BAEL-887 How to collect a Java Stream to an immutable collection?

core-java/pom.xml

@@ -375,7 +375,7 @@
         <logback.version>1.1.7</logback.version>
 
         <!-- util -->
-        <guava.version>21.0</guava.version>
+        <guava.version>22.0</guava.version>
         <commons-lang3.version>3.5</commons-lang3.version>
         <bouncycastle.version>1.55</bouncycastle.version>
         <commons-codec.version>1.10</commons-codec.version>

core-java/src/test/java/com/baeldung/stream/StreamToImmutableTest.java

@@ -0,0 +1,69 @@
+package com.baeldung.stream;
+
+import static java.util.stream.Collectors.collectingAndThen;
+import static java.util.stream.Collectors.toList;
+import static java.util.stream.Collectors.toSet;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Set;
+import java.util.stream.IntStream;
+
+import org.junit.Test;
+
+import com.baeldung.stream.mycollectors.MyImmutableListCollector;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+
+public class StreamToImmutableTest {
+
+    @Test
+    public void whenUsingCollectingToImmutableSet_thenSuccess() {
+        Set<String> mutableSet = new HashSet<>(Arrays.asList("a", "b", "c"));
+        mutableSet.add("test");
+        Set<String> immutableSet = mutableSet.stream()
+            .collect(collectingAndThen(toSet(), ImmutableSet::copyOf));
+
+        System.out.println(immutableSet.getClass());
+    }
+
+    @Test
+    public void whenUsingCollectingToUnmodifiableList_thenSuccess() {
+        List<String> givenList = new ArrayList<>(Arrays.asList("a", "b", "c"));
+        List<String> result = givenList.stream()
+            .collect(collectingAndThen(toList(), Collections::unmodifiableList));
+
+        System.out.println(result.getClass());
+    }
+
+    @Test
+    public void whenCollectToImmutableList_thenSuccess() {
+        List<Integer> list = IntStream.range(0, 9)
+            .boxed()
+            .collect(ImmutableList.toImmutableList());
+
+        System.out.println(list.getClass());
+    }
+
+    @Test
+    public void whenCollectToMyImmutableListCollector_thenSuccess() {
+        List<String> givenList = Arrays.asList("a", "b", "c", "d");
+        List<String> result = givenList.stream()
+            .collect(MyImmutableListCollector.toImmutableList());
+
+        System.out.println(result.getClass());
+    }
+
+    @Test
+    public void whenPassingSupplier_thenSuccess() {
+        List<String> givenList = Arrays.asList("a", "b", "c", "d");
+        List<String> result = givenList.stream()
+            .collect(MyImmutableListCollector.toImmutableList(LinkedList::new));
+
+        System.out.println(result.getClass());
+    }
+}

core-java/src/test/java/com/baeldung/stream/mycollectors/MyImmutableListCollector.java

@@ -0,0 +1,22 @@
+package com.baeldung.stream.mycollectors;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+
+public class MyImmutableListCollector {
+
+    public static <T, A extends List<T>> Collector<T, A, List<T>> toImmutableList(Supplier<A> supplier) {
+        return Collector.of(supplier, List::add, (left, right) -> {
+            left.addAll(right);
+            return left;
+        }, Collections::unmodifiableList);
+    }
+
+    public static <T> Collector<T, List<T>, List<T>> toImmutableList() {
+        return toImmutableList(ArrayList::new);
+    }
+
+}