Why does change one variable also change another?

[yma16]

The code has 3 class as following

import java.util.*;
import java.awt.*;

public class AList {
	public static void main(String[] args)
	{
        String s1,s2,s3;
		s1="a";s3="b";
		s2=s1;
		System.out.println(s2);
		s2=s3;
		System.out.println(s1);
        GList g = new GList();
		g.add("Luke");
    }
}

public class GList<E>
{
	private static class Node<E>
	{
		public E data;
		public Node<E> next;

		public Node(E data, Node<E> next)
		{
		this.data = data;
		this.next = next;
		}
		public E get()
		{
		    return this.data;
		}
	}
private Node<E> head;

public GList()
{
	head = new Node<E>(null, null);
}
public void add(E e)
{
	Node<E> c=head;
	while ( c.next!=null)
		c=c.next;
	Node<E> newNode=new Node<E>(e, null);
	c.next=newNode;
}
}

After s2=s3, s2 contains "b". s1 still has "a". But after
c.next=newNode, head.next is not null and contains newNode.
Why? Thank you.

[camickr]

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