Stack Min O(1)

/**
 * Stack which, in addition to push and pop,
 * also has a function min which returns the minimum element.
 *
 * push(), pop() and min() all operate in O(1) time
 *
 * @author while - mydevelopedworld.wordpress.com
 *
 */
public class StackMin {

    private Element top;
    private MinElement currentMin;

    public void push(Element item){
        /* Empty Stack = item is the new minimum */
        if(this.currentMin == null){
            this.currentMin = new MinElement(item.getValue(), null); /* lastMin = null */
            this.top = this.currentMin;
            return;
        }
        /* Found a new minimum */
        if(currentMin.getValue() > item.getValue()){ /* Item pushed < currentMin */
            MinElement newMin = new MinElement(item.getValue(), this.currentMin);
            newMin.setBelow(this.top);
            this.currentMin = newMin;
            this.top = this.currentMin;
            return;
        }
        /* Normal push case */
        item.setBelow(this.top);
        this.top = item;
    }

    public Element pop(){
        /* Empty Stack */
        if(this.currentMin == null){
            return null;
        }

        Element ret = this.top;

        /* Current minimum on the top */
        if(this.top == this.currentMin){ /* Pop of the currentMin */
            this.currentMin = this.currentMin.getLastMin(); /* currentMin = lastMin */
            this.top = this.top.getBelow();
            return ret;
        }
        /* Normal pop case */
        this.top = ret.getBelow();
        return ret;
    }

    public Element min(){
        return currentMin; /* Return null if Empty Stack */
    }

    public static void main(String args[]){
        StackMin stack = new StackMin();

        stack.push(new Element(10));
        stack.push(new Element(100));
        stack.push(new Element(5));
        stack.push(new Element(17));
        stack.push(new Element(20));

        System.out.println(stack.min()); // >> 5

        stack.push(new Element(1));

        System.out.println(stack.min()); // >> 1
        System.out.println(stack.pop()); // >> 1
        System.out.println(stack.pop()); // >> 20
        System.out.println(stack.pop()); // >> 17
        System.out.println(stack.min()); // >> 5
        System.out.println(stack.pop()); // >> 5
        System.out.println(stack.min()); // >> 10
        System.out.println(stack.pop()); // >> 100
        System.out.println(stack.min()); // >> 10
        System.out.println(stack.pop()); // >> 10
        System.out.println(stack.pop()); // >> null
        System.out.println(stack.min()); // >> null

    }



}

class Element{
    private int value;
    private Element below = null;

    public Element(int value){
        this.value = value;
    }

    public int getValue(){ return this.value; }
    public Element getBelow(){ return this.below; }

    public void setBelow(Element e){
        this.below = e;
    }

    public String toString(){
        return this.value+"";
    }
}

class MinElement extends Element{
    private MinElement lastMin;

    public MinElement(int value, MinElement lastMin){
        super(value);
        this.lastMin = lastMin;
    }

    public MinElement getLastMin(){ return this.lastMin; }
}