Question

The method Concat() does not modify the original value. It returns a new value.
like this:

String str = 'good'; str.concat('ness'); System.out.println(str);   //'good' 

But some method modify the original value. Why?

In Groovy:

def languages = ['Java', 'Groovy', 'JRuby'] languages.reverse() ===> [JRuby, Groovy, Java] println languages ===> [Java, Groovy, JRuby]   languages.sort() ===> [Groovy, JRuby, Java] println languages ===> [Groovy, JRuby, Java] 

Answer 1

String is immutable in Java. Any method that ‘modifies’ a String must return a new instance of String.

From the Java API Specifications for the String class:

Strings are constant; their values cannot be changed after they are created.

The Java Language Specifications defines this behavior in Section 4.3.3: The Class String.

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Response to the edit:

It appears that an example in Groovy has been added. (I haven’t used Groovy before, so my understanding of it may not be correct.)

From what I understand from looking at the example, there seems to be a languages list that is being reverse-ed and sort-ed — those operations themselves do not modify the String objects contained in the list, but are acting upon the list itself.

The way the list is returns a new list, or how it modifies or doesn’t modify the list is not related to the behavior of the String objects themselves.