How to sum a list of integers with java streams

Question

I want to sum a list of integers  It works as follows  but the syntax does not feel right  Could the code be optimized   Map lt String  Integer gt  integers  integers values   stream   mapToInt i - gt  i  sum

User · Answer

You can use collect method to add list of integers   List lt Integer gt  list   Arrays asList 2  4  5  6   int sum   list stream   collect Collectors summingInt Integer  intValue

User · Answer

This will work  but the i - gt  i is doing some automatic unboxing which is why it  feels  strange  Either of the following will work and better explain what the compiler is doing under the hood with your original syntax   integers values   stream   mapToInt i - gt  i intValue    sum    integers values   stream   mapToInt Integer  intValue  sum

User · Answer

I suggest 2 more options   integers values   stream   mapToInt Integer  intValue  sum    integers values   stream   collect Collectors summingInt Integer  intValue      The second one uses Collectors summingInt   collector  there is also a summingLong   collector which you would use with mapToLong     And a third option  Java 8 introduces a very effective LongAdder accumulator designed to speed-up summarizing in parallel streams and multi-thread environments  Here  here s an example use   LongAdder a   new LongAdder    map values   parallelStream   forEach a  add   sum   a intValue

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You can use reduce   to sum a list of integers   int sum   integers values   stream   reduce 0  Integer  sum

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From the docs     Reduction operations   A reduction operation  also called a fold  takes a sequence of input elements and combines them into a single summary result by repeated application of a combining operation  such as finding the sum or maximum of a set of numbers  or accumulating elements into a list  The streams classes have multiple forms of general reduction operations  called reduce   and collect    as well as multiple specialized reduction forms such as sum    max    or count         Of course  such operations can be readily implemented as simple sequential loops  as in   int sum   0  for  int x   numbers       sum    x          However  there are good reasons to prefer a reduce operation over a mutative accumulation such as the above  Not only is a reduction  more abstract  -- it operates on the stream as a whole rather than individual elements -- but a properly constructed reduce operation is inherently parallelizable  so long as the function s  used to process the elements are associative and stateless  For example  given a stream of numbers for which we want to find the sum  we can write   int sum   numbers stream   reduce 0   x y  - gt  x y         or   int sum   numbers stream   reduce 0  Integer  sum         These reduction operations can run safely in parallel with almost no modification   int sum   numbers parallelStream   reduce 0  Integer  sum      So  for a map you would use   integers values   stream   mapToInt i - gt  i  reduce 0   x y  - gt  x y     Or   integers values   stream   reduce 0  Integer  sum

User · Answer

This would be the shortest way to sum up int type array  for long array LongStream  for double array DoubleStream and so forth   Not all the primitive integer or floating point types have the Stream implementation though    IntStream of integers  sum

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May this help those who have objects on the list   If you have a list of objects and wanted to sum specific fields of this object use the below   List lt ResultSom gt  somList   MyUtil getResultSom    BigDecimal result  somList stream   map ResultSom  getNetto  reduce                                               BigDecimal ZERO  BigDecimal  add

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Most of the aspects are covered  But there could be a requirement to find the aggregation of other data types apart from Integer  Long for which specialized stream support is already present   For e g  stram with BigInteger  For such a type we can use reduce operation like  list stream   reduce  bigInteger1  bigInteger2  -  bigInteger1 add bigInteger2

User · Answer

I have declared a list of Integers    ArrayList lt Integer gt  numberList   new ArrayList lt Integer gt  Arrays asList 1  2  3  4  5      You can try using these different ways below   Using mapToInt  int sum   numberList stream   mapToInt Integer  intValue  sum      Using summarizingInt  int sum   numberList stream   collect Collectors summarizingInt Integer  intValue   getSum      Using reduce  int sum   numberList stream   reduce Integer  sum  get   intValue

User · Answer

You can use reduce method   long sum   result stream   map e - gt  e getCreditAmount    reduce 0L   x  y  - gt  x   y     or  long sum   result stream   map e - gt  e getCreditAmount    reduce 0L  Integer  sum

User · Answer

class Pojo      int num       public Pojo int num            super            this num   num             public int getNum             return num             public void setNum int num            this num   num           List lt Pojo gt  list   new ArrayList lt Pojo gt                 list add new Pojo 1                list add new Pojo 5                list add new Pojo 3                list add new Pojo 4                list add new Pojo 5                 int totalSum   list stream   mapToInt pojo - gt  pojo getNum    sum                System out println totalSum

[java] How to sum a list of integers with java streams?

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