Skillstorm Intro to Coding - Data Structures in Java

Question 1

In Java, every Object has two comparison methods. What are they?

Answer 1

equals and hashCode Methods

Question 2

The equals and hashCode methods must be _________ to work properly.

Why?

Answer 2

Overridden

Without them, you would have to create very large “if” comparisons, comparing every field from an object, making code really confusing and hard to read.

Question 3

Fill in the Blanks of the attached image

Answer 3

boolean equals Object

Question 4

Fill in the blanks of the following code:

public _____ _____ () {

final int prime = 31;

int result = 1;

result = prime * result + size;

if (topping == null) {

result = prime * result + 0;

}else {

result = prime * result + topping.hashCode();

}

return result;

}

Answer 4

int hashCode

Question 5

A List is an _________ collection that may contain _________ elements.

Answer 5

Ordered

duplicate

Question 6

What are the two types of general-purpose list implementations?

Answer 6

ArrayList and LinkedList

Question 7

Are Java LinkedList automatically Doubly LinkedList?

Answer 7

Yes

Question 8

ArrayLists are resizable. But by what size do they increase when they are full?

Answer 8

50%

Question 9

ArrayList and LinkedList are both ordered by ________ position.

Answer 9

index

Question 10

Because you can add and remove from the heads and tails of LinkedList, this makes them perfect for __________ and __________.

Answer 10

Stacks (and) Queues

Question 11

What is Last In First Out? Stacks or Queues

Answer 11

Stacks

Question 12

What is First In First Out? Stacks or Queues

Answer 12

Queues

Question 13

Manipulating LinkedList. See image…

Answer 13

add

add

remove

Question 14

Manipulating ArrayList. See attached image…

Answer 14

< String >

add

get

size

Question 15

What is a Queue?

Answer 15

A collection for holding elements prior to processing

Question 16

A ________ is a double-ended queue.

Answer 16

Deque

Question 17

A Deque supports ________, removal, and __________ of elements at both end-points!

Answer 17

insertion,

examination

Question 18

Deque interface implements both ______ and ______ at the same time.

Answer 18

Stacks

Queues

Question 19

Queue Manipulation. See attached images…

Answer 19

queue. offer
queue. peek
queue. poll

Question 20

Stack Manipulation. See attached images…

Answer 20

push

push

pop

peek

Question 21

1. Is a Map an object that maps equal keys?
2. What are mapped to unique keys?
3. Do Maps include methods for basic operations?
4. Do Maps include methods for non-bulk operations?
5. Do Maps includes methods for collection views?

Answer 21

1. False: They map unique keys
2. Values
3. True
4. False: They include methods for bulk operations
5. True

Question 22

Java contains three general-purpose Map implementations. What are they?

Answer 22

HashMap, TreeMap, and LinkedHashMap

Question 23

HashMap Manipulation. See attached image…

Answer 23

put

put

get

size

containsKey

Question 24

Can a Set contain duplicate elements?

Answer 24

No

Question 25

Sets add a stronger contract on the behavior of the _______ and _______. It allows Set instances to be compared meaningfully even if their implementation types differ.

Answer 25

equals (and) hashCode

Question 26

What are the three general-purpose Set implementations?

Answer 26

HashSet, TreeSet, and LinkedHashSet

Question 27

HashSet Manipulation. See attached images…

Answer 27

public int hashCode

public boolean equals Object obj

Question 28

LinkedList is a(n) ______ collection.

Answer 28

Ordered

Question 29

LinkedList has an underlying array that changes size as needed? True or False

Answer 29

False

Question 30

Two objects with the same hashCode must be equal? True or False

Answer 30

False

Question 31

A Queue is a first-in last-out collection? True or False

Answer 31

False

Question 32

Two objects that are equal must have the same hashCode? True or False

Answer 32

True

Question 33

The equals method in java.lang.Object compares two objects and returns a(n)______.

Answer 33

boolean

Question 34

A LinkedList in Java is _______ by default.

Answer 34

Doubly-linked

Question 35

The performance of accessing a value in a HashMap is ______________.

Answer 35

Constant Time

Question 36

An ArrayList will grow ___ in size when it reaches its maximum capacity.

Answer 36

50%

Question 37

A Stack is a ______________ collection

Answer 37

last-in first-out

Question 38

A HashSet will maintain duplicate values. True or False….

Answer 38

False

Question 39

In Java with Object Equality, you can decide when two objects are equivalent, but with what method and how is it written in Java?

Answer 39

1. The method is ==> java.lang.Object.equals method
2. In Java it is written as ==> public boolean equals(Object other)

Question 40

In Object Equality, does it check the memory location by default?

Answer 40

Yes

Question 41

In Object equality what must you override to compare object state?

