Why no compile error in this code ?

Question

strings are immutable... we can not alter it... so why the below does not create compile error ? it is an attempt to alter the string.
class test
public static void main(String args[])
    String s1 = new String("amit");
    s1.concat("Poddar");    // why no error ?
    System.out.println(s1);
}

Answer 1

strings are immutable... we can not alter it... so
why the below does not create compile error ? First of all, Strings are immuatable because the String class is coded that way - there willnever be a compiler error to stop you from altering a String's value (which is actually possible via reflection). This is similar to you writing your own class that has a private field and not providing any methods that can alter the field's value. Your new class would be considered immutable in the same way that Strings are considered immutable, and nobody can even attempt (except reflectively), to "alter" an instance of it, so there will never be a compiler error that is due to such an attempt.
it is an attempt to alter the string.No it isn't. The following is a quote from the Javadoc for String.concat:
"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."