Java String immutable

I was thinking about string in java. String are immutable. But when the code is mutable

String str = new String("abc");
System.out.println(str.toUpperCase());
System.out.println(str);

The outout is 

ABC
abc

Actually the output should be 

ABC
ABC

because String are immutable. Please explain. Thanks!

Jon Skeet
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quotationmark

No, the output is what it is precisely because strings are immutable. Calling toUpperCase() doesn't change the contents of the existing string, it creates a new one and returns a reference to that... it has to do that, due to strings being immutable.

This isn't just the case for toUpperCase() - all methods on String that sound like they might modify the string (e.g. trim()) actually return a new one instead.

Compare that with the mutable class StringBuilder, where the object is modified, and a reference to this is returned:

public class Test {
    public static void main(String[] args) throws Exception {
        StringBuilder builder = new StringBuilder("abc");
        System.out.println(builder); // abc
        System.out.println(builder.append("def")); // abcdef
        System.out.println(builder); // abcdef
    }
}

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