BAEL-4583-Generic-arrays-streams

core-java-modules/core-java-arrays-guides/src/test/java/com/baeldung/genericarrays/ListToArrayUnitTest.java

@@ -2,7 +2,6 @@ package com.baeldung.genericarrays;
 
 import org.junit.Test;
 
-import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
 

core-java-modules/core-java-arrays-guides/src/test/java/com/baeldung/genericarrays/StreamToArrayUnitTest.java

@@ -0,0 +1,77 @@
+package com.baeldung.genericarrays;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+import java.util.function.IntFunction;
+import java.util.stream.Stream;
+
+import static java.util.Collections.singletonList;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class StreamToArrayUnitTest {
+
+    @Test
+    public void givenAStream_thenCanGetArrayOfObject() {
+        Object[] strings = Stream.of("A", "AAA", "B", "AAB", "C")
+          .filter(string -> string.startsWith("A"))
+          .toArray();
+
+        assertThat(strings).containsExactly("A", "AAA", "AAB");
+        assertThat(strings).isNotInstanceOf(String[].class);
+    }
+
+    @Test
+    public void givenAStream_thenCanGetArrayOfString() {
+        String[] strings = Stream.of("A", "AAA", "B", "AAB", "C")
+          .filter(string -> string.startsWith("A"))
+          .toArray(String[]::new);
+
+        assertThat(strings).containsExactly("A", "AAA", "AAB");
+        assertThat(strings).isInstanceOf(String[].class);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void givenAStream_whenConvertToOptional_thenCanGetArrayOfOptional() {
+        Stream<Optional<String>> stream = Stream.of("A", "AAA", "B", "AAB", "C")
+                .filter(string -> string.startsWith("A"))
+                .map(Optional::of);
+        Optional<String>[] strings = stream
+          .toArray(Optional[]::new);
+
+        assertThat(strings).containsExactly(Optional.of("A"),
+          Optional.of("AAA"),
+          Optional.of("AAB"));
+        assertThat(strings).isInstanceOf(Optional[].class);
+    }
+
+    @Test
+    public void givenAStream_whenConvertToOptional_thenCanGetArrayOfOptionalWithHelper() {
+        Optional<String>[] strings = Stream.of("A", "AAA", "B", "AAB", "C")
+          .filter(string -> string.startsWith("A"))
+          .map(Optional::of)
+          .toArray(genericArray(Optional[]::new));
+
+        assertThat(strings).containsExactly(Optional.of("A"),
+          Optional.of("AAA"),
+          Optional.of("AAB"));
+        assertThat(strings).isInstanceOf(Optional[].class);
+    }
+
+    @Test
+    public void whenInvalidUseOfGenericArray_thenIllegalCast() {
+        assertThatThrownBy(() -> {
+            ArrayList<String>[] lists = Stream.of(singletonList("A"))
+                    .toArray(genericArray(List[]::new));
+        }).isInstanceOf(ClassCastException.class);
+    }
+
+    @SuppressWarnings("unchecked")
+    private static <T, R extends T> IntFunction<R[]> genericArray(IntFunction<T[]> arrayCreator) {
+        return size -> (R[])arrayCreator.apply(size);
+    }
+}