7.1 Intro to ArrayLists
-
An ArrayList is a utility from the
java.utilpackage -
To declare a variable, use the format
ArrayList<DataType> variableName = new ArrayList<DataType>(initial number of elements); -
Unlike arrays, ArrayLists are mutable (can be resized after initialization)
-
Functions as a more versatile array
- Does technically take more space than an array but for the purposes of CSA it shouldn’t matter
- Does technically take more space than an array but for the purposes of CSA it shouldn’t matter
Difference between Array and Arraylist:
| Array | Arraylist |
|---|---|
| Fixed length | Resizable length |
| Fundamental Java Feature | Part of a Framework |
| An object with no methods | A Class with many methods |
| Not very flexible | Flexible |
| Can store primitives | Cannot store primitives - stores objects instead |
Example of Array: Basketball players on court
Example of Arraylist: Dodgeball players on court
Question
When should we use ArrayLists vs Arrays?
Answer: Use ArrayLists if we want to change the size of the list in some way. Use Arrays if we don’t want to change the size of the array and do not want to take up as much space.
Showing how ArrayLists are mutable:
import java.util.ArrayList;
class ArrayListExample {
public static void main(String[] args)
{
// Size of ArrayList
int n = 5;
// Declaring ArrayList with initial size n
ArrayList<Integer> arr1 = new ArrayList<Integer>(n);
// Declaring ArrayList without initial size
ArrayList<Integer> arr2 = new ArrayList<Integer>();
// Printing ArrayList
System.out.println("Array 1:" + arr1);
System.out.println("Array 2:" + arr2);
// Appending new elements to the end of the list
for (int i = 1; i <= n; i++) {
arr1.add(i);
arr2.add(i);
}
// Printing ArrayList
System.out.println("Array 1:" + arr1);
System.out.println("Array 2:" + arr2);
}
}
ArrayListExample.main(null);
Array 1:[]
Array 2:[]
Array 1:[1, 2, 3, 4, 5]
Array 2:[1, 2, 3, 4, 5]
7.1 Popcorn Hack
Create an ArrayList that stores the integers 5, 8, 14, 18, 24, 39, and 56 and only print the numbers that are divisible by 3.
import java.util.ArrayList;
public class DivisibleBy3 {
public static void main(String[] args) {
ArrayList<Integer> nums = new ArrayList<Integer>(7);
nums.add(5);
nums.add(8);
nums.add(14);
nums.add(18);
nums.add(24);
nums.add(39);
nums.add(56);
System.out.println("Numbers divisble by 3:");
for(int num: nums){
if(num % 3 == 0){
System.out.println(num);
}
}
// write code here
}
}
DivisibleBy3.main(null);
Numbers divisble by 3:
18
24
39
Vocabulary Refresher
-
An element is a single value in the array
-
The **index** of an element is the position of the element in the array or ArrayList
- The first index of an ArrayList, like arrays is also 0
- The first index of an ArrayList, like arrays is also 0
-
The length of an array is the number of elements in the array.
7.2 Arraylist Methods
- add()
- Adding a value to the ArrayList
- Can be used to create an element at a specific index in the ArrayList - when this happens, everything at the positions of index and higher are moved to the right by 1
- get()
- Get the value of an index in the ArrayList
- set()
- Change the value at an index in the ArrayList
- remove()
- Delete the value at in index in the ArrayList
- clear()
- Remove all values in an ArrayList
- size()
- Get the length of the ArrayList
import java.util.ArrayList;
public class Test {
public static void main(String[] args) {
ArrayList<Integer> test = new ArrayList<>();
System.out.println(test.size());
test.add(1); //index 0
test.add(2); // index 1
test.add(3); // index 2
test.add(4); // index 3
test.add(5); // index 4
test.add(6); // index 5
test.add(7); // index 6
System.out.println(test);
int i = test.set(1, 200); // prints what used to be at index 1
int x = test.remove(4); // prints what was removed
System.out.println(i);
System.out.println(x);
System.out.println(test);
}
}
Test.main(null);
0
[1, 2, 3, 4, 5, 6, 7]
2
5
[1, 200, 3, 4, 6, 7]
Printing an ArrayList
- You can’t print an array without a loop
- You can print ArrayLists with just one
System.out.println()
import java.util.ArrayList;
public class Main {
public static void main(String[] args)
{
ArrayList<String> arr = new ArrayList<String>(); //initializes the arraylist object
arr.add("I agree");
arr.add(0, "You agree");
System.out.println(arr);
arr.set(0, "They agree");
arr.add("I disagree");
arr.remove(2);
System.out.println();
System.out.println(arr);
}
}
Main.main(null);
[You agree, I agree]
[They agree, I agree]
7.2 Popcorn Hack
Finish the code below so that all duplicates in the ArrayList are removed.
