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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();
}

You can read a lot more about the Stream API here.

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.

Read more about Optionals in this article and about the OptionalDouble class in the Java 8 Documentation.

4. Conclusion

In this article, we explored how to find sum/average of int array elements.

As always, the code is available over on Github.

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