relationship between pool and heap [duplicate]

This question already has an answer here:

• How do I compare strings in Java? 23 answers


• What is the difference between “text” and new String(“text”)? 6 answers

See the code.

    String xx=new String("hello world");
    String yy="hello world";

    System.out.println("xx: " + xx);
    System.out.println("yy: " + yy);
    System.out.println("xx==yy:"+(xx==yy));  // false

    //modify the value of yy using reflection
    Field yyfield=String.class.getDeclaredField("value");
    yyfield.setAccessible(true);
    char[] value=(char[])yyfield.get(yy);   // change the value of yy
    value[5]='_';

    System.out.println("xx: " + xx);   // xx's value also changed.why?
    System.out.println("yy: " + yy);
    System.out.println("xx==yy:"+(xx==yy));  //false

I've seen some posts about Literal Pool, know xx and yy point to different place. But why I change the value of yy, xx also changed. Is there any operation in reflection or some other aspects I don't know? Thanks in advance.