无向图中的顶点用数组的索引表示，该索引中的元素用bag表示，而这些元素指的是索引对应的顶点所连接的顶点。

public class Graph {
    private final int V;// 顶点个数
    private int E;// 边的数目
    private Bag<Integer> adj[];// 邻接表

//初始化邻接表
    public Graph(int v) {
        this.V = v;
        this.E = 0;
        adj = (Bag<Integer>[]) new Bag[V];
        for (int i = 0; i < V; i++) {
            adj[i] = new Bag<Integer>();
        }
    }

    public int V() {
        return V;
    }

    public int E() {
        return E;
    }

    public void addEdge(int v, int w) {
        // 每条边要添加两次
        adj[v].add(w);// 将w添加到v的链表中
        adj[w].add(v);// 将v添加到w的链表中
        E++;
    }


    public Iterable<Integer> adj(int v) {
        return adj[v];
    }
}

Bag

import java.util.Iterator;

public class Bag<Item> implements Iterable<Item> {
    private Node first;// 头结点
    // 链表的结点

    private class Node {
        Item item;
        Node next;
    }

    public void add(Item item) {
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;

    }

    public Iterator<Item> iterator() {

        return new listIterator();
    }

    private class listIterator implements Iterator<Item> {
        private Node current = first;

        public boolean hasNext() {

            return current != null;
        }

        public Item next() {
            Item item = current.item;
            current = current.next;
            return item;
        }

    }
}