这个push入栈方法是怎么实现的，每条语句都分别是什么意思
public class Stack implements Iterable{
private Node first;
private int n;
private static class Node{
private Item item;
private Node next;
}
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public void push (Item item) {
Node oldfirst = first ; //??
first = new Node(); //??
first.item=item; //??
first.next=oldfirst; //??
n++;
}
}
