[stream-2maap-dup-keys] Handle Duplicated Keys When Producing Map Usi… (#13546)

* [stream-2maap-dup-keys] Handle Duplicated Keys When Producing Map Using Java Streamt

* [stream-2maap-dup-keys] create the *-streams-maps module

* [stream-2maap-dup-keys] add the new moudle to the parent pom

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

@@ -0,0 +1 @@
+## Relevant Articles:

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

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-streams-maps</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-streams-maps</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>log4j</groupId>
+            <artifactId>log4j</artifactId>
+            <version>${log4j.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.junit</groupId>
+            <artifactId>junit-bom</artifactId>
+            <version>${junit-jupiter.version}</version>
+            <type>pom</type>
+            <scope>import</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>3.23.1</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-streams-maps</finalName>
+        <resources>
+            <resource>
+                <directory>src/main</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>${maven-compiler-plugin.version}</version>
+                <configuration>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
+                    <compilerArgument>-parameters</compilerArgument>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <!-- testing -->
+        <maven-compiler-plugin.version>3.1</maven-compiler-plugin.version>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
+    </properties>
+
+</project>

core-java-modules/core-java-streams-maps/src/test/java/com/baeldung/streams/tomap/StreamToMapDuplicatedKeysHandlingUnitTest.java

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+package com.baeldung.streams.tomap;
+
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.toMap;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+
+import org.junit.jupiter.api.Test;
+
+class City {
+    private String name;
+    private String locatedIn;
+
+    public City(String name, String locatedIn) {
+        this.name = name;
+        this.locatedIn = locatedIn;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getLocatedIn() {
+        return locatedIn;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (!(o instanceof City))
+            return false;
+
+        City city = (City) o;
+
+        if (!name.equals(city.name))
+            return false;
+        return locatedIn.equals(city.locatedIn);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = name.hashCode();
+        result = 31 * result + locatedIn.hashCode();
+        return result;
+    }
+}
+
+public class StreamToMapDuplicatedKeysHandlingUnitTest {
+    // @formatter:off
+    private final List<City> CITY_INPUT = Arrays.asList(
+      new City("New York City", "USA"),
+      new City("Shanghai", "China"),
+      new City("Hamburg", "Germany"),
+      new City("Paris", "France"),
+      new City("Paris", "Texas, USA"));
+    // @formatter:on
+
+    @Test
+    void givenCityList_whenUsingGroupingBy_shouldContainExpectedCity() {
+        Map<String, List<City>> resultMap = CITY_INPUT.stream()
+          .collect(groupingBy(City::getName));
+        assertEquals(4, resultMap.size());
+        // @formatter:off
+        assertEquals(Arrays.asList(
+          new City("Paris", "France"),
+          new City("Paris", "Texas, USA")), resultMap.get("Paris"));
+        // @formatter:on
+
+    }
+
+    @Test
+    void givenCityList_whenContainingDuplicatedNamesUsingToMap_shouldContainExpectedCity() {
+        Map<String, City> resultMap1 = CITY_INPUT.stream()
+          .collect(toMap(City::getName, Function.identity(), (first, second) -> first));
+        assertEquals(4, resultMap1.size());
+        assertEquals(new City("Paris", "France"), resultMap1.get("Paris"));
+
+        Map<String, City> resultMap2 = CITY_INPUT.stream()
+          .collect(toMap(City::getName, Function.identity(), (first, second) -> second));
+        assertEquals(4, resultMap2.size());
+        assertEquals(new City("Paris", "Texas, USA"), resultMap2.get("Paris"));
+
+        Map<String, City> resultMap3 = CITY_INPUT.stream()
+          .collect(toMap(City::getName, Function.identity(), (first, second) -> {
+              String locations = first.getLocatedIn() + " and " + second.getLocatedIn();
+              return new City(first.getName(), locations);
+          }));
+        assertEquals(4, resultMap2.size());
+        assertEquals(new City("Paris", "France and Texas, USA"), resultMap3.get("Paris"));
+    }
+
+}

core-java-modules/pom.xml

@@ -120,6 +120,7 @@
         <module>core-java-streams</module>
         <module>core-java-streams-2</module>
         <module>core-java-streams-3</module>
+        <module>core-java-streams-maps</module>
         <module>core-java-string-algorithms</module>
         <module>core-java-string-algorithms-2</module>
         <module>core-java-string-apis</module>