Can a java lambda have more than 1 parameter?
To make the use of lambda : There are three type of operation:
1. Accept parameter --> Consumer
2. Test parameter return boolean --> Predicate
3. Manipulate parameter and return value --> Function
Java Functional interface upto two parameter:
Single parameter interface
Consumer
Predicate
Function
Two parameter interface
BiConsumer
BiPredicate
BiFunction
For more than two, you have to create functional interface as follow(Consumer type):
@FunctionalInterface
public interface FiveParameterConsumer<T, U, V, W, X> {
public void accept(T t, U u, V v, W w, X x);
}
For something with 2 parameters, you could use BiFunction
. If you need more, you can define your own function interface, like so:
@FunctionalInterface
public interface FourParameterFunction<T, U, V, W, R> {
public R apply(T t, U u, V v, W w);
}
If there is more than one parameter, you need to put parentheses around the argument list, like so:
FourParameterFunction<String, Integer, Double, Person, String> myLambda = (a, b, c, d) -> {
// do something
return "done something";
};
For this case you could use interfaces from default library (java 1.8):
java.util.function.BiConsumer
java.util.function.BiFunction
There is a small (not the best) example of default method in interface:
default BiFunction<File, String, String> getFolderFileReader() {
return (directory, fileName) -> {
try {
return FileUtils.readFile(directory, fileName);
} catch (IOException e) {
LOG.error("Unable to read file {} in {}.", fileName, directory.getAbsolutePath(), e);
}
return "";
};
}}
It's possible if you define such a functional interface with multiple type parameters. There is no such built in type. (There are a few limited types with multiple parameters.)
@FunctionalInterface
interface Function6<One, Two, Three, Four, Five, Six> {
public Six apply(One one, Two two, Three three, Four four, Five five);
}
public static void main(String[] args) throws Exception {
Function6<String, Integer, Double, Void, List<Float>, Character> func = (a, b, c, d, e) -> 'z';
}
I've called it Function6
here. The name is at your discretion, just try not to clash with existing names in the Java libraries.
There's also no way to define a variable number of type parameters, if that's what you were asking about.
Some languages, like Scala, define a number of built in such types, with 1, 2, 3, 4, 5, 6, etc. type parameters.