Unit 7: ArrayList

Mastering the concept of Java’s ArrayList. AP Exam weighting: 2.5-7.5%.

7.1: ArrayList Intro

• ArrayLists are dynamic, meaning their size can grow or shrink as needed, but arrays are static in size
• Instead of creating a new array of a different size and copying the data from the initial array to the new one, we can use ArrayLists

In order to use the ArrayList class, the ArrayList class needs to be imported from the java util package. This can be done by writing import java.util.ArrayList at the top of the class file.

import java.util.ArrayList;  // Import the ArrayList class

// Declare and initialize an ArrayList of integers
ArrayList<Integer> numbersList = new ArrayList<>();

ArrayList objects are created in the same fashion as other object classes. The primary difference with ArrayLists is that the element type of the ArrayList must be specified using angled bracket <>. In this example, E represents the data type that will be used in the ArrayList. This can be replaced by an object data type:

ArrayList<E> list = new ArrayList<E>();

We can actually declare ArrayLists without specifying the type that will be included in the ArrayList, but specifying the data type is smarter because it allows the compiler to find errors before run time, so its more efficient and easy to spot errors.

ArrayList list = new ArrayList();

Quick lil popcorn hack

Create 2 ArrayLists, 1 called studentName and 1 called studentAge

public class Student
{
    public static void main(String[] args)
    {
        //Initialize your ArrayLists
        
    }
}

// new code
public class Student {
    public static void main(String[] args) {
        // Initialize the ArrayLists
        List<String> studentNames = new ArrayList<>();
        List<Integer> studentAges = new ArrayList<>();
    }
}

7.2: ArrayList Methods

Learning Objectives

Students will be able to represent collections of related object reference data using ArrayList objects.

Essential Knowledge

• Iteration statements provide a means to access all the elements stored within an ArrayList. This process is referred to as “traversing the ArrayList.”

• The following ArrayList methods, including what they do and when they are used, are part of the Java Quick Reference:

  • int size() - Returns the count of elements within the list.
  • boolean add(E obj) - Appends the object obj to the end of the list and returns true.
  • void add(int index, E obj) - Inserts obj at the specified index, shifting elements at and above that position to the right (incrementing their indices by 1) and increasing the list’s size by 1.
  • E get(int index) - Retrieves the element at the given index in the list.
  • E set(int index, E obj) - Replaces the element at the specified index with obj and returns the previous element at that index.
  • E remove(int index) - Deletes the element at the specified index, shifting all subsequent elements one index to the left, reducing the list’s size by one, and returning the removed element.

• Java allows the generic ArrayList<E>, where the generic type E specifies the type of element.

• When ArrayList<E> is specified, the types of the reference parameters and return type when using the methods are type E.

• ArrayList<E> is preferred over ArrayList because it allows the compiler to find errors that would otherwise be found at runtime.

Size of the ArrayList

• int size(); : Returns the number of elements in the list.

Consider the following code:

ArrayList<Integer> a1 = new ArrayList<>();
System.out.println(a1.size());

0

Adding Items to an ArrayList

• boolean add(E obj); : Appends obj to the end of the list and returns true.
• void add(int index, E obj) : Inserts obj at position index, as long as index is within the list’s length. It moves each element in the list 1 index higher and adds 1 to the list’s size.

Consider the following code:

ArrayList<Double> a2 = new ArrayList<>();
a2.add(1.0);
a2.add(2.0);
a2.add(3.0);
a2.add(1, 4.0);
System.out.println(a2);

[1.0, 4.0, 2.0, 3.0]

Let’s Look at an Example

Consider the following code:

ArrayList<String> h = new ArrayList<>();

h.add("Hello");
h.add("Hello");
h.add("HeLLO");
h.add("Hello");
h.add(1, "Hola");

h.add(26.2);
h.add(new String("Hello"));
h.add(false);

|   h.add(26.2);

incompatible types: double cannot be converted to java.lang.String

Now, consider this code:

ArrayList<String> g = new ArrayList<>();

g.add("Hello");
g.add("Hello");
g.add("HeLLO");
g.add("Hello");
g.add(1, "Hola");

g.add(new String("Hello"));

System.out.println(g);

[Hello, Hola, Hello, HeLLO, Hello, Hello]

Question: Why does this code work?

