Java中的链接列表数组

Question

I have to create an array of linked lists for a class in order to store a graph (adjacency list). We have to use Java. I can create the array and instantiate each linked list, but when I go to add the first elements to each one, every linked list gets changed, not just the one at the index of the array.

Node [] adjList;
for(i=0;i<adjList.length;i++)
    adjList[i] = new Node(0,0,null);

this instantiates each new linked list [ Node is my own class, with constructor Node(int head, int data, Node next ) and extends LinkedList ]

then i go to add the first values to each node:

for(i=0;i<adjList.length;i++)
    adjList[i].setHead(i+1); //  numbers 1 to end are the graph vertices

or

for(i=0;i<adjList.length;i++)
    adjList[i].add(new Node(i+1,0,null);

I use print statements to debug the code at the end of these loop I print off each Linked List, but for each one, the values come out to be the final one

ie. if adjList.length = 2 , it would print out

[3,0,null] // adjList[0]
[3,0,null] // adjList[1]
[3,0,null] // adjList[2]

edit: here is the Node class

import java.util.LinkedList;
public class Node extends LinkedList{
    private static int head;
    private static int data;
    private static Node next;

    public Node(int h,int d,Node n) {
        head = h;
        data = d;
        next = n;
    }
    public int getHead(){ // getNext() and getData() are the same
        return head;
    }
    public void setHead(int h){ // setNext() and setData() are basically the same
        head = h;
    }
}

Answer 1

when I go to add the first elements to each one, every linked list gets changed, not just the one at the index of the array

Although your code snippet doesn't show it, almost definitely you have an aliasing problem. The aliasing problem, which tends to bite beginners in almost all object-oriented languages, is the problem of referring to the same object with two different names ie two different variables pointing at the same object.

Now you may be wondering: what about array indices? The problem is with changing a variable at one array index and getting a change across all array indices , not a bunch of named variables. But, as Eric Lippert explains (for C#, which is quite similar to Java), an array really is a bunch of variables that you can refer to with an indexer expression rather than having to define a bunch of individual names. In a sense, int[] foo = new int[3] is like declaring foo0 , foo1 , and foo2 , and indexing into foo just tells the compiler to pick the appropriate variable out of foo0 , foo1 , and foo2 .

You may also be wondering how data could be shared between multiple Node instances, if your array indeed has multiple nodes in it. There are a few ways, and knowing which is pretty much impossible without the code for the Node class. As @DNA points out, there could be static data in the Node class, which is automatically shared across all instances. A Node object may also have a reference to underlying data. If you pass the same reference into all the Node constructors, they are all aliasing the same object in this way as well.

Answer 2

You have probably declared something within Node as static, so every instance ends up with the same shared value, rather than having its own value. However, this is just a guess - please post the code of Node so we can see what the problem really is...