A dynamic array data structure implementation done in java along with applicable time complexities

DynamicArray.java

/**
  * DSA Implementation Example: Array (Dynamic).
  * The time complexities are as follows:
  * 1. public E get(int index) - O(1)
  * 2. public void set(int index, E value) - O(1)
  * 3. public void add(E value) - O(1) Amortized or O(n) Worst Case
  * 4. public void remove(int index) - O(n)
  **/

import java.util.*;

public class DynamicArray<E> {
  private int size;
  private int capacity;
  private Object[] elements;

  public DynamicArray() {
    this.size = 0;
    this.capacity = 5;
    this.elements = new Object[this.capacity];
  }

  /* O(1) */
  public E get(int index)  {
    if (this.isEmpty())  {
      return null;
    }
    else if (index < 0 || index >= this.size) {
      throw new IndexOutOfBoundsException("Index " + index + " is out of bounds.");
    }
    else {
      return (E) this.elements[index];
    }
  }

  /* O(1) */
  public void set(int index, E value) {
    if (!Objects.isNull(value)) {
      if (index < 0 || index >= this.size) {
        throw new IndexOutOfBoundsException("Index " + index + " is out of bounds.");
      }
      else {
        this.elements[index] = value;
      }
    }
  }

  /* O(1) or O(n) */
  public void add(E value) {
    if (this.isFull()) {
      int capacity = (this.capacity == 0) ? 1 : this.capacity * 2;
      this.resize(capacity);
    }
    this.elements[this.size++] = value;
  }

  /* O(n) */
  public void remove(int index) {
    if (!this.isEmpty()) {
      if (index < 0 || index >= this.size) {
        throw new IndexOutOfBoundsException("Index " + index + " is out of bounds.");
      }
      else {
        for (int i = index; i < this.size - 1; ++i) {
          this.elements[i] = this.elements[i + 1];
        }
        --this.size;
        this.elements[this.size] = null;

        /* resizing - to ensure that we dont take up too much memory */
        if (this.size > 1 && (this.size <= (this.capacity/4))) {
          this.resize(this.capacity/2);
        }
      }
    }
  }

  private void resize(int capacity) {
    this.capacity = capacity;
    Object[] array = new Object[capacity];

    for (int i = 0; i < this.size; ++i) {
      array[i] = (E) this.elements[i];
    }
    this.elements = array;
  }

  public boolean isEmpty() {
    return this.size == 0;
  }

  public boolean isFull() {
    return this.size == this.capacity;
  }

  public int size() {
    return this.size;
  }

  public String toString() {
    String string = "[";
    for (int i = 0; i < this.size; ++i) {
      string += (i == this.size - 1) ? this.elements[i] : this.elements[i] + ", ";
    }
    return string += "]";
  }
}