How to mimic ArrayList in Java?

Question

I have homework to mimic the ArrayList data structure in Java. I am not allowed to use any 3rd party data structures or anything in java.Utils.package.

I got stuck on how to increase the array size dynamically.

I read the documentation at Class ArrayList and many online articles and I didn't figure out a solution.

I found a post here but the author does not recommend to use his solution. "FAKE resizable array"

This is part of my code:

//Implementation Class
package IncreaseArraySize;
public class StudentArrayList<E>{
    private Object[] elements;
    public StudentArrayList() {
            elements = new Object[]{null, null};
    }
        public StudentArrayList(int capacity) {
        elements = new Object[capacity];
        for(int i = 0; i < capacity; i++){
            elements[i] = null; 
        }
    }
    public void add(E e){
        Boolean isItAdded = false;
        for ( int i= 0; i < elements.length; i++) {
            //assuming the 1st null is the end of the list
                       if(elements[i] == null){  
                elements[i] = e;
                isItAdded = true;
                break;
            }
                        }
                        if(isItAdded == false){
                   int holdIndex = elements.length;
                   resizeArray(holdIndex+10);
                   elements[holdIndex] = e;
                }
                }
    private void resizeArray(int newCapacity){
        System.out.println("Resize the array 'elements' here");
    }
    public String toString(){
        String stringOfElements="[";
        Boolean notFirstElement = false;
        for ( int i= 0; i < elements.length; i++) {
            if(elements[i] != null){
                if(notFirstElement) stringOfElements += ", ";
              stringOfElements += elements[i];
              notFirstElement = true;
            } 
        } 
        stringOfElements += "]";
        return stringOfElements;
    }
}

//Main method
package IncreaseArraySize;

public class testMyArrayList {
    public static void main(String[] args) {
        StudentArrayList<String> myArrayList = new StudentArrayList<String>(2);
        System.out.println("Initial list of elements = " + myArrayList.toString());
        myArrayList.add("this works fine");
        myArrayList.add("this works fine too");
        System.out.println("List of elements = " + myArrayList.toString());
        myArrayList.add("This doesn't work");
        System.out.println("Array was resized: list of elements= " + myArrayList.toString());
    }
}

Any help is appreciated.

The idea is to create a new array of larger size, and either using for loop or System.arraycopy(), copy the values stored to new array then reassign the new array to the previous array variable. Also note that ArrayList maintains a variable like size/count so that insertion is O(1) and iteration is not needed for insertion — experiment unit 1998X, May 11 '23 at 07:09
Hello and welcome. I'm assuming you're talking about [this specific answer](https://stackoverflow.com/a/62850895/133203). When they say it's not recommended they mean for actual production code. For homework it's perfectly fine. Anyway, a better approach would be to double the size of the array when it's full instead of growing it by one element at each insertion. — Federico klez Culloca, May 11 '23 at 07:20

score 0 · Accepted Answer · answered May 11 '23 at 07:23

You could implement your method as

private void resizeArray(int newCapacity){
  Object[] newArray = new Object[newCapacity]; //create new extended array
  Object[] elements = this.elements; //get existing array
  System.arrayCopy(elements, 0, newArray, 0, elements.length); //copy existing data into the new array
  this.elements = newArray; // set new array as storage
}

How to mimic ArrayList in Java?

1 Answers1