Deleting Items from an ArrayList

E remove(int index) : Removes the element at position index, and moves the elements at position index + 1 and higher to the left. It also subtracts one from the list’s size. The return value is the element formerly at position index.

// If you are confused of what list g is, look back at the previous code.
g.remove(3);
String former = g.remove(0);
System.out.println(former);

Hello

Updating Items in an ArrayList

E set(int index, E obj) : Replaces the element at position index with obj and returns the element formerly at position index.

String helloFormer = g.set(1, "Bonjour");
System.out.println(helloFormer);
System.out.println(g);

Hello
[Hola, Bonjour, Hello, Hello]

Accessing Items in an ArrayList

E get(int index) Returns the element at position index in the list.

String hello = g.get(3);
System.out.println(hello);
System.out.println(g);

Hello
[Hola, Bonjour, Hello, Hello]

Passing an ArrayList as a Method Parameter

The only time that it is wise to use ArrayList instead of ArrayList<E> is when it is as a function parameter and it is only using ArrayList<>.get(E) or ArrayList<>.size(). Consider the following code:

private void accessOnly(ArrayList arr) {
    if (arr.size() > 0) {
        System.out.println(arr.get(0)); // Change the index to the one you want to access
    } else {
        System.out.println("Array is empty");
    }
}

ArrayList<Integer> myList = new ArrayList<Integer>();
accessOnly(myList);

Array is empty

Returning an ArrayList from a Method

In order for you to return an ArrayList, the data type must be specified, and the return type must be the same as the return value. Consider the following code:

private ArrayList<String> returnTheSame() {
    ArrayList<String> arr = new ArrayList<String>(); // Initialize the ArrayList
    arr.add("Hello");
    return arr;
}

ArrayList<String> result = returnTheSame();
System.out.println(result);

[Hello]

Hacks

• The learning objective is that “Students will be able to represent collections of related object reference data using ArrayList objects.” What does this mean to you?

• Answer the following questions:

  • Look back at Size of the ArrayList. What does the code output and why?
  • 0 because there are 0 items in the array list
  • Look back at Adding items to an ArrayList. What does the code output and why? What type of function is void, and what will be the return value?
  • the add() method is a void method which doesnt return anything and simply adds stuff to the array
  • Look back at Example 1. What two lines did we remove? Why?
  • If an ArrayList is being used as a parameter, what are the only two methods I can use from it? What would happen if I tried to use any other methods?

• Using the Hack Helper, write code that will:

  • Add 2 items to the list.
  • Remove an item from the list anywhere of the user’s choice.
  • Replace am item anywhere in the list of the user’s choice.
  • Get the first and last element of the list, no matter the length.
  • Return the items added, removed, replaced, and the list’s size, in one string.

Hack Helper

public class ArrayListMethodsExample {
    private String manipulateList(/* You can put parameters here if you want to... */) {
        // Your code here
    }

    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<>();
        ArrayListMethodsExample example = new ArrayListMethodsExample();
        
        String output = example.manipulateList();
        System.out.println(output);
    }
}

|       private String manipulateList(/* You can put parameters here if you want to... */) {

|           // Your code here

|       }

missing return statement

7.3: Traversing Arraylists

Learning Objectives:

• With an Arraylist you can traverse objects using a for or while loop.

• Traversing objects is similar to iterating through objects.

Essential Knowledge:

• Iteration statements can be used to accsess all the elements in an Arraylist. This is called traversing the Arraylist.

• Deleting elements during a traversal of an Arraylist requires special techniques to avoid skiping elements. This is called traversing the Arraylist.

