Bael 1611 (#3799)

* Externalizable test

* Guide to Externalizable interface +tests

* serialize Community

core-java/src/main/java/com/baeldung/externalizable/Community.java

@@ -0,0 +1,23 @@
+package com.baeldung.externalizable;
+
+import java.io.*;
+
+public class Community implements Serializable {
+
+    private int id;
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    @Override
+    public String toString() {
+        return "Community{" +
+                "id=" + id +
+                '}';
+    }
+}

core-java/src/main/java/com/baeldung/externalizable/Country.java

@@ -0,0 +1,62 @@
+package com.baeldung.externalizable;
+
+import java.io.Externalizable;
+import java.io.IOException;
+import java.io.ObjectInput;
+import java.io.ObjectOutput;
+
+public class Country implements Externalizable {
+
+    private static final long serialVersionUID = 1L;
+
+    private String name;
+    private String capital;
+    private int code;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getCapital() {
+        return capital;
+    }
+
+    public void setCapital(String capital) {
+        this.capital = capital;
+    }
+
+    public int getCode() {
+        return code;
+    }
+
+    public void setCode(int code) {
+        this.code = code;
+    }
+
+    @Override
+    public void writeExternal(ObjectOutput out) throws IOException {
+        out.writeUTF(name);
+        out.writeUTF(capital);
+        out.writeInt(code);
+    }
+
+    @Override
+    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
+        this.name = in.readUTF();
+        this.capital = in.readUTF();
+        this.code = in.readInt();
+    }
+
+    @Override
+    public String toString() {
+        return "Country{" +
+                "name='" + name + '\'' +
+                ", capital='" + capital + '\'' +
+                ", code=" + code +
+                '}';
+    }
+}

core-java/src/main/java/com/baeldung/externalizable/Region.java

@@ -0,0 +1,57 @@
+package com.baeldung.externalizable;
+
+import java.io.Externalizable;
+import java.io.IOException;
+import java.io.ObjectInput;
+import java.io.ObjectOutput;
+
+public class Region extends Country implements Externalizable {
+
+    private static final long serialVersionUID = 1L;
+
+    private String climate;
+    private Double population;
+    private Community community;
+
+    public String getClimate() {
+        return climate;
+    }
+
+    public void setClimate(String climate) {
+        this.climate = climate;
+    }
+
+    public Double getPopulation() {
+        return population;
+    }
+
+    public void setPopulation(Double population) {
+        this.population = population;
+    }
+
+    @Override
+    public void writeExternal(ObjectOutput out) throws IOException {
+        super.writeExternal(out);
+        out.writeUTF(climate);
+        community = new Community();
+        community.setId(5);
+        out.writeObject(community);
+    }
+
+    @Override
+    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
+        super.readExternal(in);
+        this.climate = in.readUTF();
+        community = (Community) in.readObject();
+    }
+
+    @Override
+    public String toString() {
+        return "Region = {" +
+                "country='" + super.toString() + '\'' +
+                "community='" + community.toString() + '\'' +
+                "climate='" + climate + '\'' +
+                ", population=" + population +
+                '}';
+    }
+}

core-java/src/test/java/com/baeldung/externalizable/ExternalizableUnitTest.java

@@ -0,0 +1,71 @@
+package com.baeldung.externalizable;
+
+import org.junit.Test;
+
+import java.io.*;
+
+import static org.junit.Assert.assertTrue;
+
+public class ExternalizableUnitTest {
+
+    private final static String OUTPUT_FILE = "externalizable.txt";
+
+    @Test
+    public void whenSerializing_thenUseExternalizable() throws IOException, ClassNotFoundException {
+
+        Country c = new Country();
+        c.setCapital("Yerevan");
+        c.setCode(374);
+        c.setName("Armenia");
+
+        FileOutputStream fileOutputStream = new FileOutputStream(OUTPUT_FILE);
+        ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
+        c.writeExternal(objectOutputStream);
+
+        objectOutputStream.flush();
+        objectOutputStream.close();
+        fileOutputStream.close();
+
+        FileInputStream fileInputStream = new FileInputStream(OUTPUT_FILE);
+        ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
+
+        Country c2 = new Country();
+        c2.readExternal(objectInputStream);
+
+        objectInputStream.close();
+        fileInputStream.close();
+
+        assertTrue(c2.getCode() == c.getCode());
+        assertTrue(c2.getName().equals(c.getName()));
+    }
+
+    @Test
+    public void whenInheritanceSerialization_then_UseExternalizable() throws IOException, ClassNotFoundException {
+
+        Region r = new Region();
+        r.setCapital("Yerevan");
+        r.setCode(374);
+        r.setName("Armenia");
+        r.setClimate("Mediterranean");
+        r.setPopulation(120.000);
+
+        FileOutputStream fileOutputStream = new FileOutputStream(OUTPUT_FILE);
+        ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
+        r.writeExternal(objectOutputStream);
+
+        objectOutputStream.flush();
+        objectOutputStream.close();
+        fileOutputStream.close();
+
+        FileInputStream fileInputStream = new FileInputStream(OUTPUT_FILE);
+        ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
+
+        Region r2 = new Region();
+        r2.readExternal(objectInputStream);
+
+        objectInputStream.close();
+        fileInputStream.close();
+
+        assertTrue(r2.getPopulation() == null);
+    }
+}