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Java ObjectOutputStream Tutorial

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  1. ObjectOutputStream
  2. Example 1
  3. Example 2
  4. writeFields()
  5. writeUnshared​(Object)

1- ObjectOutputStream

ObjectOutputStream is a subclass of the OutputStream class. It manages an OutputStream object and provides methods for writing primitive data or objects into the OutputStream that it manages.
public class ObjectOutputStream
              extends OutputStream implements ObjectOutput, ObjectStreamConstants
ObjectInputStream is used to read data sources created by ObjectOutputStream:
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ObjectOutputStream Methods
public final void writeObject(Object obj) throws IOException
public void writeBoolean(boolean val) throws IOException
public void writeByte(int val) throws IOException
public void writeShort(int val) throws IOException
public void writeChar(int val) throws IOException
public void writeInt(int val) throws IOException
public void writeLong(long val) throws IOException
public void writeFloat(float val) throws IOException
public void writeDouble(double val) throws IOException
public void writeBytes(String str) throws IOException
public void writeChars(String str) throws IOException
public void writeUTF(String str) throws IOException

public void writeUnshared(Object obj) throws IOException
public void writeFields() throws IOException

public void defaultWriteObject() throws IOException
public void useProtocolVersion(int version) throws IOException
public ObjectOutputStream.PutField putFields() throws IOException

public void reset() throws IOException

protected void writeObjectOverride(Object obj) throws IOException  
protected void annotateClass(Class<?> cl) throws IOException  
protected void annotateProxyClass(Class<?> cl) throws IOException  
protected Object replaceObject(Object obj) throws IOException
protected boolean enableReplaceObject(boolean enable) throws SecurityException
protected void writeStreamHeader() throws IOException
protected void writeClassDescriptor(ObjectStreamClass desc) throws IOException
protected void drain() throws IOException

int getProtocolVersion()  
void writeTypeString(String str) throws IOException

// Methods Inherited from OutputStream

public void write(int val) throws IOException
public void write(byte[] buf) throws IOException
public void write(byte[] buf, int off, int len) throws IOException

public void flush() throws IOException
public void close() throws IOException
ObjectOutputStream​ Constructors
ObjectOutputStream​(OutputStream out)
Objects need to be serialized before being written to ObjectOutputStream, these objects must implement the Serializable interface.

2- Example 1

The Employee class implements the Serializable interface, which is necessary so that it can be written to ObjectOutputStream.
Employee.java
package org.o7planning.beans;

import java.io.Serializable;

public class Employee implements Serializable {

    private static final long serialVersionUID = 1L;

    private String fullName;
    private float salary;

    public Employee(String fullName, float salary) {
        this.fullName = fullName;
        this.salary = salary;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String firstName) {
        this.fullName = firstName;
    }

    public float getSalary() {
        return salary;
    }

    public void setSalary(float lastName) {
        this.salary = lastName;
    }
}
For example, Use ObjectOutputStream to write Employee objects to a file.
WriteEmployeeDataEx.java
package org.o7planning.objectoutputstream.ex;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.Date;

import org.o7planning.beans.Employee;

public class WriteEmployeeDataEx {

    // Windows: C:/Data/test/employees.data
    private static String file_path = "/Volumes/Data/test/employees.data";

    public static void main(String[] args) throws IOException {
        File outFile = new File(file_path);

        outFile.getParentFile().mkdirs();

        Employee e1 = new Employee("Tom", 1000f);
        Employee e2 = new Employee("Jerry", 2000f);
        Employee e3 = new Employee("Donald", 1200f);

        Employee[] employees = new Employee[] { e1, e2, e3 };

        OutputStream os = new FileOutputStream(outFile);
        ObjectOutputStream oos = new ObjectOutputStream(os);

        System.out.println("Writing file: " + outFile.getAbsolutePath());

        oos.writeObject(new Date());
        oos.writeUTF("Employee data"); // Some informations.

