String concatenation not working, or is it a bug

Question

I have written some code and used a string that I concatentated using the += (as I only do it a couple of times.

Later on I used another string and used the concat() function. and the concatenation didn't work.

So I wrote a little method in Junit (with eclipse)...

@Test public void StingConcatTest(){ String str = ""; str += "hello "; str += " and goodbye"; String conc=""; conc.concat("hello "); conc.concat(" and goodbye"); System.out.println("str is: " + str + "\nconc is: "+ conc);

The output is...

str is: hello and goodbye conc is:

So either I'm going mad, I'm doing something wrong (most likely), there is an issue in JUNIT, or there is a problem with my JRE / eclipse or something.

Note that stringbuilders are working fine.

David.

@peter I did search google, and no responses said had a good response like the one from nambari below. — DaveM
– DaveM, Commented Aug 31, 2012 at 21:44

kosa · Accepted Answer · 2012-08-31 15:36:13Z

7

Ok, we see this question at least couple of times a day.

Strings are immutable, so all operations on String results in new String.

conc= conc.concat("hello "); you need to reassign result to string again

edited Aug 31, 2012 at 15:36

answered Aug 31, 2012 at 15:31

kosa

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1 Comment

DaveM Over a year ago

Ok I feel stupid. End of the day and not enough refreshments.

g.annunziata · Accepted Answer · 2012-08-31 15:31:37Z

You have to try with:

String conc=""; conc = conc.concat("hello "); conc = conc.concat(" and goodbye"); System.out.println("str is: " + str + "\nconc is: "+ conc);

For sake of optimization you can write:

String conc=""; conc = conc.concat("hello ").concat(" and goodbye"); System.out.println("str is: " + str + "\nconc is: "+ conc);

String is ALWAYS constant. When you use these methods (like concat()) you just create a NEW object String.

Pao · Accepted Answer · 2012-08-31 15:33:54Z

If you plan on concatenating multiple Strings you could also use StringBuilder:

StringBuilder builder = new StringBuilder(); builder.append("hello"); builder.append(" blabla"); builder.append(" and goodbye"); System.out.println(builder.toString());

Mustafa Genç · Accepted Answer · 2012-08-31 15:33:12Z

1

concat returns a String. It doesn't update the original String.

answered Aug 31, 2012 at 15:33

Mustafa Genç

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Comments

km1 · Accepted Answer · 2012-08-31 15:32:29Z

concat() returns the concatenated string.

public static void main(String [] args) { String s = "foo"; String x = s.concat("bar"); System.out.println(x); }

Philipp · Accepted Answer · 2012-08-31 15:33:53Z

String.concat doesn't change the string it is called on - it returns a new string which is the string and the argument concatenated together.

By the way: string concatenation using concat or += is not very performant. You should use the class StringBuilder instead.

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