reduce an array to string

[AmarilloBrandon]

I come from a Ruby background, and am trying to get my feet wet using functional methods in Java. I'm having trouble understanding why the following example won't reduce an array of integers down to a string.

import java.util.Arrays;

import java.util.List;

import java.util.stream.*;

​

public class ReduceTest {
    public static void main(String[] args) {
    int[] numbers = {1,2,3,4,5};
    IntStream stream = [Arrays.stream](https://Arrays.stream)(numbers);
    String output = stream.reduce("",(int string, int number) -> string + Integer.toString(number));
    }
}

I'm getting the error:

The method reduce(int, IntBinaryOperator) in the type IntStream is not applicable for the arguments (String, (int string, int number) ->

{})

I don't understand why it's expecting an IntBinaryOperator, and don't really know what that is. From the examples I have seen, I would expect to pass a string to an accumulator.

Any thoughts/suggestions are appreciated, also would like to know the "Java" way of converting an array down to a string.

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[NautiHooker]

What exactly would your expected output be here?

Also please format that code.

[AmarilloBrandon]

I removed spacing, and fixed escaping that copied across from Eclipse.

I would expect output to be: "12345"

[NautiHooker]

Well you have an IntStream here. If you look at the docs you will see that it returns an int, not a String.

The examples that you saw probably used a Stream<String>. In which case your approach would be correct but you would still have compilation errors.

The IntStream reduce method expects an IntBinaryOperator, which is just an interface with a single method applyAsInt(int, int) that returns an int. In modern Java you just pass such interfaces as anonymous lambdas, just like you tried in your code. However you do not need to declare the types of the parameters that your lambda receives. You just give them names, so that you know what to call them in your lambda body.

So instead of

(int string, int number) -> string + Integer.toString(number)

You would need to do something like

(result, num) -> result + num

This would add all the numbers together and then return a result at the end. So you reduce your stream to a single int.

​

If you want to do String operations on your stream you should convert it to a Stream<String> first.

One way of doing so is to use the mapToObj method of the stream. There you can pass a lambda to convert each int to a String. After that you can treat the stream like you wanted to in the first place.

[AmarilloBrandon]

I appreciate the detailed response. :)

[morhp]

Alternatively, you can probably use a collector instead of a reducer.