BAEL-6790: Write and Read file with HashMap (#15087)

* Add examples for reading and writing a HashMap from a file

* Add test for gson

core-java-modules/core-java-collections-maps-7/README.md

@@ -1,5 +1,7 @@
-## Relevant Articles
+## Relevant Articles:
 - [Difference Between putIfAbsent() and computeIfAbsent() in Java’s Map](https://www.baeldung.com/java-map-putifabsent-computeifabsent)
+- [How to Write and Read a File with a Java HashMap](https://www.baeldung.com/how-to-write-and-read-a-file-with-a-java-hashmap/)
 - [How to Write Hashmap to CSV File](https://www.baeldung.com/java-write-hashmap-csv)
 - [How to Get First or Last Entry From a LinkedHashMap in Java](https://www.baeldung.com/java-linkedhashmap-first-last-key-value-pair)
 - [How to Write and Read a File with a Java HashMap](https://www.baeldung.com/java-hashmap-write-read-file)
+- More articles: [[<-- prev]](/core-java-modules/core-java-collections-maps-6)

core-java-modules/core-java-collections-maps-7/pom.xml

@@ -1,11 +1,15 @@
 <?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"
+         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">
+         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-collections-maps-7</artifactId>
     <name>core-java-collections-maps-7</name>
     <packaging>jar</packaging>
+    <properties>
+        <gson.version>2.10.1</gson.version>
+        <csv.version>1.5</csv.version>
+    </properties>
 
     <parent>
         <artifactId>core-java-modules</artifactId>
@@ -13,72 +17,24 @@
         <version>0.0.1-SNAPSHOT</version>
     </parent>
 
-    <properties>
-        <spring.version>5.2.5.RELEASE</spring.version>
-    </properties>
     <dependencies>
         <dependency>
             <groupId>com.fasterxml.jackson.core</groupId>
             <artifactId>jackson-databind</artifactId>
-            <version>2.12.4</version>
+            <version>${jackson.version}</version>
-        </dependency>
-        <dependency>
-            <groupId>org.openjdk.jmh</groupId>
-            <artifactId>jmh-core</artifactId>
-            <version>1.36</version>
         </dependency>
         <dependency>
             <groupId>com.google.code.gson</groupId>
             <artifactId>gson</artifactId>
-            <version>2.8.9</version>
+            <version>${gson.version}</version>
-        </dependency>
-        <dependency>
-            <groupId>org.json</groupId>
-            <artifactId>json</artifactId>
-            <version>20230227</version>
-        </dependency>
-        <dependency>
-            <groupId>junit</groupId>
-            <artifactId>junit</artifactId>
-            <version>4.13.1</version>
-            <scope>test</scope>
-        </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter</artifactId>
-            <version>5.8.1</version>
-            <scope>test</scope>
-        </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter</artifactId>
-            <version>5.8.1</version>
-            <scope>test</scope>
-        </dependency>
-        <dependency>
-            <groupId>junit</groupId>
-            <artifactId>junit</artifactId>
-            <version>4.13.1</version>
-            <scope>test</scope>
-        </dependency>
-        <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter</artifactId>
-            <version>5.8.1</version>
-            <scope>test</scope>
-        </dependency>
-        <dependency>
-            <groupId>junit</groupId>
-            <artifactId>junit</artifactId>
-            <version>4.13.1</version>
-            <scope>test</scope>
         </dependency>
         <dependency>
             <groupId>org.apache.commons</groupId>
             <artifactId>commons-csv</artifactId>
-            <version>1.5</version>
+            <version>${csv.version}</version>
         </dependency>
     </dependencies>
+
     <build>
         <plugins>
             <plugin>
@@ -92,5 +48,4 @@
             </plugin>
         </plugins>
     </build>
-
 </project>

core-java-modules/core-java-collections-maps-7/src/main/java/com/baeldung/map/readandwritefile/Student.java

@@ -0,0 +1,51 @@
+package com.baeldung.map.readandwritefile;
+
+import java.io.Serializable;
+import java.util.Objects;
+
+public class Student implements Serializable {
+    private static final long serialVersionUID = 1L;
+    private String firstName;
+    private String lastName;
+
+    /** Default constructor for JSON serialization */
+    public Student() {
+
+    }
+
+    public Student(String firstName, String lastName) {
+        this.firstName = firstName;
+        this.lastName = lastName;
+    }
+
+    public void setFirstName(String firstName) {
+        this.firstName = firstName;
+    }
+
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+    public String getFirstName() {
+        return firstName;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        Student student = (Student) o;
+        return Objects.equals(firstName, student.firstName) && Objects.equals(lastName, student.lastName);
+    }
+
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
+}

core-java-modules/core-java-collections-maps-7/src/test/java/com/baeldung/map/readandwritefile/ReadAndWriteFileWithHashMapUnitTest.java

