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.

Answer 1

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

Answer 2

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);

Answer 3

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());

Answer 4

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

Answer 5

concat() returns the concatenated string.

public static void main(String [] args) {
    String s = "foo";

    String x = s.concat("bar");

    System.out.println(x);
}

Answer 6

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.