Does Java support anonymous functions in a similar way to scala?

Question

I am working on the Scala function below that explores the use of anonymous functions. Is there a way this could be recreated in Java? I have attached the Scala code as well as the Java code I have attempted.

def filter (lst:List[Int], fn:(Int)=>Boolean):List[Int] = 
{
var res:List[Int] = Nil
lst.foreach ((x:Int)=>if (fn(x)) res = x::res)
return res.reverse
}

val list = List.range(0, 10)
println(filter(list, x => x % 2 == 0))

Above is in Scala. I have attempted to recreate this in Java but get an error

public static void filter(List<Integer> lst, Function <Integer,Boolean> func) {
    List<Integer> res;
    for (if (func.apply(lst)):
         ) {

    }

}

Overall the code line I'm having difficulty recreating is this line: lst.foreach ((x:Int)=>if (fn(x)) res = x::res) from the scala code

Edit: I have attempted in Java again but I get an error Cannot invoke "java.util.List.stream()" because "res" is null

  Function <Integer,Boolean> fn =  x -> x % 2 == 0;
    List<Integer> list = Arrays.asList(1,2,3,5,6,7,8,9,10);
    
    System.out.println (filter(list,fn));

}
public static int filter(List<Integer> lst, Function <Integer,Boolean> func) {
    List<Integer> res = null;
    res.stream().map(func).collect(Collectors.toList());


    return 0;
}

Java has lambdas ... from Java 8 onwards.

Stephen C
– Stephen C

2021-06-24 10:50:06 +00:00
Commented Jun 24, 2021 at 10:50 — Stephen C
– Stephen C, Commented Jun 24, 2021 at 10:50
Please do not vandalize your questions.

Cody Gray
– Cody Gray ♦

2021-07-04 14:35:53 +00:00
Commented Jul 4, 2021 at 14:35 — Cody Gray
– Cody Gray ♦, Commented Jul 4, 2021 at 14:35

Sky · Accepted Answer · 2021-06-24 11:14:19Z

1

Yes, You can do like:

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;


public class TestFun {
    public static void main(String[] args) {
        Function<Integer, Boolean> fn = x -> x % 2 == 0;

        List<Integer> res = filter(Arrays.asList(1, 2, 3, 4), fn);
        System.out.println(res);
    }


    public static List<Integer> filter(List<Integer> list, Function<Integer, Boolean> func) {
        return list
                .stream() // convert into stream
                .filter(func::apply) // apply the filter
                .sorted(Collections.reverseOrder()) //reverse the order
                .collect(Collectors.toList()); // convert into List
    }
}

answered Jun 24, 2021 at 11:14

Sky

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1 Comment

Joop Eggen Over a year ago

I think reversal is not needed. In scala x::res creates the resulting list in reverse order, so there an undoing reverse was needed. Also sorted then needed naturalOrder first. Not important, but Predicate<Integer>/``test` is a bit more concise as Function<Integer, Boolean>/apply.

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