BAEL-4589: Update Guide to Java 8's Collectors (#10009)

* BAEL-4589: Move Guide to Java 8’s Collectors to core-java-11-2

* BAEL-4589: Add Java 10 unmodifiable* collectors examples

core-java-modules/core-java-11-2/README.md

@@ -4,4 +4,5 @@ This module contains articles about Java 11 core features
 
 ### Relevant articles
 - [Guide to Java 8 Optional](https://www.baeldung.com/java-optional)
+- [Guide to Java 8’s Collectors](https://www.baeldung.com/java-8-collectors)
 

core-java-modules/core-java-11-2/pom.xml

@@ -17,6 +17,11 @@
     </parent>
 
     <dependencies>
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+            <version>${guava.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
@@ -42,6 +47,7 @@
     <properties>
         <maven.compiler.source.version>11</maven.compiler.source.version>
         <maven.compiler.target.version>11</maven.compiler.target.version>
+        <guava.version>29.0-jre</guava.version>
         <assertj.version>3.17.2</assertj.version>
     </properties>
 

core-java-modules/core-java-11-2/src/test/java/com/baeldung/collectors/Java8CollectorsUnitTest.java

@@ -1,18 +1,11 @@
-package com.baeldung.streams.collectors;
+package com.baeldung.collectors;
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Sets;
 import org.junit.Test;
 
-import java.util.Arrays;
-import java.util.Comparator;
-import java.util.DoubleSummaryStatistics;
-import java.util.LinkedList;
-import java.util.List;
-import java.util.Map;
-import java.util.Optional;
-import java.util.Set;
+import java.util.*;
 import java.util.function.BiConsumer;
 import java.util.function.BinaryOperator;
 import java.util.function.Function;
@@ -20,19 +13,7 @@ import java.util.function.Supplier;
 import java.util.stream.Collector;
 
 import static com.google.common.collect.Sets.newHashSet;
-import static java.util.stream.Collectors.averagingDouble;
-import static java.util.stream.Collectors.collectingAndThen;
-import static java.util.stream.Collectors.counting;
-import static java.util.stream.Collectors.groupingBy;
-import static java.util.stream.Collectors.joining;
-import static java.util.stream.Collectors.maxBy;
-import static java.util.stream.Collectors.partitioningBy;
-import static java.util.stream.Collectors.summarizingDouble;
-import static java.util.stream.Collectors.summingDouble;
-import static java.util.stream.Collectors.toCollection;
-import static java.util.stream.Collectors.toList;
-import static java.util.stream.Collectors.toMap;
-import static java.util.stream.Collectors.toSet;
+import static java.util.stream.Collectors.*;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
@@ -48,6 +29,14 @@ public class Java8CollectorsUnitTest {
         assertThat(result).containsAll(givenList);
     }
 
+    @Test
+    public void whenCollectingToUnmodifiableList_shouldCollectToUnmodifiableList() {
+        final List<String> result = givenList.stream().collect(toUnmodifiableList());
+
+        assertThatThrownBy(() -> result.add("foo"))
+          .isInstanceOf(UnsupportedOperationException.class);
+    }
+
     @Test
     public void whenCollectingToSet_shouldCollectToSet() throws Exception {
         final Set<String> result = givenList.stream().collect(toSet());
@@ -55,6 +44,14 @@ public class Java8CollectorsUnitTest {
         assertThat(result).containsAll(givenList);
     }
 
+    @Test
+    public void whenCollectingToUnmodifiableSet_shouldCollectToUnmodifiableSet() {
+        final Set<String> result = givenList.stream().collect(toUnmodifiableSet());
+
+        assertThatThrownBy(() -> result.add("foo"))
+          .isInstanceOf(UnsupportedOperationException.class);
+    }
+
     @Test
     public void givenContainsDuplicateElements_whenCollectingToSet_shouldAddDuplicateElementsOnlyOnce() throws Exception {
         final Set<String> result = listWithDuplicates.stream().collect(toSet());
@@ -84,6 +81,15 @@ public class Java8CollectorsUnitTest {
         assertThat(result).containsEntry("a", 1).containsEntry("bb", 2).containsEntry("ccc", 3).containsEntry("dd", 2);
     }
 
+    @Test
+    public void whenCollectingToUnmodifiableMap_shouldCollectToUnmodifiableMap() {
+        final Map<String, Integer> result = givenList.stream()
+          .collect(toUnmodifiableMap(Function.identity(), String::length));
+
+        assertThatThrownBy(() -> result.put("foo", 3))
+                .isInstanceOf(UnsupportedOperationException.class);
+    }
+
     @Test
     public void whenCollectingToMapwWithDuplicates_shouldCollectToMapMergingTheIdenticalItems() throws Exception {
         final Map<String, Integer> result = listWithDuplicates.stream().collect(

core-java-modules/core-java-streams-3/README.md

@@ -6,7 +6,6 @@ This module contains articles about the Stream API in Java.
 - [The Difference Between map() and flatMap()](https://www.baeldung.com/java-difference-map-and-flatmap)
 - [How to Use if/else Logic in Java 8 Streams](https://www.baeldung.com/java-8-streams-if-else-logic)
 - [The Difference Between Collection.stream().forEach() and Collection.forEach()](https://www.baeldung.com/java-collection-stream-foreach)
-- [Guide to Java 8’s Collectors](https://www.baeldung.com/java-8-collectors)
 - [Primitive Type Streams in Java 8](https://www.baeldung.com/java-8-primitive-streams)
 - [Debugging Java 8 Streams with IntelliJ](https://www.baeldung.com/intellij-debugging-java-streams)
 - [Add BigDecimals using the Stream API](https://www.baeldung.com/java-stream-add-bigdecimals)