java - string functions are not changing the input string

Question

Following is my code:

 class program{

    public static void main(String[]args){

    String str = new String("I like Java ");
    str.concat("the most");
    str.toUpperCase();
    str.replace("LIKE","LOVE");
    System.out.println(str);

        }
   }

As I expect the output should be as:

I LOVE JAVA THE MOST

But, It isn't showing an error but also not showing the expected output...Instead it is only showing the original string:

cmd screen

Am I missing something or may be is it something else?

PS 'new String' is not needed. Do `String str = "I like Java"` instead. — Erik Pragt, Aug 10 '14 at 13:48
@JeroenVannevel what is meant by immutable? as far as I know strings are taken as object in java. — Krupal Shah, Aug 10 '14 at 13:48
@krupalshah: that's when you use google. I just gave you the term 2 minutes ago, I don't think you have read up on it yet. — Jeroen Vannevel, Aug 10 '14 at 13:49
It means you cannot change change them once they have been created. You can only create new Strings from existing Strings. But read a bit on Java Strings, it's the most covered area you could think of. — Erik Pragt, Aug 10 '14 at 13:49
@JeroenVannevel ok..I will search..and I found this: http://stackoverflow.com/questions/8798403/string-is-immutable-what-exactly-is-the-meaning — Krupal Shah, Aug 10 '14 at 13:52
@JeroenVannevel why my question is marked as duplicate? as I said I did not know about this 'immutable'. — Krupal Shah, Aug 10 '14 at 13:54
@krupalshah: that doesn't make it any less of a duplicate. Don't take it personally but do take it as a sign that you should refine your searching before you ask a question. For example when I google for *"java string doesn't change"* I get the answer in each of the first few results. — Jeroen Vannevel, Aug 10 '14 at 13:55
@JeroenVannevel I already searched and found this: http://stackoverflow.com/questions/21330961/string-length-not-working-properly-in-java but was not helpful — Krupal Shah, Aug 10 '14 at 13:58
@JeroenVannevel I am in danger of getting banned because of this question that was genuine and not the duplicate...m hopeless. — Krupal Shah, Aug 10 '14 at 14:18

enrico.bacis · Accepted Answer · 2014-08-10T13:52:41.370

The String class is immutable. This means that every method does not change the original String instance, but it creates a new object and returns it, but you are not saving their reference. You could do:

String str = "I like Java";
str = str.concat("the most");
str = str.toUpperCase();
str = str.replace("LIKE","LOVE");

Or even better:

String str = "I like Java"
    .concat("the most")
    .toUpperCase()
    .replace("LIKE","LOVE");

This works because the methods are returning new String objects, so you can concatenate them.

Now when you use System.out.println(str); the result will be I LOVE JAVATHE MOST (because you forgot the space when concatenating the two strings).

score 0 · Answer 2 · answered Aug 10 '14 at 13:48

The line:

str.replace("LIKE","LOVE");

returns a String that holds the answer that you want, which you throw away. One easy fix is to change the last line:

System.out.println(str.concat("the most").toUpperCase().replace("LIKE","LOVE"));

and delete the useless calls above it.

java - string functions are not changing the input string

2 Answers2