• The indicies of an Arraylist start at 0; If you try to use any value lower than 0, you will get an ArrayIndexOutOfBoundsException error

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> roster = new ArrayList<>();
        roster.add("Hello");
        roster.add("World");
        roster.add("Java");
        
        int sum = 0;
        for (int i = 0; i < roster.size(); i++) {
            String element = roster.get(i);
            if (element != null) {
                sum += element.length();
            }
        }
        System.out.println(sum);
    }
}

Breakdown:

• We are first declaring a new arraylist and adding a few elements.

• Next, we set the “sum” variable as 0.

• We set a for loop to traverse through the arraylist, iterating through all the indices in the arraylist and adding up the lengths of all the values.

• Lastly, we print it out.

Loop Conditions:

• There are a few diffrent loop conditions you can use to traverse an Arraylist:

First, there are three major parts of a for loop: Initialisation, in which you declare the index, can be modified to change where you want to traverse from.

Boolean condition, in which you declare the stop condition, can be modified in order to change the index you want to stop traversing in.

Update, in which you declare how many indexes to go through, can be modified to skip certain indicies and traverse in a certain direction.

Practice:

Suppose we have an arraylist named grades, and we want to remove the entries that are lower than 70. replace the question marks with code to solve the problem:

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        ArrayList<int> grades = new ArrayList<>();
        grades.add(68.9);
        grades.add(71);
        grades.add(100);
        grades.add(80);
        for(int i = 0; i<=; i){
            if(grades.get(i)<70){
                ???
            }
        }
        System.out.println(grades);
    }
}

Using Enhanced For-Loop With Traversing:

• Using Enhanced for loop is easier to read and write and is also more concise and avoids errors.

• Indexes are not explicitly used and copies of the current element are made at each iteration.

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> roster = new ArrayList<>();
        roster.add("Hello");
        roster.add("World");
        roster.add("Java");

        // Using an enhanced for loop to iterate through the ArrayList
        for (String element : roster) {
            System.out.println(element);
        }
    }
}

Common mistakes:

• Using the Wrong Data Type: Ensure that you declare your ArrayList with the correct data type. Using the wrong data type can lead to type mismatches and errors.

• Incorrect Indexing: Be cautious when using a standard for loop. Off-by-one errors or accessing elements that don’t exist can lead to runtime exceptions.

• Modifying the List During Iteration: Modifying an ArrayList (adding or removing elements) while iterating over it can lead to a ConcurrentModificationException. To avoid this, use an Iterator or create a copy of the list if modifications are needed.

• Not Checking for Null Elements: When using enhanced for loops or iterators, check for null elements if there’s a possibility that your list contains them to avoid NullPointerExceptions.

• Inefficient Searching: If you need to find a specific element, avoid using a linear search within a loop. Use appropriate methods like contains() or indexOf() to find elements efficiently.

Hacks:

• Create a scenario which requires the use of arraylist traversal and create code to solve it.
• Extra points to creative scenarios
• 90% credit to any working code and scenario
• 80% credit to incomplete code if you provide explanation of it

• -10% deduction for late submition

• Scenario: You are working for a company that sells products online. Your task is to write a program that calculates the total cost of all orders placed by customers. Each order consists of one or more products, and each product has a price. You need to traverse through the list of orders and calculate the total cost of each order, and then add up the total costs of all orders to get the grand total.

• Code:

import java.util.ArrayList;

public class OrderTotalCalculator {
    public static void main(String[] args) {
        // Create an ArrayList of orders
        ArrayList<Order> orders = new ArrayList<>();
        
        // Add some sample orders to the list
        orders.add(new Order("Product A", 10));
        orders.add(new Order("Product B", 20));
        orders.add(new Order("Product C", 30));
        
        // Calculate the total cost of all orders
        int totalCost = 0;
        for (Order order : orders) {
            totalCost += order.getPrice();
        }
        
        System.out.println("Grand Total: " + totalCost);
    }
}

class Order {
    private String name;
    private int price;
    
    public Order(String name, int price) {
        this.name = name;
        this.price = price;
    }
    
    public String getName() {
        return name;
    }
    
    public int getPrice() {
        return price;
    }
}

OrderTotalCalculator.main(null);

Grand Total: 60

7.4: Developing Algorithms Using ArrayLists

Learning Objectives

In the context of ArrayList objects, this module aims to teach the following skills:

a. Iterating through ArrayLists using for or while loops.

b. Iterating through ArrayLists using enhanced for loops.

