String字符串为什么不可变的深入理解

package com.cxy;

import sun.applet.Main;

import java.lang.reflect.Field;

public class StringDemo {
    public static void main(String[] args) throws Exception{
        String a ="aaa";
        System.out.println("第一次打印");
        System.out.println(a);

        a ="bbbbbb";
        System.out.println("第二次打印");
        System.out.println(a);
        Field value = null ;
        try {
            value = a.getClass().getDeclaredField("value");
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        value.setAccessible(true);
        char[] o = (char[])value.get(a);
        o[2]=‘3‘;
        System.out.println("第三次打印");
        System.out.println(a);


    }
}