 ## 1. Introduction

In this quick tutorial, we'll cover how we can calculate sum & average in an array using both Java standard loops and the Stream API.

## 2. Find Sum of Array Elements

### 2.1. Sum Using a For Loop

In order to find the sum of all elements in an array, we can simply iterate the array and add each element to a sum accumulating variable.

This very simply starts with a sum of 0 and add each item in the array as we go:

``````public static int findSumWithoutUsingStream(int[] array) {
int sum = 0;
for (int value : array) {
sum += value;
}
return sum;
}``````

### 2.2. Sum With the Java Stream API

We can use the Stream API for achieving the same result:

``````public static int findSumUsingStream(int[] array) {
return Arrays.stream(array).sum();
}``````

It's important to know that the sum() method only supports primitive type streams.

If we want to use a stream on a boxed Integer value, we must first convert the stream into IntStream using the mapToInt method.

After that, we can apply the sum() method to our newly converted IntStream:

``````public static int findSumUsingStream(Integer[] array) {
return Arrays.stream(array)
.mapToInt(Integer::intValue)
.sum();
}``````

## 3. Find Average in a Java Array

### 3.1. Average Without the Stream API

Once we know how to calculate the sum of array elements, finding average is pretty easy – as Average = Sum of Elements / Number of Elements:

``````public static double findAverageWithoutUsingStream(int[] array) {
int sum = findSumWithoutUsingStream(array);
return (double) sum / array.length;
}``````

Notes:

1. Dividing an int by another int returns an int result. To get an accurate average, we first cast sum to double.
2. Java Array has a length field which stores the number of elements in the array.

### 3.2. Average Using the Java Stream API

``````public static double findAverageUsingStream(int[] array) {
return Arrays.stream(array).average().orElse(Double.NaN);
}``````

IntStream.average() returns an OptionalDouble which may not contain a value and which needs a special handling.