Grouping by collector java (#10986)

* Initial Commit - Ports and Adapters

* Restructured code to implement input and outbou ports and adapters

* Added tests for groupingBy Collector

* Revert "Initial Commit - Ports and Adapters"

This reverts commit 55152dea

* Added tests for groupingBy Collector

* reverting

* corrected typo

* used BDD naming strategy for tests

* Added a test for groupingByConcurrent

* added new module and package

* added core-java-streams-4 module in main pom.xml

* updated pom.xml and changed artifactId

core-java-modules/core-java-streams-4/.gitignore

@@ -0,0 +1,26 @@
+*.class
+
+0.*
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+.resourceCache
+
+# Packaged files #
+*.jar
+*.war
+*.ear
+
+# Files generated by integration tests
+*.txt
+backup-pom.xml
+/bin/
+/temp
+
+#IntelliJ specific
+.idea/
+*.iml

core-java-modules/core-java-streams-4/README.md

@@ -0,0 +1,6 @@
+## Core Java streams
+
+This module contains articles about the Stream API in Java.
+
+### Relevant Articles: 
+- [Count Occurrences using Java groupingBy Collector](https://baeldung.com/count-occurrences-with-java-groupingby-collector)

core-java-modules/core-java-streams-4/pom.xml

@@ -0,0 +1,63 @@
+<?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-4</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-streams</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../</relativePath>
+    </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>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-streams-4</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-4/src/test/java/com/baeldung/streamcollectors/StreamGroupingByCollectorUnitTest.java

@@ -0,0 +1,99 @@
+package com.baeldung.streamcollectors;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class StreamGroupingByCollectorUnitTest
+{
+        static class Employee {
+
+                Integer employeeId;
+                String employeeName;
+                Integer departmentId;
+
+                Employee(Integer employeeId, String employeeName, Integer departmentId) {
+                        this.employeeId = employeeId;
+                        this.employeeName = employeeName;
+                        this.departmentId = departmentId;
+                }
+
+                public Integer getEmployeeId() {
+                        return employeeId;
+                }
+
+                public void setEmployeeId(Integer employeeId) {
+                        this.employeeId = employeeId;
+                }
+
+                public String getEmployeeName() {
+                        return employeeName;
+                }
+
+                public void setEmployeeName(String employeeName) {
+                        this.employeeName = employeeName;
+                }
+
+                public Integer getDepartmentId() {
+                        return departmentId;
+                }
+
+                public void setDepartmentId(Integer departmentId) {
+                        this.departmentId = departmentId;
+                }
+        }
+
+        @Test
+        public void
+        givenListOfStrings_whenGroupingEqualStrings_thenUseCollectorsGroupingByToGroupEqualStringsAndCountOfOccurrences()  {
+                List<String> list = new ArrayList<>(
+                    Arrays.asList("Foo", "Bar", "Bar", "Foo", "Bar")
+                );
+
+                Map<String, Long> result = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get("Foo"));
+                Assert.assertEquals(new Long(3), result.get("Bar"));
+
+        }
+
+        @Test
+        public void
+        givenListOfStrings_whenGroupingEqualLengthStrings_thenUseCollectorsGroupingByConcurrentToGroupEqualLengthStringsAndCountOfOccurrences()  {
+
+                List<String> list = new ArrayList<>(
+                    Arrays.asList("Adam", "Bill", "Jack", "Joe", "Ian")
+                );
+
+                Map<Integer, Long> result = list.stream()
+                    .collect(Collectors.groupingByConcurrent(String::length, Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get(3));
+                Assert.assertEquals(new Long(3), result.get(4));
+
+        }
+
+
+        @Test
+        public void
+        givenListOfEmployees_whenGroupingDepartmentId_thenUseCollectorsGroupingByDepartmentIdAndCountNumberOfEmployeesWithinEveryDepartment()  {
+
+                List<Employee> list = new ArrayList<>(
+                    Arrays.asList(new Employee(1, "Joe", 1),
+                                  new Employee(2, "Josh", 1),
+                                  new Employee(3, "Jamie", 2),
+                                  new Employee(4, "Jim", 2),
+                                  new Employee(5, "Jack", 2))
+                );
+
+                Map<Integer, Long> result = list.stream() .collect(Collectors.groupingBy(Employee::getDepartmentId, Collectors.counting()));
+                Assert.assertEquals(new Long(2), result.get(1));
+                Assert.assertEquals(new Long(3), result.get(2));
+
+        }
+
+}

core-java-modules/core-java-streams-4/src/test/resources/.gitignore

@@ -0,0 +1,13 @@
+*.class
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+
+# Packaged files #
+*.jar
+*.war
+*.ear

core-java-modules/pom.xml

@@ -103,6 +103,7 @@
         <module>core-java-streams</module>
         <module>core-java-streams-2</module>
         <module>core-java-streams-3</module>
+        <module>core-java-streams-4</module>
         <module>core-java-string-algorithms</module>
         <module>core-java-string-algorithms-2</module>
         <module>core-java-string-algorithms-3</module>