Java - ObjectOutputStream.PutField put(String name, int val) method

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Description

The Java ObjectOutputStream.PutField put(String name, int val) method puts the value of the named int field into the persistent field.

• It's used inside a custom writeObject(ObjectOutputStream oos) method of a class.

• It sets an int field by name for manual serialization.

• After using put(), you must call oos.writeFields() to flush those fields to the stream.

• Useful when you want explicit control over what gets serialized.

Declaration

Following is the declaration for java.io.ObjectOutputStream.PutField.put(String name, int val) method.

public abstract void put(String name, int val)

Parameters

• name − The name of the serializable field.

• val − The value to assign to the field.

Return Value

This method does not return a value.

Exception

• IllegalArgumentException − If name does not match the name of a serializable field for the class whose fields are being written, or if the type of the named field is not int.

Example - Usage of ObjectOutputStream.PutField put(String name, int val) method

The following example shows the usage of ObjectOutputStream.PutField put(String name, int val) 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.ObjectStreamField;
import java.io.Serializable;

public class ObjectOutputStreamDemo implements Serializable {
   public static void main(String[] args) {
      try {
         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write something in the file
         oout.writeObject(new Example());
         oout.flush();
         oout.close();

         // create an ObjectInputStream for the file we created before
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));

         // read an object from the stream and cast it to Example
         Example a = (Example) ois.readObject();

         // print var of a
         System.out.println("" + a.var);
      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }

   static public class Example implements Serializable {
      static int var = 76458;
      
      // assign a new serialPersistentFields 
      private static final ObjectStreamField[] serialPersistentFields = {
         new ObjectStreamField("var", Integer.TYPE)
      };

      private void readObject(ObjectInputStream in)
         throws IOException, ClassNotFoundException {

         // get the field and assign it at var
         ObjectInputStream.GetField fields = in.readFields();

         // get var
         var = fields.get("var", 0);
      }

      private void writeObject(ObjectOutputStream out) throws IOException {

         // write into the ObjectStreamField array the variable var
         ObjectOutputStream.PutField fields = out.putFields();
         fields.put("var", var);
         out.writeFields();
      }
   }
}

Output

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

76458

Example - Serialize a User with an explicit int ID

The following example shows the usage of ObjectOutputStream.PutField put(String name, int val) method. We're using put() to serialize the id field.

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("user1.ser"))) {
         User user = new User("Alice", 101);
         oos.writeObject(user);

         ObjectInputStream ois = new ObjectInputStream( new FileInputStream("user1.ser"));
         User u1  = (User)ois.readObject();
         System.out.println("User:: name = '" +u1.name + "'; id = " + u1.id);
      }
   }

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

      String name;
      int id;

      public User(String name, int id) {
         this.name = name;
         this.id = id;
      }

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

      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
         ObjectInputStream.GetField fields = ois.readFields();
         name = (String) fields.get("name", "Unknown");
         id = fields.get("id", -1);
      }
   }
}

Output

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

User:: name = 'Alice'; id = 101

Explanation

• We serialize the id using put("id", id).

• On deserialization, get("id", -1) provides a fallback default if the field is missing.

Example - Store a derived int value (e.g., word count)

The following example shows the usage of ObjectOutputStream.PutField put(String name, int val) method. We're storing the word count of a text in an int field called wordCount.

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("doc.ser"))) {
         Document doc = new Document("This is a test document.");
         oos.writeObject(doc);
      }

      // Read it back
      try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("doc.ser"))) {
         Document doc = (Document) ois.readObject();
         System.out.println("Deserialized: " + doc.content);
         System.out.println("Word count: " + doc.wordCount);
      } catch (ClassNotFoundException e) {
         e.printStackTrace();
      }
   }

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

      String content;
      int wordCount;

      public Document(String content) {
         this.content = content;
      }

      private void writeObject(ObjectOutputStream oos) throws IOException {
         ObjectOutputStream.PutField fields = oos.putFields();
         fields.put("content", content);
         int count = content.trim().split("\\s+").length;
         fields.put("wordCount", count);  //  derived int field
         oos.writeFields();
      }

      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
         ObjectInputStream.GetField fields = ois.readFields();
         content = (String) fields.get("content", "");
         wordCount = fields.get("wordCount", 0);
      }
   }
}

Output

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

Deserialized: This is a test document.
Word count: 5