JavaAlgorithms/DataStructures/Stacks/StackArray.java

/**
 * This class implements a Stack using a regular array.
 *
 * 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.
 *
 * @author Unknown
 *
 */
public class StackArray{

  /**
   * Main method
   *
   * @param args Command line arguments
   */
  public static void main(String[] args) {
    StackArray myStackArray = new StackArray(4); //Declare a stack of maximum size 4
    //Populate the stack
    myStackArray.push(5);
    myStackArray.push(8);
    myStackArray.push(2);
    myStackArray.push(9);

    System.out.println("*********************Stack Array Implementation*********************");
    System.out.println(myStackArray.isEmpty()); //will print false
    System.out.println(myStackArray.isFull()); //will print true
    System.out.println(myStackArray.peek()); //will print 9
    System.out.println(myStackArray.pop()); //will print 9
    System.out.println(myStackArray.peek()); // will print 2
  }

  /** 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;

  /**
   * 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){
    //private int[] transferArray = new int[newSize]; we can't put modifires here !
    int[] transferArray = new int[newSize];

    //for(int i = 0; i < stackArray.length(); i++){ the length isn't a method .
    for(int i = 0; i < stackArray.length; i++){
      transferArray[i] = stackArray[i];
      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
   *
   * 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
  }
}