4.1

class ArrayStack {
  private Object[] elements;
  private int size;
  private int capacity;

  /**
   * Creates an empty ArrayStack with capacity 1.
   */
  public ArrayStack() {
    capacity = 1;
    size = 0;
    elements = new Object[1];
  }

  /**
   * @return The size of this ArrayStack.
   */
  public int size() {
    return size;
  }

  /**
   * @return `true` iff this ArrayStack is empty, `false` otherwise.
   */
  public boolean isEmpty() {
    if(size == 0){
      return true;
    }else{
      return false;
    }
  }

  /**
   * @return `true` iff the size is equal to the capacity, `false` otherwise.
   */
  public boolean isFull() {
    if(size == capacity){
      return true;
    }else{
      return false;
    }
  }

  /**
   * @return the top element of the stack without removing it
   */
  public Object peek() throws EmptyStackException {
    if(capacity == 0 || size == 0){
      throw new EmptyStackException();
    }else{
      return elements[size - 1];
    }
  }

  /**
   * Adds `o` to the stack.
   * If capacity of stack was too small, capacity is doubled and `o` is added.
   *
   * @param o
   *     the element to add to the stack.
   */
  public void push(Object o) {
    if(isFull()){
      int capacitynew = 2 * capacity;

      Object[] temp = new Object[capacity];
      for(int i = 0; i < elements.length; i++){
        temp [i] = elements[i];
      }

      elements = new Object[capacitynew];
      for(int j = 0; j < temp.length; j++){
        elements[j] = temp[j];
      }

      capacity = capacitynew;

    }
    elements[size] = o;
    size++;
  }

  /**
   * Removes the top element from the stack.
   * If removing top would make the stack use less than 25% of its capacity,
   * then the capacity is halved.
   *
   * @return the element which was at the top of the stack.
   * @throws EmptyStackException
   *     iff the queue is empty
   */
  public Object pop() throws EmptyStackException {
    if(size == 0){
      throw new EmptyStackException();
    }else{
      Object e = elements[size - 1];
      elements[size - 1] = null;
      size--;
      if(size < (capacity / 4)){
        int newcapacity = capacity/2;

        Object[] temp = new Object[capacity];
        for(int i = 0; i < size; i++){
          temp [i] = elements[i];
        }

        elements = new Object[newcapacity];
        for(int j = 0; j < size; j++){
          elements[j] = temp[j];
        }

        capacity = newcapacity;



      }

      return e;
   }
  }

  /**
   * @return a String representation of the ArrayStack
   * Example output for ArrayStack with 2 elements and capacity 5:
   * <ArrayStack[1,2]>(Size=2, Cap=5)
   */
  public String toString() {
    if(size == 0){
      return "<ArrayStack[]>(Size=0, Cap=" + capacity + ")";
    }
     String begin = "<ArrayStack[";
     String result = begin;
     for(int i = 0; i < size; i++){
       if(i == size -1){
         result += elements[i] + "]>";
       }else{
         result += elements[i] + ",";
       }

     }

     result += "(Size=" + size + ", Cap=" + capacity + ")";
     return result;
  }

  // For testing, do not remove or change.
  public Object[] getElements() {
    return elements;
  }
}
//