# Prefix or Null with Kotlin?

Removing a prefix from a string, quite a common task, isn’t it? Let’s see how to mix with the null-safety in Kotlin for clear, nicer and shorter code.

Have a look at the following Kotlin code:

fun String.removePrefixOrNull(prefix: String) : String? = when {
startsWith(prefix) -> substring(prefix.length)
else -> null
}


This is an extension function. One can use that as-if it’s a member of a String class (but behind the scenes it’s an ordinary method that Kotlin compiler calls).

Usage is pretty easy:

"test 123".removePrefixOrNull("prefix") /// returns null
"test 123".removePrefixOrNull("test")   /// returns " 123"


The benefit could be in combining that with other operators, e.g.

arg.removePrefixOrNull("--text=")?.toLowerCase() ?: error("Incorrect param")


How would you process only a strings with a given prefix? We may start with a Java solution:

final List<String> result = new ArrayList<>();
for (String arg : args) {
if (arg.startsWith(prefix)) {
}
}


The same we can do with Java Streams like that:

args.stream()
.filter(it -> it.startsWith(prefix))
.map(it -> it.substring(prefix.length()))
.collect(Collectors.toList());


Exactly the same code is possible in Kotlin:

args.stream()
.filter { it.startsWith(prefix) }
.map { it.substring(prefix.length) }
.collect(Collectors.toList())


We can tune that a little more to make it more idiomatic, moreover, we will use Kotlin operators on collections too.

args.filter { it.startsWith(prefix) }
.map { it.substring(prefix.length) }


It is still too long and probably troublesome. I wish we could have only open operator to cut the prefix and to keep only the relevant strings. For example, we could do the following:

args.stream()
.map(it -> {
if (it.startsWith(prefix)) {
return it.substring(prefix.length());
} else {
return null;
}
})
.filter(Objects::nonNull)
.collect(Collectors.toList());


That is longer, quite similar to what we have in the very first example. The same in Kotlin could look like that:

args.mapNotNull {
if (it.startsWith(prefix)) {
it.substring(prefix.length)
} else {
null
}
}


The mapNotNull operator helps here much. We could also extract the lambda into the dedicated function to make that shorter:

fun String.removePrefixOrNull(prefix: String) : String? = when {
startsWith(prefix) -> substring(prefix.length)
else -> null
}

args.mapNotNull { it.removePrefixOrNull(prefix) }


There are so many nice and elegant functions in Kotlin that we could use to beautify and to shorten our code. Let me know if you see or use something similar.