Adding up BigDecimals using Streams

Question

I have a collection of BigDecimals  in this example  a LinkedList  that I would like to add together  Is it possible to use streams for this   I noticed the Stream class has several methods  Stream  mapToInt Stream  mapToDouble Stream  mapToLong   Each of which has a convenient sum   method  But  as we know  float and double arithmetic is almost always a bad idea   So  is there a convenient way to sum up BigDecimals   This is the code I have so far   public static void main String   args        LinkedList lt BigDecimal gt  values   new LinkedList lt  gt         values add BigDecimal valueOf  1        values add BigDecimal valueOf 1 1        values add BigDecimal valueOf 2 1        values add BigDecimal valueOf  1            Classical Java approach     BigDecimal sum   BigDecimal ZERO      for BigDecimal value   values            System out println value           sum   sum add value             System out println  Sum       sum           Java 8 approach     values forEach  value  - gt  System out println value        System out println  Sum       values stream   mapToDouble BigDecimal  doubleValue  sum         System out println values stream   mapToDouble BigDecimal  doubleValue  summaryStatistics   toString         As you can see  I am summing up the BigDecimals using BigDecimal  doubleValue    but this is  as expected  not precise   Post-answer edit for posterity   Both answers were extremely helpful  I wanted to add a little  my real-life scenario does not involve a collection of raw BigDecimals  they are wrapped in an invoice  But  I was able to modify Aman Agnihotri s answer to account for this by using the map   function for stream   public static void main String   args         LinkedList lt Invoice gt  invoices   new LinkedList lt  gt         invoices add new Invoice  C1    I-001   BigDecimal valueOf  1   BigDecimal valueOf 10         invoices add new Invoice  C2    I-002   BigDecimal valueOf  7   BigDecimal valueOf 13         invoices add new Invoice  C3    I-003   BigDecimal valueOf 2 3   BigDecimal valueOf 8         invoices add new Invoice  C4    I-004   BigDecimal valueOf 1 2   BigDecimal valueOf 7             Classical Java approach     BigDecimal sum   BigDecimal ZERO      for Invoice invoice   invoices            BigDecimal total   invoice unit price multiply invoice quantity           System out println total           sum   sum add total             System out println  Sum       sum           Java 8 approach     invoices forEach  invoice  - gt  System out println invoice total          System out println  Sum       invoices stream   map  x  - gt  x total    reduce  x  y  - gt  x add y   get        static class Invoice       String company      String invoice number      BigDecimal unit price      BigDecimal quantity       public Invoice             unit price   BigDecimal ZERO          quantity   BigDecimal ZERO             public Invoice String company  String invoice number  BigDecimal unit price  BigDecimal quantity            this company   company          this invoice number   invoice number          this unit price   unit price          this quantity   quantity             public BigDecimal total             return unit price multiply quantity              public void setUnit price BigDecimal unit price            this unit price   unit price             public void setQuantity BigDecimal quantity            this quantity   quantity             public void setInvoice number String invoice number            this invoice number   invoice number             public void setCompany String company            this company   company             public BigDecimal getUnit price             return unit price             public BigDecimal getQuantity             return quantity             public String getInvoice number             return invoice number             public String getCompany             return company

User · Answer

This post already has a checked answer  but the answer doesn t filter for null values  The correct answer should prevent null values by using the Object  nonNull function as a predicate    BigDecimal result   invoiceList stream        map Invoice  total       filter Objects  nonNull       filter i - gt   i getUnit price      null   amp  amp   i getQuantity    null        reduce BigDecimal ZERO  BigDecimal  add     This prevents null values from attempting to be summed as we reduce

User · Answer

If you don t mind a third party dependency  there is a class named Collectors2 in Eclipse Collections which contains methods returning Collectors for summing and summarizing BigDecimal and BigInteger  These methods take a Function as a parameter so you can extract a BigDecimal or BigInteger value from an object   List lt BigDecimal gt  list   mList          BigDecimal valueOf 0 1           BigDecimal valueOf 1 1           BigDecimal valueOf 2 1           BigDecimal valueOf 0 1     BigDecimal sum           list stream   collect Collectors2 summingBigDecimal e - gt  e    Assert assertEquals BigDecimal valueOf 3 4   sum    BigDecimalSummaryStatistics statistics           list stream   collect Collectors2 summarizingBigDecimal e - gt  e    Assert assertEquals BigDecimal valueOf 3 4   statistics getSum     Assert assertEquals BigDecimal valueOf 0 1   statistics getMin     Assert assertEquals BigDecimal valueOf 2 1   statistics getMax     Assert assertEquals BigDecimal valueOf 0 85   statistics getAverage       Note  I am a committer for Eclipse Collections