import java.util.ArrayList;
public class RemoveDuplicates {
public static void main(String[] args) {
ArrayList<Integer> arr = new ArrayList<>(); // creating new arraylist of integers
// adding integers to the arraylist
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(9);
arr.add(2);
System.out.println("List with duplicates: "+arr);
// iterate through the list in reverse to avoid modifying the list while iterating
for(int i = arr.size() - 1; i >= 0; i--){
for(int j = i - 1; j >= 0; j--){
if(arr.get(i).equals(arr.get(j))){
arr.remove(i);
break; // break out of inner loop once duplicate is found
}
}
}
System.out.println("List without duplicates: "+arr);
}
}
// for later: nested for loop
RemoveDuplicates.main(null);
List with duplicates: [1, 7, 9, 13, 3, 7, 9, 2]
List without duplicates: [1, 7, 9, 13, 3, 2]
Array to ArrayList
- You can use the Arrays.asList() method to convert an existing array to an ArrayList
public class ArrayListFromArray
{
public static void main(String[] args)
{
String[] names = {"Kim", "Tay", "Tran", "Ethan", "Sheng", "Raymond"};
ArrayList<String> namesList = new ArrayList<String>(Arrays.asList(names));
System.out.println(namesList);
}
}
ArrayListFromArray.main(null);
[Kim, Tay, Tran, Ethan, Sheng, Raymond]
7.3 Traversing ArrayLists
-
You can traverse an ArrayList the same way as an array, with some exceptions:
- Deleting elements in the ArrayList while iterating over the ArrayList needs to be carefully done
- Using an enhanced for loop can result in the ConcurrentModificationException error
- Do not delete elements in an ArrayList while using an enhanced for loop
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<>();
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(9);
arr.add(2);
for (int i = 0; i < arr.size(); i++) //for loop, would work the same as with an array
{
System.out.print(arr.get(i) + " ");
}
System.out.println();
for (Integer i : arr) //enhanced for loop without removing, same as an array
{
System.out.print(i + " ");
}
}
main(null);
1 7 9 13 3 7 9 2
1 7 9 13 3 7 9 2
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<>();
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(9);
arr.add(2);
for (int i = 0; i < arr.size(); i++) //throws a ConcurrentModificationException
{
if (i % 2 == 0)
{
arr.remove(i);
}
}
}
main(null);
public class Correction {
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<>();
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(9);
arr.add(2);
System.out.println("Original: "+arr);
for (int i = 0; i < arr.size(); i++) //throws a ConcurrentModificationException
{
if (arr.get(i) % 2 == 0)
{
arr.remove(i);
}
}
System.out.println("New: "+arr);
}
}
Correction.main(null);
Original: [1, 7, 9, 13, 3, 7, 9, 2]
New: [1, 7, 9, 13, 3, 7, 9]
Popcorn Hack
Traverse the following ArrayList using a loop. Remove each element that is a multiple of 4.
public class Hack {
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<>();
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(80);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(8);
arr.add(2);
arr.add(16);
System.out.println("Original: "+arr);
for(int i = 0; i < arr.size(); i++){
if(arr.get(i) % 4 == 0){
arr.remove(arr.get(i));
}
}
System.out.println("New: "+arr);
}
}
Hack.main(null);
Original: [1, 7, 9, 80, 13, 3, 7, 8, 2, 16]
New: [1, 7, 9, 13, 3, 7, 2]
7.4 Developing Algorithms Using ArrayLists
- For the most part, algorithms in ArrayLists and arrays are very similar.
