bael-2508 - Stream.peek() examples (#6132)

* bael-2508 - Stream.peek() examples

* beal-2508 additional example

* bael-2508 moving examples

java-streams/src/test/java/com/baeldung/stream/PeekUnitTest.java

@@ -0,0 +1,118 @@
+package com.baeldung.stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.StringWriter;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class PeekUnitTest {
+
+    private StringWriter out;
+
+    @BeforeEach
+    void setup() {
+        out = new StringWriter();
+    }
+
+    @Test
+    void givenStringStream_whenCallingPeekOnly_thenNoElementProcessed() {
+        // given
+        Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
+
+        // when
+        nameStream.peek(out::append);
+
+        // then
+        assertThat(out.toString()).isEmpty();
+    }
+
+    @Test
+    void givenStringStream_whenCallingForEachOnly_thenElementsProcessed() {
+        // given
+        Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
+
+        // when
+        nameStream.forEach(out::append);
+
+        // then
+        assertThat(out.toString()).isEqualTo("AliceBobChuck");
+    }
+
+    @Test
+    void givenStringStream_whenCallingPeekAndNoopForEach_thenElementsProcessed() {
+        // given
+        Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
+
+        // when
+        nameStream.peek(out::append)
+            .forEach(this::noop);
+
+        // then
+        assertThat(out.toString()).isEqualTo("AliceBobChuck");
+    }
+
+    @Test
+    void givenStringStream_whenCallingPeekAndCollect_thenElementsProcessed() {
+        // given
+        Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
+
+        // when
+        nameStream.peek(out::append)
+            .collect(Collectors.toList());
+
+        // then
+        assertThat(out.toString()).isEqualTo("AliceBobChuck");
+    }
+
+    @Test
+    void givenStringStream_whenCallingPeekAndForEach_thenElementsProcessedTwice() {
+        // given
+        Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
+
+        // when
+        nameStream.peek(out::append)
+            .forEach(out::append);
+
+        // then
+        assertThat(out.toString()).isEqualTo("AliceAliceBobBobChuckChuck");
+    }
+
+    @Test
+    void givenStringStream_whenCallingPeek_thenElementsProcessedTwice() {
+        // given
+        Stream<User> userStream = Stream.of(new User("Alice"), new User("Bob"), new User("Chuck"));
+
+        // when
+        userStream.peek(u -> u.setName(u.getName().toLowerCase()))
+            .map(User::getName)
+            .forEach(out::append);
+
+        // then
+        assertThat(out.toString()).isEqualTo("alicebobchuck");
+    }
+
+    private static class User {
+        private String name;
+
+        public User(String name) {
+            this.name = name;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+    }
+
+    private void noop(String s) {
+    }
+
+}