There are general homework guidelines you must always follow. If you fail to follow any of the following guidelines you risk receiving a O for the entire assignment.
1. All submitted code must compile under JDK 8. This includes unused code, SO don't submit extra files that don't compile. Any compile errors will result in a 0.
2. Do not include any package declarations in your classes.
3. Do not change any existing class headers, constructors, instance/global variables, or method sig- natures.
4. Do not add additional public methods.
5. Do not use anything that would trivialize the assignment. (e.g. don't import/use java.util.ArrayList for an Array List assignment. Ask if you are unsure.)
6. Always be very conscious of efficiency. Even if your method is to be O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is extremely rare).
7. You must submit your source code, the . java files, not the compiled class files.
8. After you submit your files, redownload them and run them to make sure they are what you intended to submit. You are responsible if you submit the wrong files.

You are to code an ArrayList, which is a list data structure backed by an array where all of the data is contiguous and aligned with index 0 of the array. Your ArrayList must follow the requirements stated in the javadocs of each method you must implement. A constructor stub is provided for you to fill out. Do not change headers for the constructor and for any of the methods provided. For example, many of the methods require you to throw exceptions, but you should not add "throws" to the header since they are not necessary.

You will implement three add() methods. One will add to the front, one will add to the back, and one will add to anywhere in the list. When adding to the front or the middle of the list, subsequent elements must be shifted back one position to make room for the new data. See the javadocs for more details.

Removing, just like adding, can be done from the front, the back, or anywhere in your ArrayList. When removing from the front or from the middle of the list, the element should be removed and all subsequent elements should be shifted forward by one position. When removing from the back, the last element should be set to null in the array. All unused positions in the backing array must be set to null. See the javadocs for more details.

The starting capacity of your ArrayList should be the constant INITIAL CAPACITY defined in Reference the constant as-is. Do not simply copy the value of the constant. If, while adding an element, the ArrayList does not have enough space, you should regrow the backing array to twice its old capacity. Do not regrow the backing array when removing elements. Do not change the initial capacity constant.

Amortized Efficiency
The efficiency of methods and algorithms in this course are often analyzed using a "per operation" analysis. That is, what is the worst this algorithm can do on any one instance? However, there are times where this type of analysis is unrealistically pessimistic. For example, in this homework, the addToBack method is O(1) for the most part except in the case of resizing which is O(n). However, a resize operation is rare enough that it'd be misleading to say that the method is O(n). In cases like this, we use an amortized analysis. This type of analysis adds up the cost of a series of operations and then averages the cost. Here, the resize step is O(n), but since we double the capacity whenever the array gets full, we've put off resizing for another n add operations. So, putting that together with the common, cheap O(1) operations, we get O(1) using this analysis. Whenever this type of analysis is used, we will prefix the Big-O with the word amortized.

There are two ways of defining objects as equal: reference equality and value equality. Reference equality is used when using the == operator. If two objects are equal by reference equality, that means that they have the exact same memory locations. For example, say we have a Person object with a name and id field. If you're using reference equality, two Person objects won't be considered equal unless they have the exact same memory location (are the exact same object), even if they have the same name and id. Value equality is used when using the equals( method. Here, the definition of equality is custom-made for the object. For example, in that Person example above, we may want two objects to be considered equal if they have the same name and id. Keep in mind which makes more sense to use while you are coding.

You will want to use value equality in most cases in this course when comparing objects. Notable cases where you'd use reference equality include checking for null or comparing primitives (in this case, it's just the =: operator being overloaded).

Differences between Java API and This Assignment
Some of the methods in this assignment are called different things or don't exist in Java's ArrayList class. This won't matter until you tackle coding questions on the first exam, but it's something to be aware of. The list below shows all methods with a different name and their Java API equivalent if it exists. The format is assignment method name --> Java API name.

addAtIndex (int index, T data) --> add (int index, T data)
addToFront (T data) --> no explicit method
addToBack (T data) --> add (T data)
removeAtInde (int index) --> remove (int index)
removeFromFront () --> no explicit method
removeFromBack() --> no explicit method
lastIndexOf (T data) --> lastIndexOf (Object data)

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These solutions may offer step-by-step problem-solving explanations or good writing examples that include modern styles of formatting and construction of bibliographies out of text citations and references. Students may use these solutions for personal skill-building and practice. Unethical use is strictly forbidden.

* Your implementation of an ArrayList.
* @author YOUR NAME HERE
* @version 1.0
public class ArrayList<T> {

    // Do not add new instance variables.
    private T[] backingArray;
    private int size;

    * The initial capacity of the array list.
    public static final int INITIAL_CAPACITY = 9;

    * Constructs a new ArrayList.
    * Java does not allow for regular generic array creation, so you will have
    * to cast an Object array to T[] to get the generic typing.
    public ArrayList() {
       backingArray = (T[])new Object[INITIAL_CAPACITY];

    * Adds the element to the index specified.
    * Remember that this add may require elements to be shifted.
    * Adding to index {@code size} should be amortized O(1),
    * all other adds are O(n).
    * @param index the index where you want the new element
    * @param data the data to add to the list
    * @throws IndexOutOfBoundsException if index is negative
    * or index > size
    * @throws IllegalArgumentException if data is null
    public void addAtIndex(int index, T data) {

       if(index < 0 || index > size){
            throw new IndexOutOfBoundsException("Cannot insert out of bound index into data structure.");

       if(data == null){
            throw new IllegalArgumentException("Cannot insert null data into data structure.");

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