Calculating average of an array list?

With Java 8 it is a bit easier:

OptionalDouble average = marks
            .stream()
            .mapToDouble(a -> a)
            .average();

Thus your average value is average.getAsDouble()

return average.isPresent() ? average.getAsDouble() : 0; 

From Java8 onward you can get the average of the values from a List as follows:

    List<Integer> intList = Arrays.asList(1,2,2,3,1,5);

    Double average = intList.stream().mapToInt(val -> val).average().orElse(0.0);

This has the advantage of having no moving parts. It can be easily adapted to work with a List of other types of object by changing the map method call.

For example with Doubles:

    List<Double> dblList = Arrays.asList(1.1,2.1,2.2,3.1,1.5,5.3);
    Double average = dblList.stream().mapToDouble(val -> val).average().orElse(0.0);

NB. mapToDouble is required because it returns a DoubleStream which has an average method, while using map does not.

or BigDecimals:

@Test
public void bigDecimalListAveragedCorrectly() {
    List<BigDecimal> bdList = Arrays.asList(valueOf(1.1),valueOf(2.1),valueOf(2.2),valueOf(3.1),valueOf(1.5),valueOf(5.3));
    Double average = bdList.stream().mapToDouble(BigDecimal::doubleValue).average().orElse(0.0);
    assertEquals(2.55, average, 0.000001);
}

using orElse(0.0) removes problems with the Optional object returned from the average being 'not present'.

Use a double for the sum, otherwise you are doing an integer division and you won't get any decimals:

private double calculateAverage(List <Integer> marks) {
    if (marks == null || marks.isEmpty()) {
        return 0;
    }

    double sum = 0;
    for (Integer mark : marks) {
        sum += mark;
    }

    return sum / marks.size();
}

or using the Java 8 stream API:

    return marks.stream().mapToInt(i -> i).average().orElse(0);

sum += i;

You're adding the index; you should be adding the actual item in the ArrayList:

sum += marks.get(i);

Also, to ensure the return value isn't truncated, force one operand to double and change your method signature to double:

return (double)sum / marks.size();

Using Guava, it gets syntactically simplified:

Stats.meanOf(numericList);

List.stream().mapToDouble(a->a).average()

When the number is not big, everything seems just right. But if it isn't, great caution is required to achieve correctness.

Take double as an example:

If it is not big, as others mentioned you can just try this simply:

doubles.stream().mapToDouble(d -> d).average().orElse(0.0);

However, if it's out of your control and quite big, you have to turn to BigDecimal as follows (methods in the old answers using BigDecimal actually are wrong).

doubles.stream().map(BigDecimal::valueOf).reduce(BigDecimal.ZERO, BigDecimal::add)
       .divide(BigDecimal.valueOf(doubles.size())).doubleValue();

Enclose the tests I carried out to demonstrate my point:

    @Test
    public void testAvgDouble() {
        assertEquals(5.0, getAvgBasic(Stream.of(2.0, 4.0, 6.0, 8.0)), 1E-5);
        List<Double> doubleList = new ArrayList<>(Arrays.asList(Math.pow(10, 308), Math.pow(10, 308), Math.pow(10, 308), Math.pow(10, 308)));
        // Double.MAX_VALUE = 1.7976931348623157e+308
        BigDecimal doubleSum = BigDecimal.ZERO;
        for (Double d : doubleList) {
            doubleSum =  doubleSum.add(new BigDecimal(d.toString()));
        }
        out.println(doubleSum.divide(valueOf(doubleList.size())).doubleValue());
        out.println(getAvgUsingRealBigDecimal(doubleList.stream()));
        out.println(getAvgBasic(doubleList.stream()));
        out.println(getAvgUsingFakeBigDecimal(doubleList.stream()));
    }

    private double getAvgBasic(Stream<Double> doubleStream) {
        return doubleStream.mapToDouble(d -> d).average().orElse(0.0);
    }

    private double getAvgUsingFakeBigDecimal(Stream<Double> doubleStream) {
        return doubleStream.map(BigDecimal::valueOf)
                .collect(Collectors.averagingDouble(BigDecimal::doubleValue));
    }

    private double getAvgUsingRealBigDecimal(Stream<Double> doubleStream) {
        List<Double> doubles = doubleStream.collect(Collectors.toList());
        return doubles.stream().map(BigDecimal::valueOf).reduce(BigDecimal.ZERO, BigDecimal::add)
                .divide(valueOf(doubles.size()), BigDecimal.ROUND_DOWN).doubleValue();
    }

As for Integer or Long, correspondingly you can use BigInteger similarly.

Correct and fast way compute average for List<Integer>:

private double calculateAverage(List<Integer> marks) {
    long sum = 0;
    for (Integer mark : marks) {
        sum += mark;
    }
    return marks.isEmpty()? 0: 1.0*sum/marks.size();
}

This solution take into account:

• Handle overflow
• Do not allocate memory like Java8 stream
• Do not use slow BigDecimal

It works coorectly for List, because any list contains less that 2^31 int, and it is possible to use long as accumulator.

PS

Actually foreach allocate memory - you should use old style for() cycle in mission critical parts

You can use standard looping constructs or iterator/listiterator for the same :

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
double sum = 0;
Iterator<Integer> iter1 = list.iterator();
while (iter1.hasNext()) {
    sum += iter1.next();
}
double average = sum / list.size();
System.out.println("Average = " + average);

If using Java 8, you could use Stream or IntSream operations for the same :

OptionalDouble avg = list.stream().mapToInt(Integer::intValue).average();
System.out.println("Average = " + avg.getAsDouble());

Reference : Calculating average of arraylist

Here a version which uses BigDecimal instead of double:

public static BigDecimal calculateAverage(final List<Integer> values) {
    int sum = 0;
    if (!values.isEmpty()) {
        for (final Integer v : values) {
            sum += v;
        }
        return new BigDecimal(sum).divide(new BigDecimal(values.size()), 2, RoundingMode.HALF_UP);
    }
    return BigDecimal.ZERO;
}