UnboundedArrayStack_A

package csc143.data_structures;

/**
 * @author Vita Wiebe
 * @version PA5
 */
public class UnboundedArrayStack<E> implements UnboundedStack {

   // Fields:

   // UnbAStack is our underlying array object.
   // O(n)
   Object[] unbAStack;

   // Capacity is the number of elements that can be
   // contained in bStack.
   // zero time op
   private int capacity;

   /**
    * O(n)
    * A default constructor, creates underlying array
    * with 10 elements.
    * @param None
    */
   public UnboundedArrayStack() {
      unbAStack = new Object[10];
   }

   /**
    * O(n)
    * A default constructor, creates underlying array
    * with capacity elements.
    * @param None
    */
   public UnboundedArrayStack(int capacity) {
      unbAStack = new Object[capacity];
   }

   /**
    * Push adds the object in question, o, to the top of the stack.
    * O(n^2)
    * @param Object o
    */
   public void push(Object o){

      //Temporary variable to hold new array with shifted elements.
      Object[] newStack;

      while(capacity > 2) {

         if(hasSpace()) {
            newStack = new Object[capacity];
            // Place new object at the top of the stack.
            newStack[0] = o;

            // Populates new array with elements of UnbAStack,
            // beginning with second element (index 1) of newStack.
            for (int i = 1; i < unbAStack.length; i++) {
               newStack[i] = unbAStack[i - 1];
            }
         }else{
            // Creates a new array with twice as many elements.
            newStack = new Object [capacity * 2];

            // Place new object at the top of the stack.
            newStack[0] = o;

            // Populates new array with elements of UnbAStack,
            // beginning with second element (index 1) of newStack.
            for (int i = 1; i < unbAStack.length; i++) {
               newStack[i] = unbAStack[i - 1];
            }
         }
         unbAStack = newStack;
      }
    }

    /**
     * Pop removes the last object added to the stack.
     * O(n^2)
     * @param None
     * @return unbAStack, with last element added removed.
     * @throws NullPointerException
     */
    public Object pop(){

      try {
         // Remove element on the "top" of the stack (i.e. LIFO).
         unbAStack[0] = null;
         // Shift the elements down after the "pop".
         for(int i = 0; i < unbAStack.length - 1; i++){
            unbAStack[i] = unbAStack[i + 1];
         }
      }catch(NullPointerException e){
         throw new NullPointerException("Stack contains no elements.");
      }
      return unbAStack;
    }

    /**
     * Peek lets the user see what the most-recently-added element in the stack is.
     * O(n)
     * @param None
     * @throws NullPointerException
     * @return Last object added to the stack.
     */
    public Object peek() {

      if (unbAStack.length == 0) {
         return null;
      }else{
         return unbAStack[0];
      }
    }

    /**
     * Checks to see whether the current version
     * of the stack has capacity to add more elements.
     * O(n^2)
     * @param None
     * @return boolean
     */
    public boolean hasSpace() {

      for (int i=0; i < unbAStack.length; i++){
         if (unbAStack[i] == null) {
            return true;
         }
      }
      return false;
    }

    /**
     * Checks to see whether the stack has had any elements added to it.
     * O(n^2)
     * @param None
     * @return boolean
     */
    public boolean hasContents() {

      for (int i = 0; i < unbAStack.length; i++){
         if(unbAStack[i] != null) {
            return true;
         }
      }
      return false;
    }

    /**
     * O(k)
     * @param None
     * @return The number of elements that are currently stored in the structure
     */
    public int size() {

      // Tracks the number of elements that aren't null (empty).
      int count = 0;

      for(int i = 0; i < unbAStack.length; i++) {
         if(unbAStack[i] != null){
            count++;
         }
      }
      return count;
   }
}