In the realm of algorithms, within the context of specific requirements that demand the utilization of ArrayList traversals, students will be able to:

• Recognize established algorithms.
• Customize existing algorithms.
• Create new algorithms.

Essential Knowledge

• Iteration statements provide a means to access all the elements stored within an ArrayList. This process is referred to as “traversing the ArrayList.”

• The following methods related to ArrayLists, their functions, and appropriate use are covered in the Java Quick Reference:

  • int size() - Returns the count of elements within the list.
  • boolean add(E obj) - Appends the object obj to the end of the list and returns true.
  • void add(int index, E obj) - Inserts obj at the specified index, shifting elements at and above that position to the right (incrementing their indices by 1) and increasing the list’s size by 1.
  • E get(int index) - Retrieves the element at the given index in the list.
  • E set(int index, E obj) - Replaces the element at the specified index with obj and returns the previous element at that index.
  • E remove(int index) - Deletes the element at the specified index, shifting all subsequent elements one index to the left, reducing the list’s size by one, and returning the removed element.

• There exist established algorithms for ArrayLists that make use of traversals to:

  • Insert elements.
  • Remove elements.
  • Apply the same algorithms commonly used with 1D arrays.

Popcorn Hacks:

Before you uncomment the code and run it, guess what the code will do based on what you’ve learned.

Let’s Look at an Example (Example 1)

public class ArrayListExample {
    private double findMax(double[] values) {
        // double max = values[0];
    
        //for (int index = 1; index < values.length; index++) {
        //    if (values[index] > max) {
        //        max = values[index];
        //    }
        //}
    
        // return max;
    }
    
    public static void main(String[] args) {
        double[] nums = {1.0, 3.0, 2.0, 2.0, 1.0, 69.0, 2.0, 4.0, 6.0, 2.0, 5.0, 10.0};
        ArrayListExample example = new ArrayListExample();
        double max = example.findMax(nums);
        System.out.println("Maximum value: " + max);
    }
}

ArrayListExample.main(null);

Take a closer look at the findMax() method. It takes in a list of doubles as parameters. It will then use a for loop to find the maximum value in the list. Now, using what we know, can we replace the list of doubles with an ArrayList of Doubles? We sure can! Take a look at how we can use ArrayList to do just that:

public class ArrayListExample {
    private double findMax(ArrayList<Double> values) {
        // double max = values.get(0);
    
        //for (int index = 1; index < values.size(); index++) {
        //    if (values.get(index) > max) {
        //        max = values.get(index);
        //    }
        //}
    
        //return max;
    }
    
    public static void main(String[] args) {
        ArrayList<Double> nums = new ArrayList<>();
        nums.add(1.0);
        nums.add(3.0);
        nums.add(2.0);
        nums.add(2.0);
        nums.add(1.0);
        nums.add(69.0);
        nums.add(2.0);
        nums.add(4.0);
        nums.add(6.0);
        nums.add(2.0);
        nums.add(5.0);
        nums.add(10.0);
        
        ArrayListExample example = new ArrayListExample();
        double max = example.findMax(nums);
        System.out.println("Maximum value: " + max);
    }
}

ArrayListExample.main(null);

Let’s Look at an Example (Example 2)