        oos.writeInt(employees.length); // Number of Employees

        for (Employee e : employees) {
            oos.writeObject(e);
        }
        oos.close();
        System.out.println("Finished!");
    }
}
After running the WriteEmployeeDataEx class we get a file with confusing content, to read its content you must use the ObjectInputStream class.
OK, Read the file just written in the previous step with ObjectInputStream.
ReadEmployeeDataEx.java
package org.o7planning.objectoutputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.util.Date;

import org.o7planning.beans.Employee;

public class ReadEmployeeDataEx {

    // Windows: C:/Data/test/employees.data
    private static String file_path = "/Volumes/Data/test/employees.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        File inFile = new File(file_path);
 

        InputStream is = new FileInputStream(inFile);
        ObjectInputStream ois = new ObjectInputStream(is);

        System.out.println("Reading file: " + inFile.getAbsolutePath());
        System.out.println();

        Date date = (Date) ois.readObject();
        String info = ois.readUTF();
        
        System.out.println(date);
        System.out.println(info);
        System.out.println();
        
        int employeeCount = ois.readInt();
        
        for(int i=0; i< employeeCount; i++) {
            Employee e = (Employee) ois.readObject();
            System.out.println("Employee Name: " + e.getFullName() +" / Salary: " + e.getSalary());
        }
        ois.close();
    }
}
Output: 
Reading file: /Volumes/Data/test/employees.data

Sat Mar 20 18:54:24 KGT 2021
Employee data

Employee Name: Tom / Salary: 1000.0
Employee Name: Jerry / Salary: 2000.0
Employee Name: Donald / Salary: 1200.0

3- Example 2

Most classes in Java Collection Framework implement Serializable interface, such as ArrayList, LinkedList, HashMap, LinkedHashMap, TreeMap, ... so their object is writable to ObjectOutputStream.
For example, write an ArrayList object to the file.
Note: All elements of the ArrayList must be of type Serializable.
WriteListEx1.java
package org.o7planning.objectoutputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

public class WriteListEx1 {

    // Windows: C:/Data/test/flowers.data
    private static String file_path = "/Volumes/Data/test/flowers.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        writeFile();

        readFile();
    }

    private static void writeFile() throws IOException {
        ArrayList<String> flowers = new ArrayList<String>();

        flowers.add("Tulip");
        flowers.add("Daffodil");
        flowers.add("Poppy");
        flowers.add("Sunflower");
        flowers.add("Bluebell");

        File file = new File(file_path);
        file.getParentFile().mkdirs();

        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);

        // Write a String
        oos.writeUTF("A list of flowers");
        // Write an Object
        oos.writeObject(flowers);
        oos.close();
    }
    
    @SuppressWarnings("unchecked")
    private static void readFile() throws IOException, ClassNotFoundException {
        File file = new File(file_path);
        file.getParentFile().mkdirs();

        InputStream is = new FileInputStream(file);
        ObjectInputStream ois = new ObjectInputStream(is);
        
        // Read a String
        String info = ois.readUTF();
        // Read an Object
        List<String> flowers = (List<String>) ois.readObject();

        System.out.println(info);
        System.out.println();
        for (String s : flowers) {
            System.out.println(s);
        }
        ois.close();
    }
}
Output: 
A list of flowers

Tulip
Daffodil
Poppy
Sunflower
Bluebell

4- writeFields()

Suppose you have a GameSetting object and you want to write this to ObjectOutputStream, but not all of its fields.
GameSetting.java
package org.o7planning.beans;

import java.io.IOException;
import java.io.ObjectOutputStream;

public class GameSetting implements java.io.Serializable {

    private static final long serialVersionUID = 1L;

    private int sound;
    private int bightness;
    private String difficultyLevel;

    private String userNote;

    public GameSetting(int sound, int bightness, String difficultyLevel, String userNote) {
        this.sound = sound;
        this.bightness = bightness;
        this.difficultyLevel = difficultyLevel;
        this.userNote = userNote;
    }

    public int getSound() {
        return sound;
    }

    public int getBightness() {
        return bightness;
    }

    public String getDifficultyLevel() {
        return difficultyLevel;
    }

    public String getUserNote() {
        return userNote;
    }

    // Do not change name and parameter of this method.
    private void writeObject(ObjectOutputStream out) throws IOException {

