42

TreeMap

Answer 42

Stores keys in a sorted tree structure

Adding and checking if a key is present are both O(log n).

Question 43

What is special about Map in this discussion?

Answer 43

It does not extend Collection

Question 44

Map method

void clear()

Answer 44

Removes all keys and values from the map.

Question 45

Map method

boolean isEmpty()

Answer 45

Returns whether the map is empty.

Question 46

Map method

int size()

Answer 46

Returns the number of entries (key/value pairs) in the map.

Question 47

Map method

V get(Object key)

Answer 47

Returns the value mapped by key or null if none is mapped.

Question 48

Map method

V put(K key, V value)

Answer 48

Adds or replaces key/value pair. Returns previous value or null.

Question 49

Map method

V remove(Object key)

Answer 49

Removes and returns value mapped to key. Returns null if none.

Question 50

Map method

boolean containsKey(Object key)

Answer 50

Returns whether key is in map.

Question 51

Map method

boolean containsValue(Object)

Answer 51

Returns value is in map.

Question 52

Map method

Set keySet()

Answer 52

Returns set of all keys.

Question 53

Collection values()

Answer 53

Returns Collection of all values.

Question 54

Which of the data structures discussed cannot contain null elements?

Answer 54

TreeSet cannot contain null elements

TreeMap cannot contain null keys, but CAN contain null values

ArrayDeque cannot contain null elements

Question 55

Comparable interface

Answer 55

public interface Comparable {
public int compareTo(T o);
}

Only contains the compareTo() method

Question 56

compareTo() rules

Answer 56

The number zero is returned when the current object is equal to the argument to compareTo().

A number less than zero is returned when the current object is smaller than the argument to compareTo().

A number greater than zero is returned when the current object is larger than the argument to compareTo().

Question 57

Comparator

Answer 57

A functional interface that you can use to sort a collection of objects by however you specify its compare() function.

You can define it in long form, (for example):

Comparator byWeight = new Comparator() {
     public int compare(Duck d1, Duck d2) { 
          return d1.getWeight()—d2.getWeight();
     }
};

Or as a lambda:

Comparator byWeight = (d1, d2) -> d1.getWeight()—d2.getWeight();

Question 58

How to use a comparator to sort

Answer 58

Collections.sort(listname, byWeight);

Question 59

What package is Comparable in? Comparator?

Answer 59

Comparable is in java.lang

Comparator is in java.util

Question 60

What is the method in Comparable? Comparator?

Answer 60

Comparable has compareTo(T obj)

Comparator has compare(T obj1, T obj2))

Question 61

Method References

Answer 61

A way to make the code shorter by reducing some of the code that can be inferred and simply mentioning the name of the method.

Question 62

How can you shorten the following line of code using a method reference?

Comparator byWeight = (d1, d2) -> DuckHelper.compareByWeight(d1, d2);

Answer 62

Comparator byWeight = DuckHelper::compareByWeight;

Question 63

How can you shorten the following line of code using a method reference?

String str = “abc”;
Predicate lambda2 = s -> str.startsWith(s);

Answer 63

Predicate methodRef2 = str::startsWith;

Question 64

How can you shorten the following line of code using a method reference?

Predicate lambda3 = s -> s.isEmpty();

Answer 64

Predicate methodRef3 = String::isEmpty;

Question 65

How can you shorten the following line of code using a method reference?

Supplier lambda4 = () -> new ArrayList();

Answer 65

Supplier methodRef4 = ArrayList::new;

Question 66

How to use

boolean removeIf(Predicate super E> filter)

Answer 66

If we have a List of Strings, an example is

list.removeIf(s -> s.startsWith(“A”));

Question 67

How to use

void replaceAll(UnaryOperator o)

Answer 67

List list = Arrays.asList(1, 2, 3);
list.replaceAll(x -> x*2);
System.out.println(list); // [2, 4, 6]

Note: The lambda may only take one argument because it is a UnaryOperator

Question 68

How to loop through a Collection using a lambda or method reference

Answer 68

list. forEach(c -> System.out.println(c));

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

Question 69

How to insert a new key/value pair into a Map

Answer 69

Map mymap = new HashMap<>();
mymap.put(“My key”, “My value”);

Note: you can use putIfAbsent() if you want to make sure you don’t overwrite a value that is already in the map

Question 70

merge()

Full Version:
merge(K key, V value, BiFunction super V,? super V,? extends V> remappingFunction)

Answer 70

Uses the BiFunction to make a decision on changing a the value for the specified key

Returns the result of the BiFunction

Question 71

What happens if you call merge() on a key with a null value or a key that doesn’t exist?

Answer 71

It doesn’t call the BiFunction (this would cause a NullPointerException) and it just inserts the new value in question

Question 72

What happens if you call merge() on a key and the BiFunction is called and returns null?

Answer 72

The key is removed from the map