Popcorn Hacks
Find the maximum, minimum, and sum of an ArrayList.
public class Hack1 {
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<>();
arr.add(1);
arr.add(7);
arr.add(9);
arr.add(80);
arr.add(13);
arr.add(3);
arr.add(7);
arr.add(8);
arr.add(2);
arr.add(16);
int min = arr.get(0);
for(int i = 0; i < arr.size(); i++){
if(min > arr.get(i)){
min = arr.get(i);
}
}
int max = arr.get(0);
for(int j = 0; j < arr.size(); j++){
if(max < arr.get(j)){
max = arr.get(j);
}
}
int sum = 0;
for(int k = 0; k < arr.size(); k++){
sum += arr.get(k);
}
System.out.println("Minimum: "+min);
System.out.println("Maximum: "+max);
System.out.println("Sum: "+sum);
}
}
Hack1.main(null);
Minimum: 1
Maximum: 80
Sum: 146
7.5 Searching
- Searching in an ArrayList allows you to find an element, if it exists, an index is returned
- All that is needed to search any linear structure is a standard for loop and if block
- Typically, if the element is not found then -1 is returned, however a boolean value can also be used
Example of Linear Search
import java.util.ArrayList;
public class LinearSearch {
public static int linearSearch(ArrayList<Integer> list, int target) {
for (int i = 0; i < list.size(); i++) {
if (list.get(i) == target) {
return i; // return index of target element if found
}
}
return -1; // return -1 if the target element is not in the array
}
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(5);
numbers.add(10);
numbers.add(15);
numbers.add(20);
numbers.add(25);
int target = 15; // target of search
int result = linearSearch(numbers, target);
if (result != -1) {
System.out.println(target + " found at index " + result);
} else {
System.out.println(target + " not found in the ArrayList.");
}
}
}
LinearSearch.main(null);
15 found at index 2
- The linear search algorithm searches for a specific target value (15 in this example) within the ArrayList by iterating through its elements one by one
- It checks if the element at the current index (list.get(i)) is equal to the target value
- If a match is found, the index is returned; else, it continues the loop until the end of the ArrayList is reached
Popcorn Hack
- Finish the code below, write code that checks if favoriteFlavor is in chipsBag
- Using a loop, print a message saying if favoriteFlavor is in the bag
import java.util.ArrayList;
import java.util.Scanner;
public class FavoriteChip {
public static void main(String[] args) {
ArrayList<String> chipsBag = new ArrayList<>();
Scanner scanner = new Scanner(System.in);
chipsBag.add("BBQ");
chipsBag.add("Cheddar");
chipsBag.add("Salt and Pepper");
System.out.print("Enter your favorite chip flavor: ");
String favoriteFlavor = scanner.nextLine();
System.out.println("Your input: "+favoriteFlavor);
// implement your own search logic here
for(int i = 0; i < chipsBag.size(); i++){
if(favoriteFlavor.equals(chipsBag.get(i))){
System.out.printf("Favorite flavor %s found ",favoriteFlavor);
break;
} else {
System.out.printf("Favorite flavor %s not found ",favoriteFlavor);
break;
}
}
}
}
FavoriteChip.main(null);
Enter your favorite chip flavor:
Your input: BBQ
Favorite flavor BBQ found
7.6 Sorting
- There are 2 main sorting algorithms that Collegeboard focuses on, selection sort and insertion sort
Selection sort
- Selection sort divides the ArrayList into two “subarrays,” the first is sorted and the second is unsorted.
- Selection sort is easy to implement and is useful for sorting through small datasets.
Example of Selection sort
import java.util.ArrayList;
public class SelectionSort {
public static void main(String[] args) {
ArrayList<Integer> arrayList = new ArrayList<>();
arrayList.add(64);
arrayList.add(25);
arrayList.add(12);
arrayList.add(22);
arrayList.add(11);
selectionSort(arrayList);
System.out.println("Sorted ArrayList: " + arrayList);
}
public static void selectionSort(ArrayList<Integer> arr) {
int n = arr.size();
for (int i = 0; i < n - 1; i++) {
int minIndex = i;
for (int j = i + 1; j < n; j++) {
if (arr.get(j) < arr.get(minIndex)) {
minIndex = j;
}
}
// Swap the found minimum element with the element at index i
int temp = arr.get(i);
arr.set(i, arr.get(minIndex));
arr.set(minIndex, temp);
}
}
}
SelectionSort.main(null);
Sorted ArrayList: [11, 12, 22, 25, 64]
-
Start with an ArrayList, [64, 25, 12, 22, 11]
-
We want to arrange these numbers in order from smallest to largest, like [11, 12, 22, 25, 64]
-
selectionSort starts at the beginning of the list and looks for the smallest number in the whole list. In this case, it’s 11
-
It swaps 11 with the first number in the list (which is 64) so that 11 comes first
-
Now, the list looks like [11, 25, 12, 22, 64]
-
The function repeats this process for the remaining numbers, finding the next smallest (which is 12) and putting it in the second position
-
This continues until all numbers are in order
-
The final sorted list is [11, 12, 22, 25, 64]
Insertion Sort
- In insertion sort, we assume that the first element is already sorted
- The second element is then taken and is either inserted before the first element or kept in place to make the first 2 elements sorted
- Insertion sort is adaptive as it does not need to perform as many comparisons and swaps as selection sort. If you expect the data to be partially ordered, insertion sort is the better option.
