Java: How to properly manipulate BigDecimal array using functional programming?

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So, how to get outcome of this code using functional programming:

public static final List<BigDecimal> numbers = Arrays.asList(     new BigDecimal("15"), new BigDecimal("10"), new BigDecimal("17"),     new BigDecimal("30"), new BigDecimal("18"), new BigDecimal("23"),     new BigDecimal("5"), new BigDecimal("12") );  BigDecimal totalOfReducedNumbers = BigDecimal.ZERO;    for(BigDecimal number : numbers) {        if(number.compareTo(BigDecimal.valueOf(20)) > 0)             totalOfReducedNumbers =           totalOfReducedNumbers.add(number.multiply(BigDecimal.valueOf(0.9)));    } System.out.println("Total of reduced numbers: " + totalOfReducedNumbers); 

Which throws out "Total of reduced numbers: 47.7"

How to get the same outcome using functional programming tools like map(), reduce() etc?


The for loop implementation is a typical "reduce" pattern (also known as "fold", "Aggregate" pattern in some other languages)

You're looking for filter --> reduce method:

BigDecimal reduce =                  .filter(n -> n.compareTo(BigDecimal.valueOf(20)) > 0)                 .reduce(BigDecimal.ZERO,                         (a, b) -> a.add(b.multiply(BigDecimal.valueOf(0.9)))); 

You can further minimise the number of objects contructed by caching BigDecimal.valueOf(20) and BigDecimal.valueOf(0.9) as shown in @Elliot's answer i.e.:

BigDecimal TWENTY = BigDecimal.valueOf(20); BigDecimal POINT9 = BigDecimal.valueOf(0.9); BigDecimal reduce =                          .filter(n -> n.compareTo(TWENTY) > 0)                     .reduce(BigDecimal.ZERO,                             (a, b) -> a.add(b.multiply(BigDecimal.valueOf(POINT9)))); 


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