        ObjectOutputStream.PutField fields = out.putFields();
        // Write this object with custom fields

        fields.put("sound", this.sound < 20 ? 20 : this.sound);
        fields.put("bightness", this.bightness < 30 ? 30 : this.bightness);
        fields.put("difficultyLevel", this.difficultyLevel);
        
        // Do not write "userNote".
        // fields.put("userNote", this.userNote);

        out.writeFields();
    }
}
ObjectOutputStream_writeFields.java
package org.o7planning.objectoutputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.Date;

import org.o7planning.beans.GameSetting;

public class ObjectOutputStream_writeFields {

    // Windows: C:/Data/test/game_setting.data
    private static String file_path = "/Volumes/Data/test/game_setting.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        GameSetting setting = new GameSetting(10, 80, "Hard", "Try game again!");

        writeGameSetting(setting);
        readGameSetting();
    }

    private static void writeGameSetting(GameSetting setting) throws IOException {
        File file = new File(file_path);
        file.getParentFile().mkdirs();

        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);

        // Write a String
        oos.writeUTF("Game Settings, Save at " + new Date());
        // Write Object
        oos.writeObject(setting);

        oos.close();
    }

    private static void readGameSetting() throws IOException, ClassNotFoundException {
        File file = new File(file_path);
        file.getParentFile().mkdirs();

        InputStream is = new FileInputStream(file);
        ObjectInputStream ois = new ObjectInputStream(is);

        // Read a String
        String info = ois.readUTF();

        // Read fields
        GameSetting setting = (GameSetting) ois.readObject();

        System.out.println("sound: " + setting.getSound());
        System.out.println("bightness: " + setting.getBightness());
        System.out.println("difficultyLevel: " + setting.getDifficultyLevel());
        System.out.println("userNote: " + setting.getUserNote());  // null.

        ois.close();
    }
}
Output: 
sound: 20
bightness: 80
difficultyLevel: Hard
userNote: null
See more about the ObjectInputStream.readFields() method:

5- writeUnshared​(Object)

The writeUnshared(Object) method works similarly to the writeObject(Object) method but differs in the following situation:
Suppose you want to write the "X" object twice to an ObjectOutputStream, what will happen?
  1. objectOutputStream.writeObject(X);
  2. objectOutputStream.writeObject(X);
  1. The object X will be serialized to get the result Y, then write Y to the target.
  2. Writes a reference of Y to the target.
  1. objectOutputStream.writeUnshared(X);
  2. objectOutputStream.writeUnshared(X);
  1. The object X will be serialized to get the result Y, then write Y to the target.
  2. The object X will be serialized to get the result Y, then write Y to the target.
For example, use the writeObject method to write an ArrayList object twice to a file. And use the writeUnshared method to write an ArrayList object twice to another file. The second file will have a larger size.
ObjectOutputStream_writeUnshared​.java
package org.o7planning.objectoutputstream.ex;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.ArrayList;

public class ObjectOutputStream_writeUnshared​ {

    // Windows: C:/Data/test/test1.data
    private static String file_path1 = "/Volumes/Data/test/test1.data";
    private static String file_path2 = "/Volumes/Data/test/test2.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        writeObjectTest();
        
        writeUnsharedTest();
    }
    
    private static void writeObjectTest() throws IOException  {
        File file = new File(file_path1);
        file.getParentFile().mkdirs();
        
        ArrayList<String> list = new ArrayList<String>();
        list.add("One");
        list.add("Two");
        
        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);
        
        oos.writeObject(list); // Write the first time
        oos.writeObject(list); // Write the second time
        oos.close();
    }

    private static void writeUnsharedTest() throws IOException  {
        File file = new File(file_path2);
        file.getParentFile().mkdirs();
        
        ArrayList<String> list = new ArrayList<String>();
        list.add("One");
        list.add("Two");
        
        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);
        
        oos.writeUnshared(list); // Write the first time
        oos.writeUnshared(list); // Write the second time
        oos.close();
    }
}
Output:

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