链表有环的情况一般是链表的尾指向前面的节点而不是null，

如head->node1->node2->node3->node4->tail->node2，该链表存在环。

判断环是否存在可以借助两个指针，一个指针每次迭代只移动一步，第二个指针每次迭代移动两步。如果存在环，两个指针最终一定会相撞的，即指向同一个节点。其它情况则说明环不存在，比如两个指针中的任一个指向null。

代码如下：

Node.java

public class Node {
public char data;
public Node next;
public Node(char data, Node next)
{
this.data = data;
this.next = next;
}
public Node(char data)
{
this.data = data;
this.next = null;
}
}

LinkedListLoop.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class LinkedListLoop {
private Node head;
private Node tail;
public LinkedListLoop() {
this.head = null;
this.tail = null;
}
public boolean isEmpty() {
return (this.head == null);
}
public void CreateLinkedList() {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str = null;
Node node = null;
char data;
try {
str = br.readLine();
}catch (IOException e) {
e.printStackTrace();
}
for(int i=0; i<str.length(); i++) {
data = str.charAt(i);
if(data != ' ') {
node = new Node(data);
if(i==0) {
head = node;
tail = node;
}
else {
tail.next = node;
tail = node;
}
}
}
}
public Node nodeAt(int i) {
Node node = head;
int pos = 0;
while(node!=null) {
if(pos==i) break;
pos++;
node = node.next;
}
return node;
}
public void createLoop(int i) {
Node node = nodeAt(i);
if(node==null) return;
tail.next = node;
}
public void destroyLoop() {
tail.next = null;
}
public boolean hasNode()
{
Node p1 = head;
Node p2 = head;
while(p1!=null) {
p1 = p1.next;
p2 = p2.next;
if(p2==null) return false;
p2 = p2.next;
if(p2==null) return false;
if(p2 == p1) return true;
}
return false;
}
}

TestDriver.java

public class TestDriver {
public static void main(String arg[]) {
LinkedListLoop forTest = new LinkedListLoop();
forTest.CreateLinkedList();
if(forTest.hasNode()) System.out.println("It has loop!");
else System.out.println("It has no loop.");
forTest.createLoop(3);
if(forTest.hasNode()) System.out.println("It has loop!");
else System.out.println("It has no loop.");
forTest.destroyLoop();
if(forTest.hasNode()) System.out.println("It has loop!");
else System.out.println("It has no loop.");
}
}

测试例子：