Answer 41

The method

Question 42

True or False, in Object Equality, some data structures require unique objects…

Answer 42

True

Question 43

True or False, the hashcode isn’t used to give a consistent integer that represents the object…

Answer 43

False

Question 44

How is the hashCode method written in code?

Answer 44

public int hashCode()

Question 45

Is hashCode loved by HashMap, HashSet, and others?

Answer 45

Yes

Question 46

True or False, HashCode puts objects into a smaller group to optimize searching…

Answer 46

True

Question 47

Fill in the blanks of this code:

public _____ _____ ( _____ obj) {

Pizza other = (Pizza) obj;

if (size != other.size)

return false;

if (topping == null)

if (other.topping != null)

return false;

} else if (!topping.equals(other.topping))

return false;

return true;

}

Answer 47

boolean equals ( Object obj)

Question 48

True or False, ArrayList is a class in the Collections API in Java…

Answer 48

True

Question 49

What does ArrayList do in Java?

Answer 49

1. Creates an array
2. Provides methods for working with ArrayList

Question 50

What is the ArrayList default size when it is created?

Answer 50

16

Question 51

True or False, ArrayList is not mutable…

Answer 51

False

Question 52

What happens if an ArrayList fills up?

Answer 52

It grows 50% larger to accommodate more elements

Question 53

True or False, ArrayList does not allow for duplicate elements…

Answer 53

False

Question 54

True or False, in ArrayList order of insertion is preserved…

Answer 54

True

Question 55

LinkedList is a Class in the Collections API in Java, True or False?

Answer 55

True

Question 56

What is the LinkedList a series of?

Answer 56

Nodes

Question 57

What are Nodes in LinkedList?

Answer 57

A Node wraps up a value (String, Pizza, etc.)

Question 58

How are Nodes in LinkedList connected?

Answer 58

They are “Linked” by a reference to the next Node

Question 59

Are LinkedList singly-linked or doubly-linked?

Answer 59

Doubly-linked

Question 60

How are doubly-linked Nodes connected?

Answer 60

The Nodes have a previous and next reference.

Question 61

LinkedLists allow duplicate elements, True or False?

Answer 61

True

Question 62

Order of insertion is not preserved in LinkedList, True or False?

Answer 62

False

Question 63

Fill in the blanks of the following code:

public class LinkedListExample {

public static void main(String[] args) {

List list = new LinkedList();

// write Dill pickle to the list

list.______ (new Pickle(“Dill”));

// write Sweet pickle to the list

list.______ (new Pickle(“Sweet”));

// delete the pickle at index 1

list.______ (1);

}

}

class Pickle{

String flavor;

public Pickle(String flavor) {

this.flavor = flavor;

}

}

Answer 63

1. add
2. add
3. remove

Question 64

Fill in the blanks in the following code:

// create an ArrayList of Strings

ArrayList _____ list = new ArrayList<>();

// add XYZ to the list

list.______ (“XYZ”);

// print element at index 2

System.out.println(list.______ (2));

// print how many Strings are in the list

System.out.println(list.______ ());

Answer 64

1. 2. add
2. get
3. size

Question 65

Is the Queue an interface in the Collections API in Java?

Answer 65

Yes

Question 66

True or False, the queue is First-in/First-out…

Answer 66

True

Question 67

1. When you Push in a Queue, what happens?
2. What you Pop in a Queue, what happens?

Answer 67

1. Node goes to the tail
2. Node goes to the head

Question 68

True or False, the Queue doesn’t allow for duplicate elements…

Answer 68

False

Question 69

True or False, in a Queue, the order of insertion is preserved…

Answer 69

True

Question 70

True or False, the Stack is a Class in the Collections API in Java…

Answer 70

True

Question 71

Stack is Last-in/First-out, True or False?

Answer 71

False and True, it can be First-in/Last-out and Last-In/First-Out!

Question 72

1. In a Stack what happens when we Push?
2. In a Stack what happens when we Pop?

Answer 72

1. Data/Node goes to the tail
2. Data/Node comes from the tail

Question 73

True or False, Stacks don’t allow for duplicate elements…

Answer 73

False

Question 74

In Stacks order of insertion is preserved, True or False?

Answer 74

True

Question 75

Fill in the following blanks in the code:

public class QueueExample {

public static void main(String[] args) {

Queue queue = new LinkedList();

// add new Person to tail ‘push’

______._____ (new Person());

// check what’s at the head of the queue

______._____();

// ‘pop’ the head of the queue

System.out.println( _____._____ ());

}

}

class Person{}

Answer 75

1. queue.offer
2. queue.peek
3. queue.poll

Question 76

Fill in the blanks in the following code:

public class StackExample {

public static void main(String[] args) {

Stack stack = new Stack();

// add two cards to the stack

stack. _____ (new Card(“A of Spades”));
stack. _____ (new Card(“K of Diamonds”));

// remove the top of the stack

System.out.println(stack._____ ());

// look at (but not remove) top of the stack

System.out.println(stack._____ ());

}

}

class Card {

String value;

public Card(String value) {

this.value = value;

}

}

Answer 76

1. push
2. push
3. pop
4. peek

Question 77

A HashMap is not a class in a Collections API that stores key-value pairs, True or False?

