BAEL-7062 - Fetching Index from List using Streams (#14914)

* Primary Commit

* More Cases covered

* Indentation Corrections on POM

* Merge Conflict Fixed

* Updating as per Jira comments

* Correction

* Review Comment Implementation

* Indentation fixes

* Review Comment Implementation

* Method removal for Iterators

* Test Case Update

* Review Comment changes

* formatting change

* Updating map method

* Review comment update

* Review Comments

* Review Comments

core-java-modules/core-java-streams-5/pom.xml

@@ -48,6 +48,11 @@
             <artifactId>guava</artifactId>
             <version>${guava.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>${commons-collections4.version}</version>
+        </dependency>
     </dependencies>
 
     <build>

core-java-modules/core-java-streams-5/src/main/java/com/baeldung/streams/firstmatchingelement/User.java

@@ -0,0 +1,21 @@
+package com.baeldung.streams.firstmatchingelement;
+
+public class User {
+
+    private String userName;
+    private Integer userId;
+
+    public User(Integer userId, String userName) {
+        this.userId = userId;
+        this.userName = userName;
+    }
+
+    public String getUserName() {
+        return userName;
+    }
+
+    public Integer getUserId() {
+        return userId;
+    }
+
+}

core-java-modules/core-java-streams-5/src/test/java/com/baeldung/streams/firstmatchingelement/FirstMatchingElementUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.streams.firstmatchingelement;
+
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.IntStream;
+
+import org.apache.commons.collections4.IterableUtils;
+import org.junit.Test;
+
+import com.google.common.collect.Iterables;
+
+public class FirstMatchingElementUnitTest {
+
+    private List<User> userList = List.of(new User(1, "David"), new User(2, "John"), new User(3, "Roger"), new User(4, "John"));
+    private String searchName = "John";
+
+    @Test
+    public void whenUsingStream_thenFindFirstMatchingUserIndex() {
+        AtomicInteger counter = new AtomicInteger(-1);
+        int index = userList.stream()
+          .filter(user -> {
+              counter.getAndIncrement();
+              return searchName.equals(user.getUserName());
+          })
+          .mapToInt(user -> counter.get())
+          .findFirst()
+          .orElse(-1);
+
+        assertEquals(1, index);
+    }
+
+    @Test
+    public void whenUsingIntStream_thenFindFirstMatchingUserIndex() {
+        int index = IntStream.range(0, userList.size() - 1)
+          .filter(streamIndex -> searchName.equals(userList.get(streamIndex).getUserName()))
+          .findFirst()
+          .orElse(-1);
+        assertEquals(1, index);
+    }
+
+    @Test
+    public void whenUsingTakeWhile_thenFindFirstMatchingUserIndex() {
+        long predicateIndex = userList.stream()
+          .takeWhile(user -> !user.getUserName().equals(searchName))
+          .count();
+        assertEquals(1, predicateIndex);
+    }
+    
+    @Test
+    public void whenUsingTakeWhile_thenFindIndexFromNoMatchingElement() {
+        List<User> userList = List.of(new User(1, "David"), new User(2, "Vick"), new User(3, "Roger"), new User(4, "James"));
+        long predicateIndex = userList.stream()
+          .takeWhile(user -> !user.getUserName().equals(searchName))
+          .count();
+        assertEquals(4, predicateIndex);
+    }
+    
+    @Test
+    public void whenUsingGoogleGuava_thenFindFirstMatchingUserIndex() {
+        int index = Iterables.indexOf(userList, user -> searchName.equals(user.getUserName()));
+        assertEquals(1, index);
+    }
+
+    @Test
+    public void whenUsingApacheCommons_thenFindFirstMatchingUserIndex() {
+        int index = IterableUtils.indexOf(userList, user -> searchName.equals(user.getUserName()));
+        assertEquals(1, index);
+    }
+}