User · Answer

You can sum up the values of a BigDecimal stream using a reusable Collector named summingUp   BigDecimal sum   bigDecimalStream collect summingUp       The Collector can be implemented like this   public static Collector lt BigDecimal     BigDecimal gt  summingUp         return Collectors reducing BigDecimal ZERO  BigDecimal  add

User · Answer

Use this approach to sum the list of BigDecimal   List lt BigDecimal gt  values          List of BigDecimal objects BigDecimal sum   values stream   reduce  x  y  - gt  x add y   get      This approach maps each BigDecimal as a BigDecimal only and reduces them by summing them  which is then returned using the get   method   Here s another simple way to do the same summing   List lt BigDecimal gt  values          List of BigDecimal objects BigDecimal sum   values stream   reduce BigDecimal  add  get      Update  If I were to write the class and lambda expression in the edited question  I would have written it as follows   import java math BigDecimal  import java util LinkedList   public class Demo     public static void main String   args          LinkedList lt Invoice gt  invoices   new LinkedList lt  gt         invoices add new Invoice  C1    I-001   BigDecimal valueOf  1   BigDecimal valueOf 10         invoices add new Invoice  C2    I-002   BigDecimal valueOf  7   BigDecimal valueOf 13         invoices add new Invoice  C3    I-003   BigDecimal valueOf 2 3   BigDecimal valueOf 8         invoices add new Invoice  C4    I-004   BigDecimal valueOf 1 2   BigDecimal valueOf 7             Java 8 approach  using Method Reference for mapping purposes      invoices stream   map Invoice  total  forEach System out  println       System out println  Sum       invoices stream   map Invoice  total  reduce  x  y  - gt  x add y   get               This is just my style of writing classes  Yours can differ    static class Invoice         private String company      private String number      private BigDecimal unitPrice      private BigDecimal quantity       public Invoice               unitPrice   quantity   BigDecimal ZERO             public Invoice String company  String number  BigDecimal unitPrice  BigDecimal quantity              setCompany company         setNumber number         setUnitPrice unitPrice         setQuantity quantity              public BigDecimal total               return unitPrice multiply quantity              public String getCompany               return company             public void setCompany String company              this company   company             public String getNumber               return number             public void setNumber String number              this number   number             public BigDecimal getUnitPrice               return unitPrice             public void setUnitPrice BigDecimal unitPrice              this unitPrice   unitPrice             public BigDecimal getQuantity               return quantity             public void setQuantity BigDecimal quantity              this quantity   quantity

User · Answer

Original answer  Yes  this is possible   List lt BigDecimal gt  bdList   new ArrayList lt  gt       populate list BigDecimal result   bdList stream            reduce BigDecimal ZERO  BigDecimal  add     What it does is    Obtain a List lt BigDecimal gt   Turn it into a Stream lt BigDecimal gt  Call the reduce method   3 1  We supply an identity value for addition  namely BigDecimal ZERO   3 2  We specify the BinaryOperator lt BigDecimal gt   which adds two BigDecimal s  via a method reference BigDecimal  add    Updated answer  after edit  I see that you have added new data  therefore the new answer will become   List lt Invoice gt  invoiceList   new ArrayList lt  gt       populate Function lt Invoice  BigDecimal gt  totalMapper   invoice - gt  invoice getUnit price   multiply invoice getQuantity     BigDecimal result   invoiceList stream            map totalMapper           reduce BigDecimal ZERO  BigDecimal  add     It is mostly the same  except that I have added a totalMapper variable  that has a function from Invoice to BigDecimal and returns the total price of that invoice   Then I obtain a Stream lt Invoice gt   map it to a Stream lt BigDecimal gt  and then reduce it to a BigDecimal   Now  from an OOP design point I would advice you to also actually use the total   method  which you have already defined  then it even becomes easier   List lt Invoice gt  invoiceList   new ArrayList lt  gt       populate BigDecimal result   invoiceList stream            map Invoice  total           reduce BigDecimal ZERO  BigDecimal  add     Here we directly use the method reference in the map method

[java] Adding up BigDecimals using Streams

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