BAEL-6790: Write and Read file with HashMap (#15087)
* Add examples for reading and writing a HashMap from a file * Add test for gson
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Lucian Snare
GitHub
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## Relevant Articles
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## Relevant Articles:
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- [Difference Between putIfAbsent() and computeIfAbsent() in Java’s Map](https://www.baeldung.com/java-map-putifabsent-computeifabsent)
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- [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/)
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- [How to Write Hashmap to CSV File](https://www.baeldung.com/java-write-hashmap-csv)
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- [How to Get First or Last Entry From a LinkedHashMap in Java](https://www.baeldung.com/java-linkedhashmap-first-last-key-value-pair)
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- [How to Write and Read a File with a Java HashMap](https://www.baeldung.com/java-hashmap-write-read-file)
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- More articles: [[<-- prev]](/core-java-modules/core-java-collections-maps-6)
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@ -6,6 +6,10 @@
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<artifactId>core-java-collections-maps-7</artifactId>
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<name>core-java-collections-maps-7</name>
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<packaging>jar</packaging>
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<properties>
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<gson.version>2.10.1</gson.version>
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<csv.version>1.5</csv.version>
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</properties>
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<parent>
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<artifactId>core-java-modules</artifactId>
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@ -13,72 +17,24 @@
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<version>0.0.1-SNAPSHOT</version>
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</parent>
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<properties>
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<spring.version>5.2.5.RELEASE</spring.version>
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</properties>
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<dependencies>
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<dependency>
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<groupId>com.fasterxml.jackson.core</groupId>
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<artifactId>jackson-databind</artifactId>
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<version>2.12.4</version>
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</dependency>
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<dependency>
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<groupId>org.openjdk.jmh</groupId>
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<artifactId>jmh-core</artifactId>
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<version>1.36</version>
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<version>${jackson.version}</version>
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</dependency>
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<dependency>
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<groupId>com.google.code.gson</groupId>
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<artifactId>gson</artifactId>
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<version>2.8.9</version>
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</dependency>
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<dependency>
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<groupId>org.json</groupId>
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<artifactId>json</artifactId>
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<version>20230227</version>
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</dependency>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<version>4.13.1</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.1</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.1</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<version>4.13.1</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.1</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<version>4.13.1</version>
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<scope>test</scope>
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<version>${gson.version}</version>
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</dependency>
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<dependency>
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<groupId>org.apache.commons</groupId>
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<artifactId>commons-csv</artifactId>
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<version>1.5</version>
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<version>${csv.version}</version>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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@ -92,5 +48,4 @@
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</plugin>
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</plugins>
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</build>
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</project>
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@ -0,0 +1,51 @@
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package com.baeldung.map.readandwritefile;
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import java.io.Serializable;
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import java.util.Objects;
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public class Student implements Serializable {
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private static final long serialVersionUID = 1L;
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private String firstName;
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private String lastName;
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/** Default constructor for JSON serialization */
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public Student() {
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}
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public Student(String firstName, String lastName) {
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this.firstName = firstName;
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this.lastName = lastName;
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}
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public void setFirstName(String firstName) {
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this.firstName = firstName;
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}
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public void setLastName(String lastName) {
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this.lastName = lastName;
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}
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public String getFirstName() {
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return firstName;
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}
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public String getLastName() {
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return lastName;
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}
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@Override
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public boolean equals(Object o) {
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if (this == o)
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return true;
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if (o == null || getClass() != o.getClass())
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return false;
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Student student = (Student) o;
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return Objects.equals(firstName, student.firstName) && Objects.equals(lastName, student.lastName);
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}
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@Override
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public int hashCode() {
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return super.hashCode();
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}
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}
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@ -0,0 +1,135 @@
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package com.baeldung.map.readandwritefile;
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import static org.assertj.core.api.Assertions.assertThat;
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import java.io.File;
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import java.io.FileInputStream;
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import java.io.FileOutputStream;
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import java.io.FileReader;
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import java.io.FileWriter;
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import java.io.IOException;
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import java.io.InputStream;