Take a look at this code:

public class ArrayListExample {
    private int findMin(int[] values) {
        //int min = Integer.MAX_VALUE;
        //for (int currentValue : values) {
        //    if (currentValue < min) {
        //        min = currentValue;
        //    }
        //}
        return min;
    }

    public static void main(String[] args) {
        int[] nums = {420, 703, 2034, 582, 1047, 4545};
        ArrayListExample example = new ArrayListExample();
        int min = example.findMin(nums);
        System.out.println("Minimum value: " + min);
    }
}

ArrayListExample.main(null);

Now, can we use ArrayLists to make this code better? We sure can! Take a look at the new and improved code that uses ArrayLists:

public class ArrayListExample {
    private int findMin(ArrayList<Integer> values) {
        //int min = Integer.MAX_VALUE;
        //for (int currentValue : values) {
        //    if (currentValue < min) {
        //        min = currentValue;
        //    }
        //}
        return min;
    }

    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<>();
        nums.add(420);
        nums.add(703);
        nums.add(2034);
        nums.add(582);
        nums.add(1047);
        nums.add(4545);
        ArrayListExample example = new ArrayListExample();
        int min = example.findMin(nums);
        System.out.println("Minimum value: " + min);
    }
}

ArrayListExample.main(null);

Hacks

• Answer the questions:
  • Look back at the examples. What’s similar? What’s different?
  • Why do we use ArrayList? Why not just regular lists?
• Demonstrate at least two ArrayList methods that aren’t ArrayList<>.size() and ArrayList<>.get().
• Write the method findSum() using the Hack Helper and incorporating ArrayList.

Hack Helper

public class ArrayListHacks {
    private int findSum(ArrayList<Integer> values) {
        // Your code here
        return 0;
    }

    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<>();
        nums.add(0);
        nums.add(1);
        nums.add(2);
        nums.add(3);
        nums.add(5);
        nums.add(8);

        ArrayListHacks hacks = new ArrayListHacks();
        hacks.findSum(nums);
    }
}

ArrayListHacks.main(null);

7.5 Searching

Learning Objectives

• Apply sequential/linear search algorithms to search for specific information in array or arraylist objects

Essential Knowledge:

• Sequential/linear search alogorithms check each element in order untill the desired value is found or all elementsin the array or arraylist have been checked

Search Process

• Linear searching fits a standard for loop perfectly! We need to specify each element, one at a time, and do not need to track the index after execution

• Inside the for loop, we retrieve the value from the structure at the specified index and compare it to the searched value

• If it matches we return the index, otherwise we keep looking!

Searching Linear Structures

Finding information with a computer is something we need to know how to do. Linear search algorithms are BEST used when we do not have any idea about the order of the data and so we need to look at each element to determine if what we are looking for is in fact inside the array or ArrayList.

When searching, we do need to remember that different data types require comparisons!

• When looking at int values, the == operator is the tool to use!
• When searching for a double value, we need to make sure the value is close enough by doing some math!
• Object instances should always use the .equals(otheThing) method to check for a match!

Searching an ArrayList of Double

public int where(double magicNumber, ArrayList<Double> realNumbers, double delta)
{
    for (int index = 0; index < realNumbers.size(); index++)
    {
        if (Math.abs(magicNumber - realNumbers.get(index)) < delta)
        {
            return index;
        }
    }
    return -1;
}

Explanation

The where function searches through a list of numbers to find and return the position of the first number that is very close to a specific target number, known as magicNumber. If no number in the list is close enough to the target number, the function returns -1, indicating that no match was found.

Searching an ArrayList of book for a String

public int findTheWord(String searchedPhrase, ArrayList<Book> myBooks)
{
    for (int index = 0; index < myBooks.size(); index++)
    {
        Book currentBook = myBooks.get(index);
        String currentPhrase = currentBook.getDescription();
        if(currentPhrase.equals(searchedPhrase))
        {
            return index;
        }
    }
    return -1;
}

Explanation

This little code snippet is like a treasure hunt through a collection of books; it’s on a mission to find the one book whose description matches exactly with a special phrase you’re looking for. If it finds the perfect match, it’ll excitedly tell you where it is in the collection, but if not, it’ll sadly let you know with a -1 that the search was a bust.

