Let us start with a simple helloworld.dart example and use it to see how string concatenation works in Dart.

void main() {
  print("hello, world!");
}

The above works, obviously. Now, if we were given this greeting as two strings, “hello, ” and “world!”, and asked to join them together, we might be tempted to do:

"hello, " + "world!"

This works in lots of languages, but not in Dart. The + operator has not been overloaded in the String class, the above code throws a NoSuchMethodError.

Not a problem: Dart gives us lots of ways to contcatenate strings. I list the most common ways below. Above the examples you will see some crude benchmarks that I calculated by running each example a million times on my MacBook. These can give a general sense of the relative efficiency of each method.

The easiest, most efficient way to concat strings is by using adjacent string literals:

.041 seconds
String a = "hello, " "world!";

This still works if the adjacent strings are on different lines:

.040 seconds
String b = "hello, "
    'world!';

Dart also has a StringBuffer class, and this can be used to build up a StringBuffer object and convert it to a string by calling toString() on it:

.689 seconds
var sb = new StringBuffer();
["hello, ", "world!"].forEach((item) {
  sb.add(item);
  });
String c = sb.toString();

The Strings class (notice the plural) gives us 2 methods, join() and concatAll() that can also be used. Strings.join() takes a delimiter as a second argument:

.408 seconds
String d = Strings.join(["hello", "world!"], ", ");

.385 seconds
String e = Strings.concatAll(["hello", "world"]);

All of the above work, but if you are looking for a + substitute, use adjacent string literals; if you need to join a list of strings using a delimiter, use Strings.join(). If you plan on building a very long string, the StringBuffer class can gather the components quite efficiently and convert them to a string only when needed.

You can also use string interpolation; that will be the subject of my next post.