"Error: no suitable method found for reduce" when reducing an int[] array using streams

Question

Given this attempt to implement a sum :

int[] nums = { 1,3,4,5,7};
var sum = Arrays.asList(nums).stream().reduce(0,(a,b)->a+b);

Problems like the following are typically due to insufficient type information provided to the compiler for type inference. But here we have a non-ambiguous int[] array as the source. Why is this error then happening?

Line 3: error: no suitable method found for reduce(int,(a,b)->a + b)
        var sum = Arrays.asList(nums).stream().reduce(0,(a,b)->a+b);
                                              ^
    method Stream.reduce(int[],BinaryOperator<int[]>) is not applicable
      (argument mismatch; int cannot be converted to int[])
    method Stream.<U>reduce(U,BiFunction<U,? super int[],U>,BinaryOperator<U>) is not applicable
      (cannot infer type-variable(s) U
        (actual and formal argument lists differ in length))
  where U,T are type-variables:
    U extends Object declared in method <U>reduce(U,BiFunction<U,? super T,U>,BinaryOperator<U>)
    T extends Object declared in interface Stream

I cannot believe this kind of problem was not asked before... — user85421
– user85421, Commented Jan 1 at 9:55
kind of sure now: Arrays.asList() not working as it should? or Arrays.stream(array) vs Arrays.asList(array).stream() or java : Understanding Arrays.asList(T...array) method for primitive types or Lambda not resolving type or Collecting value of int array using normal JAVA Stream - . . . — user85421
– user85421, Commented Jan 1 at 10:24
@user85421 Yea well I had a reduce() problem. Wow how bad of me not to translate it to those other ones. — WestCoastProjects
– WestCoastProjects, Commented Jan 1 at 14:40
@WestCoastProjects I agree with you that it's not immediately obvious from the error messaging. — M. Justin
– M. Justin, Commented Jan 1 at 17:04

M. Justin · Accepted Answer · 2025-01-01 04:38:39Z

3

var sum = Arrays.asList(nums) returns a List<int[]> and consequently the reduce method adds int[] to int[]. This is not allowed and leads to a compilation error.

This is a possible solution:

int[] nums = { 1,3,4,5,7};
var sum= Arrays.stream(nums).reduce(0,(a,b)->a + b);

or

var result = Arrays.stream(nums).sum();

edited Jan 1 at 4:38

M. Justin

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answered Apr 19, 2021 at 23:14

Shems Oubouziane

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1 Comment

WestCoastProjects Over a year ago

ah nice catch. That's a pretty quick first accepted answer - congrats!

M. Justin · Accepted Answer · 2025-01-01 08:43:04Z

Since nums is an array of primitives (int[]), not an array of objects (e.g. Integer[]), its elements are not treated as varargs arguments to Arrays.asList(T... a). Rather, the whole int[] is treated as a single argument. This results in a list of type List<int[]> with a single element: the int[] value nums.

Since this is a Stream<int[]>, the compilation fails since the reduce lambda (a, b)->a+b is attempting to add two int[] values together using +, which is not a valid operator for two arrays.

Changing nums to an array of objects causes Arrays.asList to work as originally intended:

Integer[] nums = {1,3,4,5,7};
var sum = Arrays.asList(nums).stream().reduce(0,(a, b)->a+b);

That said, this results in needless boxing and unboxing between Integer and int. It would be better to use an IntStream of the array using the Arrays.stream method:

int[] nums = {1,3,4,5,7};
var sum = Arrays.stream(nums).reduce(0,(a, b)->a+b);

The Arrays.stream(nums) call creates an IntStream of the array's elements. This is then reduced to the sum using its reduce(int identity, IntBinaryOperator op) method.

Summing values of an IntStream can be simplified further to use sum instead of reduce:

int[] nums = {1,3,4,5,7};
var sum = Arrays.stream(nums).sum();

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