代码中有两个String对象："java"和"world"。它们都是不可变的（除非使用反射并依赖JDK内部）。s变量首先指向第一个，然后指向第二个，但它们是独立的对象。
之后：

String s="java";

你的记忆里有这样的东西

+−−−−−−−−−−+
s:Ref3243−−−−−>| (string) |
               +−−−−−−−−−−+      +−−−−−−−−−+
               | value:   |−−−−−>| (array) |
               | ...      |      +−−−−−−−−−+
               +−−−−−−−−−−+      | 0: 'j'  |
                                 | 1: 'a'  |
                                 | 2: 'v'  |
                                 | 3: 'a'  |
                                 +−−−−−−−−−+

然后

s="world";

您拥有：

+−−−−−−−−−−+
               | (string) |
               +−−−−−−−−−−+      +−−−−−−−−−+
               | value:   |−−−−−>| (array) |
               | ...      |      +−−−−−−−−−+
               +−−−−−−−−−−+      | 0: 'j'  |
                                 | 1: 'a'  |
                                 | 2: 'v'  |
                                 | 3: 'a'  |
                                 +−−−−−−−−−+
               +−−−−−−−−−−+   
s:Ref6449−−−−−>| (string) |   
               +−−−−−−−−−−+      +−−−−−−−−−+
               | value:   |−−−−−>| (array) |
               | ...      |      +−−−−−−−−−+
               +−−−−−−−−−−+      | 0: 'w'  |
                                 | 1: 'o'  |
                                 | 2: 'r'  |
                                 | 3: 'l'  |
                                 | 4: 'd'  |
                                 +−−−−−−−−−+

因此，末尾的println显示了第二个字符串的内容。
上面显示的value成员在任何给定的JDK实现中都可以称为value，也可以不称为value，它是一个私有数据成员。