Use LinkedList to Implement Stack

public class UseLinkedListImplementStack {
    private ListNode head;
    private int length ;
    public UseLinkedListImplementStack() {
        this.head = null ;
        this.length = 0;
    }
    /*             H
         (last in) 1->2->3->4 (1st in)
                      o/H
     */
    public void push(int val){
        ListNode node = new ListNode(val) ;
        node.next = head ;
        head = node ;
        length++;
    }

    public Integer pop(){
        if (head == null)  return null;
        int value = head.val ;
        head = head.next;
        length-- ;
        return value ;
    }

    public Integer peek(){
        if (head == null) return null ;
        return head.val ;
    }

    public boolean isEmpty(){
        return this.length <= 0 ;
    }

    public int getSize(){
        return this.length ;
    }

    public static void main(String[] args) {
        UseLinkedListImplementStack stack = new UseLinkedListImplementStack();
        stack.push(1);
        stack.push(3);
        System.out.println(stack.getSize());
        System.out.println(stack.peek());
        System.out.println(stack.pop());
        stack.push(4);
        System.out.println(stack.peek());
    }
}