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import java.io.ObjectInputStream;
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import java.io.ObjectOutputStream;
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import java.io.OutputStream;
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import java.lang.reflect.Type;
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import java.nio.file.Files;
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import java.util.HashMap;
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import java.util.Map;
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import java.util.Properties;
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import java.util.stream.Collectors;
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import com.fasterxml.jackson.core.type.TypeReference;
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import com.fasterxml.jackson.databind.ObjectMapper;
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import com.google.gson.Gson;
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import com.google.gson.reflect.TypeToken;
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import org.junit.After;
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import org.junit.Test;
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import org.junit.Before;
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public class ReadAndWriteFileWithHashMapUnitTest {
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private static final Map<Integer, Student> STUDENT_DATA = new HashMap<>();
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static {
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STUDENT_DATA.put(1234, new Student("Henry", "Winter"));
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STUDENT_DATA.put(5678, new Student("Richard", "Papen"));
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}
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private File file;
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@Before
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public void createFile() throws IOException {
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file = File.createTempFile("student", ".data");
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}
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@After
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public void deleteFile() {
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file.delete();
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}
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@Test
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public void givenHashMap_whenWrittenAsPropertiesFile_thenReloadedMapIsIdentical() throws IOException {
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// Given a map containing student data
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Map<String, String> studentData = new HashMap<>();
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studentData.put("student.firstName", "Henry");
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studentData.put("student.lastName", "Winter");
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// When converting to a Properties object and writing to a file
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Properties props = new Properties();
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props.putAll(studentData);
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try (OutputStream output = Files.newOutputStream(file.toPath())) {
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props.store(output, null);
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}
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// Then the map resulting from loading the Properties file is identical
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Properties propsFromFile = new Properties();
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try (InputStream input = Files.newInputStream(file.toPath())) {
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propsFromFile.load(input);
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}
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Map<String, String> studentDataFromProps = propsFromFile.stringPropertyNames()
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.stream()
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.collect(Collectors.toMap(key -> key, props::getProperty));
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assertThat(studentDataFromProps).isEqualTo(studentData);
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}
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@Test
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public void givenHashMap_whenSerializedToFile_thenDeserializedMapIsIdentical() throws IOException, ClassNotFoundException {
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// Given a map containing student data (STUDENT_DATA)
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// When serializing the map to a file
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try (FileOutputStream fileOutput = new FileOutputStream(file); ObjectOutputStream objectStream = new ObjectOutputStream(fileOutput)) {
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objectStream.writeObject(STUDENT_DATA);
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}
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// Then read the file back into a map and check the contents
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Map<Integer, Student> studentsFromFile;
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try (FileInputStream fileReader = new FileInputStream(file); ObjectInputStream objectStream = new ObjectInputStream(fileReader)) {
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studentsFromFile = (HashMap<Integer, Student>) objectStream.readObject();
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}
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assertThat(studentsFromFile).isEqualTo(STUDENT_DATA);
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}
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@Test
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public void givenHashMap_whenSerializedToFileWithJackson_thenDeserializedMapIsIdentical() throws IOException {
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// Given a map containing student data (STUDENT_DATA)
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// When converting to JSON with Jackson and writing to a file
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ObjectMapper mapper = new ObjectMapper();
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try (FileOutputStream fileOutput = new FileOutputStream(file)) {
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mapper.writeValue(fileOutput, STUDENT_DATA);
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}
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// Then deserialize the file back into a map and check that it's identical
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Map<Integer, Student> mapFromFile;
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try (FileInputStream fileInput = new FileInputStream(file)) {
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// Create a TypeReference so we can deserialize the parameterized type
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TypeReference<HashMap<Integer, Student>> mapType = new TypeReference<HashMap<Integer, Student>>() {
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};
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mapFromFile = mapper.readValue(fileInput, mapType);
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}
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assertThat(mapFromFile).isEqualTo(STUDENT_DATA);
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}
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@Test
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public void givenHashMap_whenSerializedToFileWithGson_thenDeserializedMapIsIdentical() throws IOException {
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// Given a map containing student data (STUDENT_DATA)
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// When converting to JSON using Gson and writing to a file
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Gson gson = new Gson();
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try (FileWriter writer = new FileWriter(file)) {
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gson.toJson(STUDENT_DATA, writer);
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}
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// Then deserialize the file back into a map and check that it's identical
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Map<Integer, Student> studentsFromFile;
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try (FileReader reader = new FileReader(file)) {
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Type mapType = new TypeToken<HashMap<Integer, Student>>() {
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}.getType();
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studentsFromFile = gson.fromJson(reader, mapType);
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}
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assertThat(studentsFromFile).isEqualTo(STUDENT_DATA);
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}
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}
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