Java - ObjectOutputStream.PutField Class

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Description

The Java ObjectOutputStream.PutField class provide programmatic access to the persistent fields to be written to ObjectOutput.

Class declaration

Following is the declaration for Java.io.ObjectOutputStream.PutField class −

public abstract static class ObjectOutputStream.PutField
   extends Object

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes −

• Java.io.Object

Example - Serialize a User object with a custom boolean field

The following example shows the usage of ObjectOutputStream.PutField put(String name, boolean val) method. We're serializing only the isActive field explicitly, along with the name.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException, ClassNotFoundException {
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user_bool1.ser"))) {
         User user = new User("Alice", true);
         oos.writeObject(user);

         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user_bool1.ser"));
         User u = (User)ois.readObject();
         System.out.println(" User:: name= " + u.name + " isActive= "  + u.isActive);
      }
   }

   static class User implements Serializable {
      private static final long serialVersionUID = 1L;

      String name;
      boolean isActive;

      public User(String name, boolean isActive) {
         this.name = name;
         this.isActive = isActive;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("name", name);
         fields.put("isActive", isActive); // write boolean field manually
         oos.writeFields();
      }

      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
         ObjectInputStream.GetField fields = ois.readFields();
         name = (String) fields.get("name", "Unknown");
         isActive = fields.get("isActive", false);
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

User:: name= Alice isActive= true

Explanation

• The put() call explicitly writes the isActive boolean field.

• On deserialization, the value is restored properly.

Example - Serialize a Student object with a grade (char)

The following example shows the usage of ObjectOutputStream.PutField put(String name, char val) method. We're writing a student's grade using put() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException, ClassNotFoundException {
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("student1.ser"))) {
         Student s = new Student("Alice", 'A');
         oos.writeObject(s);

         ObjectInputStream ois = new ObjectInputStream( new FileInputStream("student1.ser"));
         Student s1 = (Student)ois.readObject();
         System.out.println("Student::  name = '" + s1.name + "' , grade = " + s1.grade);
      }
   }

   static class Student implements Serializable {
      private static final long serialVersionUID = 1L;

      String name;
      char grade;

      public Student(String name, char grade) {
         this.name = name;
         this.grade = grade;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("name", name);
         fields.put("grade", grade); // putting a char field
         oos.writeFields();
      }

      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
         ObjectInputStream.GetField fields = ois.readFields();
         name = (String) fields.get("name", "Unknown");
         grade = fields.get("grade", 'F');
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Student::  name = 'Alice' , grade = A

Explanation

• The grade field is written manually as a char using put().

Example - Serialize a User with a manually set String email field

The following example shows the usage of ObjectOutputStream.PutField put(String name, Object val) method. We're manually writing a String object field using put() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException {
      try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user_obj1.ser"))) {
         User user = new User("alice@example.com");
         oos.writeObject(user);
      }

      // Deserialize
      try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user_obj1.ser"))) {
         User user = (User) ois.readObject();
         System.out.println("Deserialized email: " + user.email);
      } catch (ClassNotFoundException e) {
         e.printStackTrace();
      }
   }

   static class User implements Serializable {
      private static final long serialVersionUID = 1L;

      String email;

      public User(String email) {
         this.email = email;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("email", email);  // write object field (String)
         oos.writeFields();
      }

      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
         ObjectInputStream.GetField fields = ois.readFields();
         email = (String) fields.get("email", "unknown@example.com");
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Deserialized email: alice@example.com

Explanation

• A String object (email) is written using put("email", email).

• It's a basic use of put() with an object-type field.