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up vote 12 down vote favorite
2

Is there a way to initialize an array or a collection by using a simple lambda expression?

Something like

// What about this?
Person[] persons = new Person[15];
persons = () -> {return new Person()};

Or

// I know, you need to say how many objects
ArrayList<Person> persons = () -> {return new Person()};
share|improve this question
up vote 18 down vote accepted

Sure - I don't know how useful it is, but it's certainly doable:

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Test
{
    public static void main(String[] args)
    {
        Supplier<Test> supplier = () -> new Test();
        List<Test> list = Stream
            .generate(supplier)
            .limit(10)
            .collect(Collectors.toList());
        System.out.println(list.size()); // 10
        // Prints false, showing it really is calling the supplier
        // once per iteration.
        System.out.println(list.get(0) == list.get(1));
    }
}
share|improve this answer
1  
Or directly .generate(() -> new Test()), just to use the lambda operator requested by OP... would rather use .generate(Test::new). – Jean-François Savard yesterday
4  
@Jean-FrançoisSavard: Yes, I was only separating out the declaration to show the types involved. – Jon Skeet yesterday
up vote 12 down vote

If you already have a pre-allocated array, you can use a lambda expression to populate it using Arrays.setAll or Arrays.parallelSetAll:

Arrays.setAll(persons, i -> new Person()); // i is the array index

To create a new array, you can use 

Person[] persons = IntStream.range(0, 15)  // 15 is the size
    .mapToObj(i -> new Person())
    .toArray(Person[]::new);
share|improve this answer
up vote 2 down vote

If you want to initialize it using Java 8, you don't really need to use a lambda expression. You can achieve that using Stream:

Stream.of(new Person()).collect(Collectors.toList());
share|improve this answer
2  
I think the OP wants to call the lambda expression multiple times to populate the list - see if my answer makes sense to you. – Jon Skeet yesterday
    
@JonSkeet Totally makes sense. – Maroun Maroun yesterday
2  
This will create a list containing a single Person instance, thus, it’s a complicated way to do the same as Collections.singletonList(new Person()) – Holger yesterday
    
@Holger singletonList return an immutable list. – Jean-François Savard yesterday
    
@Jean-FrançoisSavard: Collectors.toList() gives no guarantees about mutability or immutability, so that's not really a point in favor of this answer. – user2357112 yesterday

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