Questions

Should we use == when looking for an Object?

No, that only will return true if the variable and the element stored at that index point to the same memory, are aliases of each other

Why did we subtract the double values?

To make sure that the lack of preciosin that is inherit in the data type is handled within our code

Why does order sometimes matter?

When searching for a value to remove from a list, if we search forward we have to make sure to adjust the loop control variable, or we might skip what we are looking for when removing!

7.6 Sorting

Learning Objectives

• Apply selection sort and insertion sort algorithms to sort the elements of array or ArrayList objects.

Essential Knowledge:

• Selection sort and insertion sort are iterative sorting algorithms that can be used to sort elements in an array or ArrayList.

Selection Sort

This is one of the easiest sorts to demonstrate. The selection sort identifies either the maximum or minimum of the compared values and iterates over the structure checking if the item stored at the index matches that condition, if so, it will swap the value stored at that index and continue. This implementation uses a helper method to perform the swap operation since variables can hold only one value at a time!

Example:

// with normal arrays
for (int outerLoop = 0; outerLoop < myDucks.length; outerLoop ++)
{
    int minIndex = outerLoop;
    for (int inner = outerLoop +1; inner < myDucks.length; inner++)
    {
        if (myDucks[inner].compareT(myDucks[minIndex]) < 0)
        {
            minIndex = inner;
        }
    }
    if (minIndex != outerLoop)
    {
        swapItems(minIndex, outerloop, myDucks);
    }
}

// with array lists
for (int outerLoop = 0; outerLoop < myDucks.size(); outerLoop++) {
    int minIndex = outerLoop;
    for (int inner = outerLoop + 1; inner < myDucks.size(); inner++) 
    {
        if (myDucks.get(inner).compareT(myDucks.get(minIndex)) < 0) 
        {
            minIndex = inner;
        }
    }
    if (minIndex != outerLoop) {
        swapItems(minIndex, outerLoop, myDucks); 
    }
}
/*
This code performs a selection sort on the myDucks ArrayList, ordering its elements based on the compareT method. 
During each iteration of the outer loop, it finds the index of the minimum element in the unsorted portion of the list and swaps it with the first element of the unsorted portion.
 */ 

Insertion Sort Algorithm

The insertion sort is characterized by building a sorted structure as it proceeds. It inserts each value it finds at the appropriate location in the data structure. This is often accomplished by using a while loop as the inner loop.

Example:

for (int outer = 1; outer < randomList.size(); outer++)
{
    DebugDuck tested = randomList.get(outer);
    int inner = outer -1;

    while ( innter >= 0 && tested.compareTo(randomList.get(inner)) < 0)
    {
        ramdomList.set(inner +1, randomList.get(inner));
        inner--;
    }
    randomList.set(inner +1, tested)
}
// This code arranges a list of DebugDuck objects in order using the insertion sort method, 
// by moving through the list and putting each item in its proper place one by one.

7.7: Ethical Issues around Data Collection

Learning Objectives:

• Explaining the risks of privacy from collecting and storing personal data on computer systems.

Essential Knowledge:

• When using the computer, personal privacy is at risk. Programmers should attempt to safeguard personal privacy.

Privacy Protection:

• A simple way to protect privacy is to delete personal user info after done using it.
• Another way is to minimize the amount of data used by the program in order to protect privacy.
• Anonymizing personal data via the object method hashCode() is another way.

Hacks:

• Think of other ways to safeguard user privacy (any other method not listed is fine)
• Extra points to creative solutions
• 90% credit to any solution
• 80% credit to incomplete solution (how did you do that?)
• -10% deduction for late submition

Grading:

• 7.2 Hacks (0.19)
• 7.3 Hacks (0.19)
• 7.4 Hacks (0.19)
• 7.7 Hacks (0.19)
• Popcorn Hacks (0.19)
• Anything else creative or above-and-beyond showing your understanding of ArrayLists (0.05)