I am trying to add elements of int array using Stream API.

import java.util.*;

public class Main {
    public static void main(String[] args) {
        int[] a = {1,2,3,4,5};
        int sum = Arrays.asList(a).stream().reduce(0, (psum, x) -> psum x);
        System.out.println(sum);
    }
}

But its is giving this error message.

E:\Java\StreamAPI\src\Main.java:6:44
java: no suitable method found for reduce(int,(psum,x)->psum   x)
    method java.util.stream.Stream.reduce(int[],java.util.function.BinaryOperator<int[]>) is not applicable
      (argument mismatch; int cannot be converted to int[])
    method java.util.stream.Stream.<U>reduce(U,java.util.function.BiFunction<U,? super int[],U>,java.util.function.BinaryOperator<U>) is not applicable
      (cannot infer type-variable(s) U
        (actual and formal argument lists differ in length))

I even tried doing this

import java.util.*;

public class Main {
    public static void main(String[] args) {
        int[] a = {1,2,3,4,5};
        int sum = Arrays.asList(a).stream().reduce(0, Integer::sum);
        System.out.println(sum);
    }
}

I got this error message:

E:\Java\StreamAPI\src\Main.java:6:44
java: no suitable method found for reduce(int,Integer::sum)
    method java.util.stream.Stream.reduce(int[],java.util.function.BinaryOperator<int[]>) is not applicable
      (argument mismatch; int cannot be converted to int[])
    method java.util.stream.Stream.<U>reduce(U,java.util.function.BiFunction<U,? super int[],U>,java.util.function.BinaryOperator<U>) is not applicable
      (cannot infer type-variable(s) U
        (actual and formal argument lists differ in length))

Both of these examples were given in this Baeldung article about reduce in stream api.

CodePudding user response：

Your issue is that Arrays.asList(int[]) doesn't do what you think it does. It creates a list with a single element, an integer array. It does not create a list containing several integers. (And note that the Baeldung article you link doesn't use Array.asList on an int[], either.)

Instead, write Arrays.stream(a).reduce(0, Integer::sum), or even Arrays.stream(a).sum().

CodePudding user response：

The problem you are facing is, that you input a int[] into the Arrays.ofList-method.

The return type of the call Arrays.ofList(int[]) would be List<int[]>, which is currently not supported. Thus it is being interpreted you wanting a list with only a single element of type int[].

To circumvent that, you can either declare the list using the boxed types (Integer, Double, etc.) or use different Methods to generate a stream of integers.

Solution 1: Use Boxed Types

Integer[] a = {1,2,3,4,5};
int sum = Arrays.asList(a).stream().reduce(0, (psum, x) -> psum x);
System.out.println(sum);
// prints 15

Solution 2: Use Arrays.asList with varargs directly

int sum = Arrays.asList(1,2,3,4,5).stream().reduce(0, (psum, x) -> psum x);
System.out.println(sum);
// prints 15

Solution 3: Use Stream with varargs directly

Similar to the previous solution but generating directly the stream without an intermediate list.

int sum = Stream.of(1,2,3,4,5).reduce(0, (psum, x) -> psum x);
System.out.println(sum);
// prints 15

Solution 4: Use IntStream with varargs directly

Java offers multiple different types of Stream one of them being IntStream. This offers an unboxed version for primitives.

int sum = IntStream.of(1,2,3,4,5).reduce(0, (psum, x) -> psum x);
System.out.println(sum);
// prints 15

Bonus

An alternative to using reduce(0, (psum, x) -> psum x) would be using some adding-operation.

You could use Integer.sum(int a, int b) which fulfills the contract of reduce.

int sum = IntStream.of(1,2,3,4,5).reduce(0, Integer::sum);
System.out.println(sum);
// prints 15

Using an IntStream offers an additional benefit having the method sum().

int sum = IntStream.of(1,2,3,4,5).sum();
System.out.println(sum);
// prints 15

In case you are not able to generate an IntStream yourself and need to work with a Stream<Integer>, you can also map to an IntStream.

int sum = Stream.of(1,2,3,4,5).mapToInt(Integer::intValue).sum()
System.out.println(sum);
// prints 15