如何在Java中实现动态数组堆栈？

Question

I need to modify a class to create a dynamic array stack. My code at this point looks something like this:

public class DynamicArrayStack<E> implements Stack<E> {                                                                                                                                                                                                                                                                                                                                      

  private E[] elems;  //used to store the elements
  public static final int defaultIncrement = 25;
  private final int increment;

  private int top;  

  @SuppressWarnings( "unchecked" )  
  public DynamicArrayStack( int increment ) {
     this.increment = increment;

     elems = (E[]) new Object[ increment ];
     top = 0;

  }


  /** 
   * Constructor with no parameter that will initialize
   * the stack to have an array whose size is the value
   * of increment and memorise that value as the value
   * of increment.
   */  
  public void ArraySize() { }

  public boolean isEmpty() {
      return top == 0;
  }  

  public E peek() {
      return elems[ top-1 ];
  }  

  public E pop() {
      // save the top element
      E saved = elems[ --top ];
      // scrub the memory, then decrements top
      elems[ top ] = null;
      return saved;
  }  

  public void push( E elem ) {
      // stores the element at position top, then increments top
      elems[ top++ ] = elem;
  }  

  public String toString() {

      StringBuffer b;
      b = new StringBuffer( "DynamicArrayStack: {" );

      for ( int i=top-1; i>=0; i-- ) {
          if ( i!=top-1 ) {
              b.append( "," );
          }
          b.append( elems[ i ] );
      }

      b.append( "}" );

      return b.toString();
  }  
}

How do I edit the first constructor to set increment as the initial size of the stack and that same value to be used when increasing or decreasing the size of the array. My method for doing this seems way too simple. Parameter must be > 0 and a fixed number of cells are added or removed when the size of the array changes.

The second constructor should set the stack to have an array whose size is the value of increment. I keep getting errors here because I can't figure out how to do that because I thought that was already set in the first constructor. Also the size of the array as the value of increment.

Also how do I make this class capable of changing the capacity of the stack and into which method should I place that code?

Answer 1

Here is the simple java code to implement it:

1)Stack based:

  public class DynamicArrayStack {
    public static void main(String[] args) {

        DynamicStack dstack=new DynamicStack(2);
        System.out.println("--Pushing--");
        dstack.push(1);
        dstack.push(2);
        dstack.display();
        dstack.push(3);
        dstack.push(2);
        dstack.push(5);
        dstack.display();
        System.out.println("--Popping--");
        dstack.pop();
        dstack.pop();
        dstack.pop();
        dstack.display();

    }

}

class DynamicStack {
    private int top;
    private int capacity;
    private int[] array;

    public DynamicStack(int cap) {
        capacity = cap;
        array = new int[capacity];
        top = -1;
    }

    public void push(int data) {
        if (isFull()){
            expandArray();      //if array is full then increase its capacity
        }
        array[++top] = data;    //insert the data
    }

    public void expandArray() {
        int curr_size = top + 1;
        int[] new_array = new int[curr_size * 2];
        for(int i=0;i<curr_size;i++){
            new_array[i] = array[i];
        }
        array = new_array;              //refer to the new array 
        capacity = new_array.length;
    }

    public boolean isFull() {
        if (capacity == top+1)
            return true;
        else
            return false;
    }

    public int pop() {
        if (isEmpty()) {
            System.out.println("Stack is empty");
            return -1;
        } else {
            reduceSize();                 //function to check if size can be reduced
            return array[top--];
        }
    }

    public void reduceSize() {
        int curr_length = top+1;
        if (curr_length < capacity / 2) {
            int[] new_array = new int[capacity / 2];
            System.arraycopy(array, 0, new_array, 0, new_array.length);
            array = new_array;
            capacity = new_array.length;
        }
    }

    public boolean isEmpty() {
        if (top == -1)
            return true;
        else
            return false;
    }

    public void display() {
        for (int i = 0; i <= top; i++) {
            System.out.print(array[i] + "=>");
        }
        System.out.println();
        System.out.println("ARRAY SIZE:" + array.length);
    }

}

OUTPUT:

--Pushing--

1=>2=>

ARRAY SIZE:2

1=>2=>3=>2=>5=>

ARRAY SIZE:8

--Popping--

1=>2=>

ARRAY SIZE:4

2)Link List based:

   public class LinkListStack {
    public static void main(String[] args) {

        StackList stack = new StackList();
        System.out.println("--Pushing--");
        stack.push(1);
        stack.push(2);
        stack.push(3);
        stack.push(4);
        stack.push(5);
        stack.push(6);
        stack.display();
        System.out.println("--Popping--");
        stack.pop();
        stack.pop();
        stack.display();

    }
}

class Node {
    private int data;
    private Node next;

    public Node(int d) {
        data = d;
        next = null;
    }

    public int getData() {
        return data;
    }

    public void setData(int data) {
        this.data = data;
    }

    public Node getNext() {
        return next;
    }

    public void setNext(Node next) {
        this.next = next;
    }

}

class StackList {
    private Node top;
    private int length;

    public StackList() {
        length = 0;
        top = null;
    }

    public void push(int data) {
        Node temp = new Node(data);
        if (top == null) {
            top = temp;
        } else {
            temp.setNext(top);
            top = temp;
        }
        length++;
    }

    public int pop() {
        Node temp=top;
        int data = top.getData();
        top = top.getNext();
        temp=null;
        length--;
        return data;
    }

    public void display() {
        Node temp = top;
        if (isEmpty()) {
            System.out.println("Stack is empty");
        } else {
            while (temp != null) {
                System.out.print(temp.getData() + "=>");
                temp = temp.getNext();
            }
        }
        System.out.println();
    }

    public boolean isEmpty() {
        return (top == null);
    }

}

OUTPUT:

--Pushing--

6=>5=>4=>3=>2=>1=>

--Popping--

4=>3=>2=>1=>

Answer 2

Default constructor

Your default constructor could simply call your other constructor with a default increment value. For example:

public DynamicArrayStack() {
    this(defaultIncrement);
}

Expanding the array

The correct place to expand the array is within the push method. When attempting to add a new element you can check if the array is large enough, and if not create a new larger array. For example you could do the following:

@Override
public E push(final E elem) {
    // Check if we need to expand the array
    if (elems.length - 1 == top) {
        @SuppressWarnings("unchecked")
        final E[] newElems = (E[]) new Object[elems.length + increment];
        System.arraycopy(elems, 0, newElems, 0, elems.length);
        elems = newElems;
    }
    // stores the element at position top, then increments top
    elems[top++] = elem;
    return elem;
}

If you want to shrink the array the sensible place to do this would be in the pop() method. You might want to consider only reducing the length when (top + (increment*2))<elems.length to avoid repeatedly copying arrays when you're on the boundary.