Example of Insertion Sort
import java.util.ArrayList;
public class InsertionSort {
public static void insertionSort(ArrayList<Integer> arr) {
int n = arr.size();
for (int i = 1; i < n; i++) {
int key = arr.get(i);
int j = i - 1;
// Move elements of arr[0..i-1] that are greater than key
// to one position ahead of their current position
while (j >= 0 && arr.get(j) > key) {
arr.set(j + 1, arr.get(j));
j = j - 1;
}
arr.set(j + 1, key);
}
}
public static void main(String[] args) {
ArrayList<Integer> arrayList = new ArrayList<>();
arrayList.add(12);
arrayList.add(11);
arrayList.add(13);
arrayList.add(5);
arrayList.add(6);
System.out.println("Original ArrayList: " + arrayList);
insertionSort(arrayList);
System.out.println("Sorted ArrayList: " + arrayList);
}
}
InsertionSort.main(null);
Original ArrayList: [12, 11, 13, 5, 6]
Sorted ArrayList: [5, 6, 11, 12, 13]
-
Start with an ArrayList of numbers, [12, 11, 13, 5, 6]
-
We want to arrange these numbers in order from smallest to largest, like [5, 6, 11, 12, 13]
-
insertionSort starts with the second element (11)
-
It compares 11 to the elements on its left. It finds that 11 is smaller than 12, so it shifts 12 one position to the right to make space for 11
-
The ArrayList now looks like [11, 12, 13, 5, 6]
-
The function repeats this process for each element in the list. It moves 13 to the right place and then 5 and 6
-
After sorting all the elements, the ArrayList is now sorted in ascending order: [5, 6, 11, 12, 13]
7.6 Popcorn Hack
Given an ArrayList of String objects, sort the following ArrayLists on ascending order of word length.
1)
import java.util.ArrayList;
public class SortByLength {
public static void main(String[] args) {
ArrayList<String> words = new ArrayList<>(); // creating new arraylist
// adding words to the list
words.add("theater");
words.add("connection");
words.add("seasonal");
words.add("feast");
words.add("meeting");
int n = words.size();
for (int i = 0; i < n - 1; i++) {
int minIndex = i;
for (int j = i + 1; j < n; j++) {
if (words.get(j).length() < words.get(minIndex).length()) {
minIndex = j;
}
}
// Swap the found minimum element with the element at index i
String temp = words.get(i);
words.set(i, words.get(minIndex));
words.set(minIndex, temp);
}
for(String word : words){
System.out.println(word);
}
}
}
SortByLength.main(null);
feast
theater
meeting
seasonal
connection
7.7 Ethical Issues Around Data Collection
When collecting data in a Java program, data security is VERY IMPORTANT. This involves any program that deals with
- Necessary personal information
- Information that is associated with results
Removing Data
- When collected data is no longer needed, be sure to remove the information from your application
- Removal of unnecessary data reduces risk of data breaches and protects sensitive information from unauthorized access
Anonymizing Data
- When collecting user information, avoid asking for private information to identify users ie. phone numbers, social security numbers, etc.
Collegeboard Example
Scanner inputScanner = new Scanner(System.in);
String fullName = inputScanner.nextLine();
// implementation of code
int identifier = fullName.hashCode();
fullName = null;
// further implementation
In this example…
- The user would type their full name in which it is used in the program
- After, the identifier equates to the hash value of the original string and the fullName is set to null
- Using the hash value instead of the actual name adds a degree of anonymity for the user
- Turning the fullName to null also indicates that the code no longer needs to store the name of the user
This minimizes the possible impact of the data by reducing the likelihood of unintentional exposure of the user’s name within the program.
