Why can't I declare an array of functions in Java? [duplicate]

Question

I'm used to declaring array inline like this:

    String s1 = "world";
    String[] strings = { "world" };

Why can't I do the same for functions? Given I have a class Book with getTitle() and getAuthor() methods, this is valid:

    Function<Book, String> f1 = Book::getTitle;
    System.out.println(f1.apply(myBook));

However this is not:

    Function<Book, String>[] funcs = { Book::getTitle, Book::getAuthor };
    for (Function<Book, String> f2 : funcs) {
        System.out.println(f2.apply(myBook));
    }

It doesn't like the inline array declaration and the compiler error is "Cannot create a generic array of Function"

Edit I'd argue my question isn't a duplicate of the suggestion as I want to statically define a set of functions using the array initializer syntax

Try with a map, like this Map<String, Function<Book, String>> mapToFunctions = new HashMap<>(); — Manuel Perez Heredia
– Manuel Perez Heredia, Commented Jan 29, 2018 at 15:49
If is a generic type, you need to use funcs = new Function<Book, String>[] { Book::getTitle }; — Héctor Valls
– Héctor Valls, Commented Jan 29, 2018 at 15:51
Function[] funcs = { (Function<Book, String >) Book::getTitle };. — Anonymous
– Anonymous, Commented Jan 29, 2018 at 16:16
If you want to simply apply the same method call to all of your books(assuming books is a List), you can simply do List<String> titles = books.stream().map(Book::getTitle).collect(Collectors.toList()); and then titles.stream().forEach(System.out::println); to print them. It is redundant to create an array of the same function. — ichantz
– ichantz, Commented Jan 29, 2018 at 19:26

Manuel Perez Heredia · Accepted Answer · 2018-01-30 10:08:55Z

2

You could try this:

public static void testFunction() {
    Map<String, Function<Book, String>> mapToFunctions = new HashMap<>();

    Function<Book, String> myFunction = x -> new String(x.getTitle());

    mapToFunctions.put("firstfunction", myFunction);

    for (Function<Book, String> f : mapToFunctions.values()) {
        System.out.println(f.apply(new Book("my title")));
    }
}

UPDATE:

Using Set<Function<Book, String>> it would be something like:

package com.victor.main;

import java.util.HashSet;
import java.util.Set;
import java.util.function.Function;

public class FunctionalTest {
    public static void testFunction() {
        Set<Function<Book, String>> mapToFunctions = new HashSet<>();

        Function<Book, String> myFunction = x -> new String(x.getTitle());

        mapToFunctions.add(myFunction);

        for (Function<Book, String> f : mapToFunctions) {
            System.out.println(f.apply(new Book("my title")));
        }
    }

    public static void main(String[] args) {
        testFunction();
    }
}

edited Jan 30, 2018 at 10:08

answered Jan 29, 2018 at 16:17

Manuel Perez Heredia

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4 Comments

batwad Over a year ago

That will get difficult to maintain when there are many functions, as you have to give them a name and a lambda. I really want a simple list.

Manuel Perez Heredia Over a year ago

Try with a Set<Function<Book, String>> funSet = new HashSet<>();

Manuel Perez Heredia Over a year ago

I updated the answer with a version using Set<Function<Book, String>>

batwad Over a year ago

Thanks. Looks like this falls into the category of "not possible because of type erasure"

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