Mastering the concept of Java’s ArrayList. AP Exam weighting: 2.5-7.5%.
Arrays | ArrayLists |
---|---|
Fixed Length | Resizable Length |
Fundamental Java feature | Part of a framework |
An object with no methods | Class with many methods |
Not as flexible | Designed to be very flexible |
Can store primitive data | Not designed to store primitives |
Slightly slower than arrays | |
Need an import statement |
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();
Create 2 ArrayLists, 1 called studentName
and 1 called studentAge
public class Student
{
public static void main(String[] args)
{
//Initialize your ArrayLists
ArrayList<String> studentName = new ArrayList<String>();
ArrayList<integer> studentAge = new ArrayList<integer>();
}
}
Students will be able to represent collections of related object reference data using ArrayList
objects.
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.
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
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]
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);
System.out.println(h);
[Hello, Hola, Hello, HeLLO, Hello, Hello]
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?
All the added elements are of the same data type
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
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]
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]
ArrayList
as a Method ParameterThe 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
ArrayList
from a MethodIn 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]
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:
ArrayList
. What does the code output and why?
ArrayList
. What does the code output and why? What type of function is void
, and what will be the return value?
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:
import java.util.ArrayList;
public class ArrayListMethodsExample {
private ArrayList<String> shoppingList = new ArrayList<>();
private String manipulateList(String item1, String item2, int remove, int replace, String replaceItem) {
//initialize some values
shoppingList.add("strawberries");
shoppingList.add("water");
shoppingList.add("flour");
shoppingList.add("cheese");
// Add 2 items to the list.
shoppingList.add(item1);
shoppingList.add(item2);
// Remove an item from the list anywhere of the user's choice.
String removedItem = shoppingList.remove(remove);
// Replace an item anywhere in the list of the user's choice.
String replacedItem = shoppingList.set(replace, replaceItem);
// Get the first and last element of the list
String firstItem = shoppingList.get(0);
String lastItem = shoppingList.get(shoppingList.size() - 1);
// Return the items added, removed, replaced, and the list's size, in one string
return "Added: " + item1 + ", " + item2 + "\n" +
"Removed: " + removedItem + "\n" +
"Replaced: " + replacedItem + " at index " + replace + "\n" +
"First Item: " + firstItem + "\n" +
"Last Item: " + lastItem + "\n" +
"List Size: " + shoppingList.size();
}
public static void main(String[] args) {
ArrayListMethodsExample example = new ArrayListMethodsExample();
// Sample inputs for the method
String list = example.manipulateList("milk", "chocolate", 0, 0, "butter");
System.out.println(list);
}
}
ArrayListMethodsExample.main(null)
Added: milk, chocolate
Removed: strawberries
Replaced: water at index 0
First Item: butter
Last Item: chocolate
List Size: 5
With an Arraylist you can traverse objects using a for or while loop.
Traversing objects is similar to iterating through objects.
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);
}
}
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.
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.
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<Double> grades = new ArrayList<>(); // Fix the arraylist
grades.add((double)68.9);
grades.add((double)71); // Casting
grades.add((double)100);
grades.add((double)80);
for(int i = 0; i < grades.size(); i++ ){
if(grades.get(i)<70){
grades.remove(i);
}
}
System.out.println(grades);
}
}
Main.main(null)
[71.0, 100.0, 80.0]
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);
}
}
}
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.
You are going on a rollercoaster ride at Six Flags. You see that there is a height limit on one of the rides. You ask the worker and they say they create a cap on the heights because people who are too tall will get their heads cut off. Therefore, the worker must check to see that all riders are below 6 foot (72 inches). Thus, we want to remove all the heights that are equal to 72 or taller. However, if the riders are too short, they will fall out and die. We must also check that all riders are taller than 4 feet (48 inches).
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) {
ArrayList<Double> heights = new ArrayList<>(); // Fix the arraylist
heights.add((double)43);
heights.add((double)89);
heights.add((double)59);
heights.add((double)58);
heights.add((double)81);
heights.add((double)47);
heights.add((double)39);
heights.add((double)98);
heights.add((double)85);
heights.add((double)54);
heights.add((double)60);
for(int i = 0; i < heights.size(); i++ ){
if(heights.get(i)>71 || heights.get(i)<48){
heights.remove(i);
}
}
System.out.println(heights);
}
}
Main.main(null)
[89.0, 59.0, 58.0, 47.0, 98.0, 54.0, 60.0]
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:
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:
Before you uncomment the code and run it, guess what the code will do based on what you’ve learned.
It will find the max
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);
Maximum value: 69.0
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);
Maximum value: 69.0
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);
Minimum value: 420
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);
Minimum value: 420
all hve a class called ArrayList Example
ArrayList
? Why not just regular lists?
ArrayList
methods that aren’t ArrayList<>.size()
and ArrayList<>.get()
.findSum()
using the Hack Helper and incorporating ArrayList
.ArrayList<Integer> nums = new ArrayList<>();
nums.add(420);
nums.add(703);
nums.add(2034);
nums.add(582);
nums.add(1047);
nums.add(4545);
System.out.println(nums);
nums.add(69);
nums.remove(1);
System.out.println(nums);
[420, 703, 2034, 582, 1047, 4545]
[420, 2034, 582, 1047, 4545, 69]
public class ArrayListHacks {
private int findSum(ArrayList<Integer> values) {
int sum = 0;
for (int value : values) {
sum += value;
}
return sum;
}
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();
int sum = hacks.findSum(nums);
System.out.println("The sum of the elements in the ArrayList is: " + sum);
}
}
ArrayListHacks.main(null);
The sum of the elements in the ArrayList is: 19
arraylist
objectsarraylist
have been checkedLinear 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!
ArrayList
.int
values, the == operator is the tool to use!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!ArrayList
of Doublepublic 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;
}
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.
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;
}
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.
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
To make sure that the lack of preciosin that is inherit in the data type is handled within our code
ArrayList
objects.ArrayList
.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.
*/
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.