Guide to Stream.reduce() (#6372)

* Initial Commit * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java
2019-02-23 23:20:37 -03:00
parent 37f9a70441
commit ca4440d792
4 changed files with 265 additions and 0 deletions
@@ -0,0 +1,126 @@
+package com.baeldung.streamreduce.tests;
+
+import com.baeldung.streamreduce.entities.User;
+import com.baeldung.streamreduce.utilities.NumberUtils;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Test;
+
+public class StreamReduceUnitTest {
+
+    @Test
+    public void givenIntegerList_whenReduceWithSumAccumulatorLambda_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        int result = numbers.stream().reduce(0, (a, b) -> a + b);
+        
+        assertThat(result).isEqualTo(21);
+    }
+
+    @Test
+    public void givenIntegerList_whenReduceWithSumAccumulatorMethodReference_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        int result = numbers.stream().reduce(0, Integer::sum);
+        
+        assertThat(result).isEqualTo(21);
+    }
+
+    @Test
+    public void givenStringList_whenReduceWithConcatenatorAccumulatorLambda_thenCorrect() {
+        List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
+        
+        String result = letters.stream().reduce("", (a, b) -> a + b);
+        
+        assertThat(result).isEqualTo("abcde");
+    }
+
+    @Test
+    public void givenStringList_whenReduceWithConcatenatorAccumulatorMethodReference_thenCorrect() {
+        List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
+        
+        String result = letters.stream().reduce("", String::concat);
+        
+        assertThat(result).isEqualTo("abcde");
+    }
+
+    @Test
+    public void givenStringList_whenReduceWithUppercaseConcatenatorAccumulator_thenCorrect() {
+        List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
+        
+        String result = letters.stream().reduce("", (a, b) -> a.toUpperCase() + b.toUpperCase());
+        
+        assertThat(result).isEqualTo("ABCDE");
+    }
+
+    @Test
+    public void givenUserList_whenReduceWithAgeAccumulatorAndSumCombiner_thenCorrect() {
+        List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
+        
+        int result = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        
+        assertThat(result).isEqualTo(65);
+    }
+
+    @Test
+    public void givenStringList_whenReduceWithParallelStream_thenCorrect() {
+        List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
+        
+        String result = letters.parallelStream().reduce("", String::concat);
+        
+        assertThat(result).isEqualTo("abcde");
+    }
+
+    @Test
+    public void givenNumberUtilsClass_whenCalledDivideListElements_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        assertThat(NumberUtils.divideListElements(numbers, 1)).isEqualTo(21);
+    }
+
+    @Test
+    public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlock_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21);
+    }
+
+    @Test
+    public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlockAndListContainsZero_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(0, 1, 2, 3, 4, 5, 6);
+        
+        assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21);
+    }
+    
+    @Test
+    public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlockAndDividerIsZero_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 0)).isEqualTo(0);
+    }
+    
+    @Test
+    public void givenStream_whneCalleddivideListElementsWithApplyFunctionMethod_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        
+        assertThat(NumberUtils.divideListElementsWithApplyFunctionMethod(numbers, 1)).isEqualTo(21);
+    }
+    
+    @Test
+    public void givenTwoStreams_whenCalledReduceOnParallelizedStream_thenFasterExecutionTime() {
+        List<User> userList = new ArrayList<>();
+        for (int i = 0; i <= 1000000; i++) {
+            userList.add(new User("John" + i, i));
+        }
+        long currentTime1 = System.currentTimeMillis();
+        userList.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        long sequentialExecutionTime = System.currentTimeMillis() -currentTime1;
+        long currentTime2 = System.currentTimeMillis();
+        userList.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        long parallelizedExecutionTime = System.currentTimeMillis() - currentTime2;
+        
+        assertThat(parallelizedExecutionTime).isLessThan(sequentialExecutionTime);
+    }
+}