How can we call String immutable?

Question

Hello there
String is immutable, that is, it can not be extended. Then how concat() function works.. or how two strings are appended together with a + sign.
What is the logic behind?
Also, what happens when a StringBuffer calls append method, how can a StringBuffer be called mutable?
Thanks

Answer 1

String is immutable, that is, it can not be extended.No. String is final so it can't be extended (subclassed). You can't have:    // Won't compile
class MyString extends String {}Immutable is quite different. It means that once a string is created it will never change its contents.
Then how concat() function works.concat() works just like the API documentation says it works: It {color:#3333FF}"Concatenates the specified string to the end of this string.
If the length of the argument string is 0, then this String object is returned. Otherwise, a new String object is created, representing a character sequence that is the concatenation of the character sequence represented by this String object and the character sequence represented by the argument string"{color}.
http://java.sun.com/javase/6/docs/api/java/lang/String.html#concat(java.lang.String)
Notice the bit that says {color:#3333FF}a new String object is created{color} - that's because Strings are immutable. Neither of the strings you start with are (or can be) changed. Instead a new String is created.
Also, what happens when a StringBuffer calls append methodYou can read for yourself here: http://java.sun.com/javase/6/docs/api/java/lang/StringBuffer.html#append(java.lang.String) The important thing is that no new StringBuffer is created. The string being appended is ... appended and the string buffer's contents are changed (it's now longer).
how can a StringBuffer be called mutable?Since the contents of a StringBuffer can change - for instance if append() is invoked with a non-empty String argument.
For other classes the contents might be called state.
StringBuffer, by the way, is final like String. But it's not immutable.