Fixed MapUsingReduce

ARascius · ARascius · commit 6c3e4c7afa09 · 2014-09-29T11:15:13.000+03:00
Contract of Stream.reduce method requires that accumulator function
does not mutate its arguments. Previous implementation of
MapUsingReduce.map method was using accumulator and combiner
functions that were mutating list passed as argument. Mutating
parameters in accumulator and combiner functions results in
incorrect behaviour when parallel stream is processed.

New implementation of MapUsingReduce.map copies argument lists
in accumulator and combiner functions. This is very inefficient.
Stream.collect method could be used to implement more efficient
mutable reduction, but exercise 1 of chapter 3 asks to use reduce
method explicitly.

Unit test MapUsingReduceTest was updated to check functionality of
MapUsingReduce.map with parallel streams.
diff --git a/src/main/java/com/insightfullogic/java8/answers/chapter3/MapUsingReduce.java b/src/main/java/com/insightfullogic/java8/answers/chapter3/MapUsingReduce.java
@@ -12,11 +12,19 @@ public class MapUsingReduce {
 
     public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper) {
         return stream.reduce(new ArrayList<O>(), (acc, x) -> {
-            acc.add(mapper.apply(x));
-            return acc;
+        	// We are copying data from acc to new list instance. It is very inefficient,
+        	// but contract of Stream.reduce method requires that accumulator function does
+        	// not mutate its arguments.
+        	// Stream.collect method could be used to implement more efficient mutable reduction,
+        	// but this exercise asks to use reduce method.
+        	List<O> newAcc = new ArrayList<>(acc);
+        	newAcc.add(mapper.apply(x));
+            return newAcc;
         }, (List<O> left, List<O> right) -> {
-            left.addAll(right);
-            return left;
+        	// We are copying left to new list to avoid mutating it. 
+        	List<O> newLeft = new ArrayList<>(left);
+        	newLeft.addAll(right);
+            return newLeft;
         });
     }
 
diff --git a/src/test/java/com/insightfullogic/java8/answers/chapter3/MapUsingReduceTest.java b/src/test/java/com/insightfullogic/java8/answers/chapter3/MapUsingReduceTest.java
@@ -1,14 +1,13 @@
 package com.insightfullogic.java8.answers.chapter3;
 
-import com.insightfullogic.java8.answers.chapter3.MapUsingReduce;
-import org.junit.Test;
+import static java.util.Arrays.asList;
+import static org.junit.Assert.assertEquals;
 
 import java.util.Collections;
 import java.util.List;
 import java.util.function.Function;
 
-import static java.util.Arrays.asList;
-import static org.junit.Assert.assertEquals;
+import org.junit.Test;
 
 public class MapUsingReduceTest {
 
@@ -30,6 +29,9 @@ public void incrementingNumbers() {
     private <I, O> void assertMapped(Function<I, O> mapper, List<I> input, List<O> expectedOutput) {
         List<O> output = MapUsingReduce.map(input.stream(), mapper);
         assertEquals(expectedOutput, output);
+
+        List<O> parallelOutput = MapUsingReduce.map(input.parallelStream(), mapper);
+        assertEquals(expectedOutput, parallelOutput);
     }
 
 }
diff --git a/src/test/java/com/insightfullogic/java8/exercises/chapter3/MapUsingReduceTest.java b/src/test/java/com/insightfullogic/java8/exercises/chapter3/MapUsingReduceTest.java
@@ -1,13 +1,13 @@
 package com.insightfullogic.java8.exercises.chapter3;
 
-import org.junit.Test;
+import static java.util.Arrays.asList;
+import static org.junit.Assert.assertEquals;
 
 import java.util.Collections;
 import java.util.List;
 import java.util.function.Function;
 
-import static java.util.Arrays.asList;
-import static org.junit.Assert.assertEquals;
+import org.junit.Test;
 
 public class MapUsingReduceTest {
 
@@ -29,6 +29,9 @@ public void incrementingNumbers() {
     private <I, O> void assertMapped(Function<I, O> mapper, List<I> input, List<O> expectedOutput) {
         List<O> output = MapUsingReduce.map(input.stream(), mapper);
         assertEquals(expectedOutput, output);
+
+        List<O> parallelOutput = MapUsingReduce.map(input.parallelStream(), mapper);
+        assertEquals(expectedOutput, parallelOutput);
     }
 
 }