@@ -0,0 +1,135 @@
+package com.baeldung.map.readandwritefile;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.FileReader;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.OutputStream;
+import java.lang.reflect.Type;
+import java.nio.file.Files;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Properties;
+import java.util.stream.Collectors;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.gson.Gson;
+import com.google.gson.reflect.TypeToken;
+
+import org.junit.After;
+import org.junit.Test;
+import org.junit.Before;
+
+public class ReadAndWriteFileWithHashMapUnitTest {
+
+    private static final Map<Integer, Student> STUDENT_DATA = new HashMap<>();
+
+    static {
+        STUDENT_DATA.put(1234, new Student("Henry", "Winter"));
+        STUDENT_DATA.put(5678, new Student("Richard", "Papen"));
+    }
+
+    private File file;
+
+    @Before
+    public void createFile() throws IOException {
+        file = File.createTempFile("student", ".data");
+    }
+
+    @After
+    public void deleteFile() {
+        file.delete();
+    }
+
+    @Test
+    public void givenHashMap_whenWrittenAsPropertiesFile_thenReloadedMapIsIdentical() throws IOException {
+        // Given a map containing student data
+        Map<String, String> studentData = new HashMap<>();
+        studentData.put("student.firstName", "Henry");
+        studentData.put("student.lastName", "Winter");
+
+        // When converting to a Properties object and writing to a file
+        Properties props = new Properties();
+        props.putAll(studentData);
+        try (OutputStream output = Files.newOutputStream(file.toPath())) {
+            props.store(output, null);
+        }
+
+        // Then the map resulting from loading the Properties file is identical
+        Properties propsFromFile = new Properties();
+        try (InputStream input = Files.newInputStream(file.toPath())) {
+            propsFromFile.load(input);
+        }
+
+        Map<String, String> studentDataFromProps = propsFromFile.stringPropertyNames()
+            .stream()
+            .collect(Collectors.toMap(key -> key, props::getProperty));
+        assertThat(studentDataFromProps).isEqualTo(studentData);
+    }
+
+    @Test
+    public void givenHashMap_whenSerializedToFile_thenDeserializedMapIsIdentical() throws IOException, ClassNotFoundException {
+        // Given a map containing student data (STUDENT_DATA)
+
+        // When serializing the map to a file
+        try (FileOutputStream fileOutput = new FileOutputStream(file); ObjectOutputStream objectStream = new ObjectOutputStream(fileOutput)) {
+            objectStream.writeObject(STUDENT_DATA);
+        }
+
+        // Then read the file back into a map and check the contents
+        Map<Integer, Student> studentsFromFile;
+        try (FileInputStream fileReader = new FileInputStream(file); ObjectInputStream objectStream = new ObjectInputStream(fileReader)) {
+            studentsFromFile = (HashMap<Integer, Student>) objectStream.readObject();
+        }
+        assertThat(studentsFromFile).isEqualTo(STUDENT_DATA);
+    }
+
+    @Test
+    public void givenHashMap_whenSerializedToFileWithJackson_thenDeserializedMapIsIdentical() throws IOException {
+        // Given a map containing student data (STUDENT_DATA)
+
+        // When converting to JSON with Jackson and writing to a file
+        ObjectMapper mapper = new ObjectMapper();
+        try (FileOutputStream fileOutput = new FileOutputStream(file)) {
+            mapper.writeValue(fileOutput, STUDENT_DATA);
+        }
+
+        // Then deserialize the file back into a map and check that it's identical
+        Map<Integer, Student> mapFromFile;
+        try (FileInputStream fileInput = new FileInputStream(file)) {
+            // Create a TypeReference so we can deserialize the parameterized type
+            TypeReference<HashMap<Integer, Student>> mapType = new TypeReference<HashMap<Integer, Student>>() {
+            };
+            mapFromFile = mapper.readValue(fileInput, mapType);
+        }
+        assertThat(mapFromFile).isEqualTo(STUDENT_DATA);
+    }
+
+    @Test
+    public void givenHashMap_whenSerializedToFileWithGson_thenDeserializedMapIsIdentical() throws IOException {
+        // Given a map containing student data (STUDENT_DATA)
+
+        // When converting to JSON using Gson and writing to a file
+        Gson gson = new Gson();
+        try (FileWriter writer = new FileWriter(file)) {
+            gson.toJson(STUDENT_DATA, writer);
+        }
+
+        // Then deserialize the file back into a map and check that it's identical
+        Map<Integer, Student> studentsFromFile;
+        try (FileReader reader = new FileReader(file)) {
+            Type mapType = new TypeToken<HashMap<Integer, Student>>() {
+            }.getType();
+            studentsFromFile = gson.fromJson(reader, mapType);
+        }
+        assertThat(studentsFromFile).isEqualTo(STUDENT_DATA);
+    }
+}