Hacks for Unit 7
- Complete all questions and popcorn hacks
- Write a Java program that creates an ArrayList of integers. Create methods for adding, removing, setting, getting, etc numbers and also sort the list. Then, utilize user input to determine which methods will actually be run and enable the user to also determine the specific index that a number will be added, removed.
Challenge
You are given an ArrayList of Student objects. Each Student has a name (String) and a GPA (double). Create a program that sorts the Student objects in descending order of GPA.
ArrayList Methods Program
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
public class IntegerList {
private ArrayList<Integer> numbers = new ArrayList<>();
public void addNumber(int num, int index) {
numbers.add(index, num);
}
public void removeNumber(int index) {
numbers.remove(index);
}
public void setNumber(int num, int index) {
numbers.set(index, num);
}
public int getNumber(int index) {
return numbers.get(index);
}
public void sortNumbers() {
Collections.sort(numbers);
}
public void printNumbers() {
System.out.println(numbers);
}
public static void main(String[] args) {
IntegerList list = new IntegerList();
Scanner scanner = new Scanner(System.in);
while (true) {
System.out.println("Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit");
System.out.println("Enter your choice: ");
int choice = scanner.nextInt();
switch (choice) {
case 1:
System.out.println("Enter number to add: ");
int numToAdd = scanner.nextInt();
System.out.println("Enter index to add at: ");
int addIndex = scanner.nextInt();
list.addNumber(numToAdd, addIndex);
break;
case 2:
System.out.println("Enter index to remove from: ");
int removeIndex = scanner.nextInt();
list.removeNumber(removeIndex);
break;
case 3:
System.out.println("Enter number to set: ");
int numToSet = scanner.nextInt();
System.out.println("Enter index to set at: ");
int setIndex = scanner.nextInt();
list.setNumber(numToSet, setIndex);
break;
case 4:
System.out.println("Enter index to get number from: ");
int getIndex = scanner.nextInt();
System.out.println("Number at index " + getIndex + ": " + list.getNumber(getIndex));
break;
case 5:
list.sortNumbers();
break;
case 6:
list.printNumbers();
break;
case 7:
scanner.close();
return;
default:
System.out.println("Invalid choice! Try again.");
}
}
}
}
IntegerList.main(null);
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Enter number to add:
Enter index to add at:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Enter number to add:
Enter index to add at:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Enter number to add:
Enter index to add at:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
[16, 25, 44]
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Enter index to remove from:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
Enter index to remove from:
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
[44]
Hello! What would you like to do with your list? 1) Add, 2) Remove, 3) Set, 4) Get, 5) Sort, 6) Print, 7) Exit
Enter your choice:
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
class Student {
String name;
double gpa;
public Student(String name, double gpa) {
this.name = name;
this.gpa = gpa;
}
@Override
public String toString() {
return "Name: " + this.name + ", GPA: " + this.gpa;
}
}
public class SortByGPA {
public static void selectionSort(ArrayList<Student> list){
for(int i = 0; i < list.size() - 1; i++){
int maxIndex = i;
for(int j = i+1; j < list.size(); j++){
if(list.get(j).gpa > list.get(maxIndex).gpa){
maxIndex = j;
}
}
Student temp = list.get(maxIndex);
list.set(maxIndex, list.get(i));
list.set(i, temp);
}
}
public static void main (String[] args) {
// creating new arraylist to store student objects
ArrayList<Student> students = new ArrayList<>();
// add student objects to ArrayList
students.add(new Student("John Doe", 3.8));
students.add(new Student("Monkey Monkey", 4.6));
students.add(new Student("Emaad Mir", 0.0));
students.add(new Student("Jane Doe", 4.3));
System.out.println("Original List:");
for (Student student : students) {
System.out.println(student);
}
System.out.println("\nSorted List:");
selectionSort(students);
for (Student student : students) {
System.out.println(student);
}
}
}
SortByGPA.main(null);
Original List:
Name: John Doe, GPA: 3.8
Name: Monkey Monkey, GPA: 4.6
Name: Emaad Mir, GPA: 0.0
Name: Jane Doe, GPA: 4.3
Sorted List:
Name: Monkey Monkey, GPA: 4.6
Name: Jane Doe, GPA: 4.3
Name: John Doe, GPA: 3.8
Name: Emaad Mir, GPA: 0.0