Java ObjectInputStream readField() Method

The readField() method of ObjectInputStream class is used to read the persistent fields from the stream and makes them available by name.

Syntax

public ObjectInputStream.GetField readFields()throws IOException, ClassNotFoundException

Parameter

No parameter is passed.

Returns

the GetField object representing the persistent fields of the object being deserialized

Throws

ClassNotFoundException - if the class of a serialized object could not be found.

IOException - if an I/O error occurs.

NotActiveException - if the stream is not currently reading objects.

Example 1

import java.io.*;
import static java.lang.System.out;

public class ObjectInputStreamreadFieldExample1 {
   public static void main(String[] args) {
      try {
         // create a new file with an ObjectOutputStream
         FileOutputStream outstream = new FileOutputStream("file1.txt");
         ObjectOutputStream objoutstream = new ObjectOutputStream(outstream);

         // write something in the file
         objoutstream.writeObject(new Task());
         objoutstream.flush();

         // create an ObjectInputStream for the file we created before
         ObjectInputStream objinstream = new ObjectInputStream(new FileInputStream("file1.txt"));

         // read the object and print the string
          Task  a = (Task) objinstream.readObject();

         // print the string that is in Example class
         System.out.println("" + a.st);

      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }

   static class Task implements Serializable {
      String st = "Welcome to javaTpoint";

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

         // call readFields in readObject
         ObjectInputStream.GetField g = in.readFields();

         // save the string and return it
         return (String) g.get("st", null);
      }
   }
}

Output:

Welcome to javaTpoint

Example 2

import java.io.*; 
  public class ObjectInputStreamreadFieldExample2 
{ 
    public static void main(String[] args) 
    { 
        String str1 = "JavaTpoint"; 
        try
        { 
            // create a new file with an ObjectOutputStream and ObjectInputStream 
            FileOutputStream fileout = new FileOutputStream("file1.txt"); 
            ObjectOutputStream filein = new ObjectOutputStream(fileout); 
            ObjectInputStream InStream  = new ObjectInputStream(new FileInputStream("file1.txt")); 
            // These are ObjectOutputStream methods, Don't bother about these methods 
            // As they are covered in other articles you can refer them 
            filein.writeObject(new Test()); 
            filein.writeUTF(str1); 
            filein.flush(); 
  
            // read the object and print the string 
            Test a = (Test) InStream.readObject(); 
  
            // String value using readFields() methods : 
            System.out.println("Using readFields() : " + a.str); 
  
            // Checking th use of registerValidation 
            System.out.println("----------------------------------------------------------"); 
            if (Test.str.equals("Welcome to javaTpoint")) 
            { 
                System.out.println("javaTpoint"); 
            } 
            
        } 
  
        catch (Exception excpt) 
        { 
            System.out.println("Error"); 
            excpt.printStackTrace(); 
        } 
    } 
  
    static class Test implements Serializable, ObjectInputValidation 
    { 
        static String str = "Welcome to javaTpoint";
        private String readObject(ObjectInputStream in) 
        throws IOException, ClassNotFoundException 
        { 
  
            // use of readFields() method : 
            ObjectInputStream.GetField getfield = in.readFields(); 
  
            // USe of registerValidation() method : 
            in.registerValidation(this, 0); 
  
            // Returning String "geek" to the main 
            return (String) getfield.get("str", null); 
        } 
  
        @Override
        public void validateObject() throws InvalidObjectException 
        { 
            throw new UnsupportedOperationException("Not supported yet."); 
        } 
    } 
}

Output:

Using readFields() : Welcome to javaTpoint
----------------------------------------------------------
javaTpoint

Next TopicJava ObjectInputStream

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