BAEL-3918 - Adding BigDecimal numbers using the Stream API (#8900)

* BAEL-3918 - Adding BigDecimal numbers using the Stream API

* BAEL-3918 - Adding BigDecimal numbers using the Stream API

* BAEL-3918 - Adding BigDecimal numbers using the Stream API

* BAEL-3918 - Updating test methods names to be compliant with Baeldung standards

* Minor name change

Co-authored-by: ashleyfrieze <ashley@incredible.org.uk>

core-java-modules/core-java-streams-3/src/test/java/com/baeldung/streams/bigdecimals/AddNumbersUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.streams.bigdecimals;
+
+import static org.junit.Assert.assertEquals;
+
+import java.math.BigDecimal;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+public class AddNumbersUnitTest {
+
+    @Test
+    public void givenIntStream_whenSum_thenResultIsCorrect() {
+        IntStream intNumbers = IntStream.range(0, 3);
+        assertEquals(3, intNumbers.sum());
+    }
+
+    @Test
+    public void givenCollectionOfDouble_whenUsingMapToDoubleToSum_thenResultIsCorrect() {
+        List<Double> doubleNumbers = Arrays.asList(23.48, 52.26, 13.5);
+        double result = doubleNumbers.stream()
+            .mapToDouble(Double::doubleValue)
+            .sum();
+        assertEquals(89.24, result, .1);
+    }
+
+    public void givenStreamOfIntegers_whenUsingReduceToSum_thenResultIsCorrect() {
+        Stream<Integer> intNumbers = Stream.of(0, 1, 2);
+        int result = intNumbers.reduce(0, Integer::sum);
+        assertEquals(106, result);
+    }
+
+    public void givenStreamOfBigDecimals_whenUsingReduceToSum_thenResultIsCorrect() {
+        Stream<BigDecimal> bigDecimalNumber = Stream.of(BigDecimal.ZERO, BigDecimal.ONE, BigDecimal.TEN);
+        BigDecimal result = bigDecimalNumber.reduce(BigDecimal.ZERO, BigDecimal::add);
+        assertEquals(11, result);
+    }
+
+}