String hello = "Hello World!";

String xx = new String("Hello World!");

/**
*字符串中对于内容和地址的判定可以用下面两种方式，但侧重点不一样。
*/
equals // 判断 两个字符串的内容是否一致
==  // 判断两个字符串的内存地址是否一致

public static void simple() {
  String s1 = "Hello World!";
  String s2 = "Hello World!";
  String s3 = new String("Hello World!");
  String s4 = new String("Hello World!");

  // 下面开始比较引用和内容的比较
  System.out.println("字符串赋值方式：");
  System.out.println(s1==s2);
  System.out.println(s1.equals(s2));

  System.out.println("\n字符串赋值方式和new方式：");
  System.out.println(s1==s3);
  System.out.println(s1.equals(s3));

  System.out.println("\nnew 方式：");
  System.out.println(s3==s4);
  System.out.println(s3.equals(s4));
 }

字符串赋值方式：
true
true

字符串赋值方式和new方式：
false
true

new 方式：
false
true

/**
  * Returns a string resulting from replacing all occurrences of
  * {@code oldChar} in this string with {@code newChar}.
  * <p>
  * If the character {@code oldChar} does not occur in the
  * character sequence represented by this {@code String} object,
  * then a reference to this {@code String} object is returned.
  * Otherwise, a {@code String} object is returned that
  * represents a character sequence identical to the character sequence
  * represented by this {@code String} object, except that every
  * occurrence of {@code oldChar} is replaced by an occurrence
  * of {@code newChar}.
  * <p>
  * Examples:
  * <blockquote><pre>
  * "mesquite in your cellar".replace('e', 'o')
  *   returns "mosquito in your collar"
  * "the war of baronets".replace('r', 'y')
  *   returns "the way of bayonets"
  * "sparring with a purple porpoise".replace('p', 't')
  *   returns "starring with a turtle tortoise"
  * "JonL".replace('q', 'x') returns "JonL" (no change)
  * </pre></blockquote>
  *
  * @param oldChar the old character.
  * @param newChar the new character.
  * @return a string derived from this string by replacing every
  *   occurrence of {@code oldChar} with {@code newChar}.
  */
 public String replace(char oldChar, char newChar) {
  if (oldChar != newChar) {
   int len = value.length;
   int i = -1;
   char[] val = value; /* avoid getfield opcode */

   while (++i < len) {
    if (val[i] == oldChar) {
     break;
    }
   }
   if (i < len) {
    char buf[] = new char[len];
    for (int j = 0; j < i; j++) {
     buf[j] = val[j];
    }
    while (i < len) {
     char c = val[i];
     buf[i] = (c == oldChar) ? newChar : c;
     i++;
    }
    return new String(buf, true);
   }
  }
  return this;
 }

new String(buf, true);

private static void deep() throws NoSuchFieldException, IllegalAccessException {
  String hello = "Hello World!";
  String xx = new String("Hello World!");
  String yy = "Hello World!";

  /**
   * 判断字符串是否相等，默认以内存引用为标准
   */
  System.out.println(hello == xx);
  System.out.println(hello == yy);
  System.out.println(xx == yy);

  // 查看hello, xx, yy 三者所指向的value数组的真实位置
  Field hello_field = hello.getClass().getDeclaredField("value");
  hello_field.setAccessible(true);
  char[] hello_value = (char[]) hello_field.get(hello);
  System.out.println( hello_field.get(hello));

  Field xx_field = xx.getClass().getDeclaredField("value");
  xx_field.setAccessible(true);
  char[] xx_value = (char[]) xx_field.get(xx);
  System.out.println(xx_field.get(xx));

  Field yy_field = yy.getClass().getDeclaredField("value");
  yy_field.setAccessible(true);
  char[] yy_value = (char[]) yy_field.get(yy);
  System.out.println(yy_field.get(yy));
  /**
   * 经过反射获取到这三个字符串对象的最底层的引用数组value，发现如果一开始内容一致的话，java底层会将创建的字符串对象指向同一个字符数组
   * 
   */

  // 通过反射修改字符串引用的value数组
  Field field = hello.getClass().getDeclaredField("value");
  field.setAccessible(true);
  char[] value = (char[]) field.get(hello);
  System.out.println(value);
  value[5] = '^';
  System.out.println(value);

  // 验证xx是否被改变
  System.out.println(xx);
 }

false
true
false
[C@6d06d69c
[C@6d06d69c
[C@6d06d69c
Hello World!
Hello^World!
Hello^World!