Answer 77

False

Question 78

In HashMaps, how do you identify the value?

Answer 78

By the Key

For example, a Social Security Number maps to a U.S. Citizen.

Question 79

True or False, Keys do not have to be unique whereas values must be unique…

Answer 79

False, keys must be unique while values have no restriction

Question 80

What happens if a key already has a value and then you add a different value?

Answer 80

It updates the value for that key.

For example, if the key is 79 and the value assigned to it is “Joe Rogan takes horse dewormer!” and then you change the value of 79 to “Joe Rogan actually takes Soolantra (generic brand for Ivermectin for humans for the last 16 years),” then the value for the key 79 updates to the last quote!

Question 81

In a HashMap, stores “Entry” objects in a Queue, True or False?

Answer 81

False, the “Entry” objects are stored in an array.

Question 82

What does the Object’s HashCode do?

Answer 82

It helps calculate the array index to store the object in.

Question 83

In HashMaps, if the calculation for 2 indices yields the same value, what is the outcome?

Answer 83

HashMap collisions occur and then Java appends the Entry in a a LinkedList.

Question 84

In HashMaps, if the LinkedList gets too long, what problem occurs?

Answer 84

Read operations are much slower.

Question 85

What is the time complexity of HashMap Reads?

Answer 85

O(1) - Constant Time

Question 86

In HashMaps, when LinkedLists get too long, what happens?

Answer 86

When the LinkedList is 8+, it is converted to a red-black tree.

Question 87

Fill in the blanks of the following code for HashMap:

public class HashMapExample {

public static void main(String[] args) {

HashMap citizens = new HashMap<>();

// add two entries to the map

citizens. _____ (“5001”, new Citizen(“Brian”));
citizens. _____ (“4524”, new Citizen(“Diane”));

// retrieve the value for the key 5001

System.out.println(citizens._____ (“5001”));

// print how many entries are in the map

System.out.println(citizens._____ ());

// print if the key 5001 is in the map

citizens._____ (“5001”);

}

}

class Citizen{

String name;

Citizen(String name) {

this.name = name;

}

}

Answer 87

put

put

get

size

containsKey

Question 88

True or False, HashSet is a class in the Collections API…

Answer 88

True

Question 89

In the HashSet, are duplicates are allowed?

Answer 89

No, HashSets only store unique objects.

Question 90

True or False, in HashSets order of elements is guaranteed…

Answer 90

False, Order of elements is not guaranteed.

Question 91

Do HashSets have indices available?

Answer 91

No, there is no index available unlike in Lists.

Question 92

Where do HashSets store values?

Answer 92

HashSets store values inside of a HashMap.

Use set.add();

to store values tied to a key in a HashMap.

Question 93

Fill in the blanks of the following HashSet Code:

public class HashSetExample {

public static void main(String[] args)

{

HashSetExample friends = new HashSet();

friends. add(new Friend(“Dan”, 24));
friends. add(new Friend(“Dan”, 24));

}

}

class Friend{

String name;

Friend(String name, int age){

this. name = name;
this. age = age;

}

// hashcode method

_____ _____ _____ () {

return this.age;

}

// equals method

_____ ______ ______ ( _____ ______ ) {

Friend other = (Friend) obj;

if (age != other.age)

return false

if (name == null) {

if (other.name != null)

return false

} else if (!name.equals(other.name))

return false

return true

}

}

Answer 93

1. public int hashCode
2. public boolean equals ( Object obj )

Question 94

LinkedList is a(n) ______ collection

Answer 94

Ordered

Question 95

True or False, LinkedList has an underlying array that changes size as needed…

Answer 95

False

Question 96

True or False, Two objects with the same hashcode must be equal…

Answer 96

False

Question 97

True or False, A Queue is a first-in last-out collection…

Answer 97

False

Question 98

True or False, Two objects that are equal must have the same hashcode…

Answer 98

True

Question 99

The equals method in java.lang.Object compares two objects and returns a(n) _____ .

Answer 99

boolean

Question 100

A LinkedList in Java is _____ by default.

Answer 100

Doubly-linked

Question 101

The performance of accessing a value in a HashMap is _____ .

Answer 101

Constant time

Question 102

An ArrayList will grow _____ in size when it reaches its maximum capacity.

Answer 102

50%

Question 103

A Stack is a _____ collection.

Answer 103

last-in first-out

Question 104

A HashSet will maintain duplicate values, True or False?

Answer 104

False

Question 105

Why does the main method have the static modifier?

Answer 105

Because the static modifier allows us to call all members in main without having to create an Object!