JavaAlgorithms/DataStructures/Stacks/StackArray.java

package DataStructures.Stacks;

/**
 * This class implements a Stack using a regular array.
 *
 * <p>A stack is exactly what it sounds like. An element gets added to the top of the stack and only
 * the element on the top may be removed. This is an example of an array implementation of a Stack.
 * So an element can only be added/removed from the end of the array. In theory stack have no fixed
 * size, but with an array implementation it does.
 */
public class StackArray {

  /** Driver Code */
  public static void main(String[] args) {
    // Declare a stack of maximum size 4
    StackArray myStackArray = new StackArray(4);

    assert myStackArray.isEmpty();
    assert !myStackArray.isFull();

    // Populate the stack
    myStackArray.push(5);
    myStackArray.push(8);
    myStackArray.push(2);
    myStackArray.push(9);

    assert !myStackArray.isEmpty();
    assert myStackArray.isFull();
    assert myStackArray.peek() == 9;
    assert myStackArray.pop() == 9;
    assert myStackArray.peek() == 2;
    assert myStackArray.size() == 3;
  }

  /** Default initial capacity. */
  private static final int DEFAULT_CAPACITY = 10;

  /** The max size of the Stack */
  private int maxSize;

  /** The array representation of the Stack */
  private int[] stackArray;

  /** The top of the stack */
  private int top;

  /** init Stack with DEFAULT_CAPACITY */
  public StackArray() {
    this(DEFAULT_CAPACITY);
  }

  /**
   * Constructor
   *
   * @param size Size of the Stack
   */
  public StackArray(int size) {
    maxSize = size;
    stackArray = new int[maxSize];
    top = -1;
  }

  /**
   * Adds an element to the top of the stack
   *
   * @param value The element added
   */
  public void push(int value) {
    if (!isFull()) { // Checks for a full stack
      top++;
      stackArray[top] = value;
    } else {
      resize(maxSize * 2);
      push(value); // don't forget push after resizing
    }
  }

  /**
   * Removes the top element of the stack and returns the value you've removed
   *
   * @return value popped off the Stack
   */
  public int pop() {
    if (!isEmpty()) { // Checks for an empty stack
      return stackArray[top--];
    }

    if (top < maxSize / 4) {
      resize(maxSize / 2);
      return pop(); // don't forget pop after resizing
    } else {
      System.out.println("The stack is already empty");
      return -1;
    }
  }

  /**
   * Returns the element at the top of the stack
   *
   * @return element at the top of the stack
   */
  public int peek() {
    if (!isEmpty()) { // Checks for an empty stack
      return stackArray[top];
    } else {
      System.out.println("The stack is empty, cant peek");
      return -1;
    }
  }

  private void resize(int newSize) {
    int[] transferArray = new int[newSize];

    for (int i = 0; i < stackArray.length; i++) {
      transferArray[i] = stackArray[i];
    }
    // This reference change might be nice in here
    stackArray = transferArray;
    maxSize = newSize;
  }

  /**
   * Returns true if the stack is empty
   *
   * @return true if the stack is empty
   */
  public boolean isEmpty() {
    return (top == -1);
  }

  /**
   * Returns true if the stack is full
   *
   * @return true if the stack is full
   */
  public boolean isFull() {
    return (top + 1 == maxSize);
  }

  /**
   * Deletes everything in the Stack
   *
   * <p>Doesn't delete elements in the array but if you call push method after calling makeEmpty it
   * will overwrite previous values
   */
  public void makeEmpty() { // Doesn't delete elements in the array but if you call
    top = -1; // push method after calling makeEmpty it will overwrite previous values
  }

  /**
   * Return size of stack
   *
   * @return size of stack
   */
  public int size() {
    return top + 1;
  }
}