Hints for java.lang.String.replace problem? [duplicate]

Question

I would like to replace "." by "," in a String/double that I want to write to a file.

Using the following Java code

double myDouble = myObject.getDoubleMethod(); // returns 38.1882352941176
System.out.println(myDouble);

String myDoubleString = "" + myDouble;
System.out.println(myDoubleString);

myDoubleString.replace(".", ",");
System.out.println(myDoubleString);

myDoubleString.replace('.', ',');
System.out.println(myDoubleString);

I get the following output

38.1882352941176
38.1882352941176
38.1882352941176
38.1882352941176

Why isn't replace doing what it is supposed to do? I expect the last two lines to contain a ",".

Do I have to do/use something else? Suggestions?

I have re-read the docs of replace and it clearly starts with "Returns a new string ..." ;-) — CL23
– CL23, Commented Jul 22, 2009 at 17:36

Joe · Accepted Answer · 2015-02-16 18:49:21Z

20

You need to assign the new value back to the variable.

double myDouble = myObject.getDoubleMethod(); // returns 38.1882352941176
System.out.println(myDouble);

String myDoubleString = "" + myDouble;
System.out.println(myDoubleString);

myDoubleString = myDoubleString.replace(".", ",");
System.out.println(myDoubleString);

myDoubleString = myDoubleString.replace('.', ',');
System.out.println(myDoubleString);

edited Feb 16, 2015 at 18:49

Joe

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answered Jul 22, 2009 at 17:30

AlbertoPL

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2 Comments

CL23 Over a year ago

As usual the problem sits in front of the computer ;-) Thx

Powerlord Over a year ago

The reason it returns a new string is that Strings are immutable in Java.

ngrashia · Accepted Answer · 2015-01-08 04:09:28Z

11

The original String isn't being modified. The call returns the modified string, so you'd need to do this:

String modded = myDoubleString.replace(".",",");
System.out.println( modded );

edited Jan 8, 2015 at 4:09

ngrashia

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answered Jul 22, 2009 at 17:31

Chris Kessel

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Comments

rink.attendant.6 · Accepted Answer · 2014-09-29 17:07:46Z

10

The bigger question is why not use DecimalFormat instead of doing String replace?

edited Sep 29, 2014 at 17:07

rink.attendant.6

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answered Jul 22, 2009 at 19:46

vh.

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1 Comment

CL23 Over a year ago

Very good point! Your answer goes beyond my question, but solves the problem that I essentially had. I didn't know DecimalFormat, thanks!

dfa · Accepted Answer · 2009-07-22 17:32:09Z

5

replace returns a new String (since String is immutable in Java):

String newString = myDoubleString.replace(".", ",");

answered Jul 22, 2009 at 17:32

dfa

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1 Comment

Raedwald Over a year ago

But you need not introduce newString, because although String objects are immutable, references to String objects can be changed (if they are not final): stackoverflow.com/questions/1552301/…

Brian Beckett · Accepted Answer · 2009-07-22 17:49:33Z

3

Always remember, Strings are immutable. They can't change. If you're calling a String method that changes it in some way, you need to store the return value. Always.

I remember getting caught out with this more than a few times at Uni :)

answered Jul 22, 2009 at 17:49

Brian Beckett

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