Simple HashMap implementation during interviews, without key-collision

MarcellosHashMap.java

/*
 * Click `Run` to execute the snippet below!
 */

import java.io.*;
import java.util.*;

/*
 * To execute Java, please define "static void main" on a class
 * named Solution.
 *
 * This is a HashMap implementation where 
    K -> V
 */

  interface AppleHashMap {
    void upsert(Object k, Object v);
    void remove(Object k);
    int size();
    List<Object> keys();
    Object value(Object k);
  }

class Solution implements AppleHashMap {

  private Object[] keys;
  private Object[] values;

  private int numberOfElements = 0;

  public Solution(int initialSize) {
    this.keys = new Object[initialSize];
    this.values = new Object[initialSize];
  }

  public void upsert(Object k, Object v) {
    // key collesions will be acceptable as updates
    this.keys[this.numberOfElements] = k;
    this.values[this.numberOfElements] = v;
    this.numberOfElements++;
  }

  private int index(Object k) {
    int foundIndex = -1;
    for (Object value : this.keys) {
      ++foundIndex;
      if (value.equals(k)) {
        return foundIndex;
      }
    }
    return foundIndex;
  }

  public void remove(Object k) {
    if (this.numberOfElements == 0) {
      throw new IllegalStateException("There's nothing to be removed");
    }
    int keyIndex = this.index(k);
    if (keyIndex == -1) {
      throw new IllegalArgumentException("There's no key '" + k + "' on the map");
    }

    this.keys[this.numberOfElements] = null;
    this.values[this.numberOfElements] = null;

    // implement removal
    this.numberOfElements--;
  }

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

  public List<Object> keys() {
    return Arrays.asList(this.keys);
  }

  public Object value(Object k) {
    if (k == null) {
      throw new IllegalArgumentException("Pass a key");
    }

    int keyIndex = this.index(k);
    if (keyIndex == -1) {
      throw new IllegalArgumentException("There's no value associated with key '" + k + "' on the map");
    }
    return this.values[keyIndex];
  }

  public static void main(String[] args) {
    AppleHashMap interviewMap = new Solution(2);
    System.out.println("Init Current size: " + interviewMap.size());

    interviewMap.upsert("Marcello", "San Diego");
    interviewMap.upsert("Stanley", "San Francisco");

    System.out.println("Current size: " + interviewMap.size());

    for (Object personName: interviewMap.keys()) {
      System.out.println("Key: " + personName + " = " + interviewMap.value(personName));
    }
  }
}

output.log

Marcello DeSales ran 105 lines of Java (finished in 2.58s):

Init Current size: 0
Current size: 2
Key: Marcello = San Diego
Key: Leandro = Maceio