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Java 8 - sum List of Integers with Stream
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In this article, we would like to show you how to sum List of Integers with Stream in Java 8.
In this example, we use Stream.reduce() method where the accumulator function is a lambda expression that adds two Integer values.
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import java.util.*;2
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public class Example {4
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public static void main(String[] args) {6
// create list of integers7
List<Integer> numbers = Arrays.asList(1, 2, 3);8
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// sum elements10
Integer sum = numbers.stream()11
.reduce(0, Integer::sum);12
13
// display result14
System.out.println("sum = " + sum);15
}16
}Output:
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sum = 6Note:
xxxxxxxxxx1Integer sum = numbers.stream()2.reduce(0, Integer::sum);is equivalent to:
xxxxxxxxxx1Integer sum = numbers.stream()2.reduce(0, (a, b) -> a + b);
In this example, to calculate the sum of a list of integers we use:
mapToInt()that converts the stream to an IntStream object,sum()method to calculate the sum of the IntStream's elements.
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import java.util.*;2
3
public class Example {4
5
public static void main(String[] args) {6
// create list of integers7
List<Integer> numbers = Arrays.asList(1, 2, 3);8
9
// sum elements10
int sum = numbers.stream()11
.mapToInt(Integer::intValue) // convert to IntStream object12
.sum(); // sum elements13
14
// display result15
System.out.println("sum = " + sum);16
}17
}Output:
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sum = 6Note:
In the same way, we can use the
mapToLong()andmapToDouble()methods to calculate the sums of longs and doubles.xxxxxxxxxx1double sum = numbers.stream()2.mapToDouble(Double::doubleValue)3.sum();
In this example, we use collect() method to calculate the sum of a list of integers.
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import java.util.*;2
import java.util.stream.Collectors;3
4
public class Example {5
6
public static void main(String[] args) {7
// create list of integers8
List<Integer> numbers = Arrays.asList(1, 2, 3);9
10
// sum elements11
Integer sum = numbers.stream()12
.collect(Collectors.summingInt(Integer::intValue));13
14
// display result15
System.out.println("sum = " + sum);16
}17
}Output:
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sum = 6