Java Coding Problems

Is String completely immutable?

Well, behind the scenes, String uses private final char[] to store each character of the string. By using the Java Reflection API, in JDK 8, the following code will modify this char[] (the same code in JDK 11 will throw java.lang.ClassCastException):

String user = "guest";System.out.println("User is of type: " + user);Class<String> type = String.class;Field field = type.getDeclaredField("value");field.setAccessible(true);char[] chars = (char[]) field.get(user);chars[0] = 'a';chars[1] = 'd';chars[2] = 'm';chars[3] = 'i';chars[4] = 'n';System.out.println("User is of type: " + user);

So, in JDK 8, String is effectively immutable, but not completely.