After all Read the objects from the file in java, there are many issues that you need to write more data on file. Usually we just declare

FileOutputStream f = new FileOutputStream(fileName,true);

But after many attempts I still do not. Search for a while, apart from the extra stuff 2 such is true, then we need to override the method writeStreamHeader as follows:

ObjectOutputStream oStream = new ObjectOutputStream(f) {
	protected void writeStreamHeader() throws IOException {
		reset();
	}
};

1. Quick Code to test

package vietSource.net.IOFile;

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

public class ReadWriteObject {

	public static void main(String[] args) {

		File f = new File("student.dat");
		FileOutputStream fo;
		ObjectOutputStream oStream = null;

		// ghi file lan 1
		try {
			fo = new FileOutputStream(f);
			oStream = new ObjectOutputStream(fo);

			oStream.writeObject(new MyStudent("pham ha", 22));
			oStream.close();
		} catch (FileNotFoundException e1) {
			e1.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}

		// ghi file lan 2
		try {
			fo = new FileOutputStream(f, true);
			oStream = new ObjectOutputStream(fo) {
				protected void writeStreamHeader() throws IOException {
					reset();
				}
			};
			oStream.writeObject(new MyStudent("nguyenvanquan7826", 22));
			oStream.close();
		} catch (FileNotFoundException e1) {
			e1.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}

		// doc file
		try {
			FileInputStream fis = new FileInputStream(f);
			ObjectInputStream inStream = new ObjectInputStream(fis);

			System.out.println(inStream.readObject());
			System.out.println(inStream.readObject());

			inStream.close();
		} catch (ClassNotFoundException e) {
			System.out.println("Class not found");
			e.printStackTrace();
		} catch (IOException e) {
			System.out.println("Error Read file");
			e.printStackTrace();
		}
	}
}

class MyStudent implements Serializable {
	String name;
	int age;

	public MyStudent(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}

	public String toString() {
		return "MyStudent [name=" + name + ", age=" + age + "]";
	}
}

2. Complete code should be used

Do not exist if no object file or files will be added error, therefore to ensure that all cases you see the following code:

package vietSource.net.IOFile;

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

public class ReadWriteObject {

	// kiem tra file co object hay khong
	public static boolean hasObject(File f) {
		// thu doc xem co object nao chua
		FileInputStream fi;
		boolean check = true;
		try {
			fi = new FileInputStream(f);
			ObjectInputStream inStream = new ObjectInputStream(fi);
			if (inStream.readObject() == null) {
				check = false;
			}
			inStream.close();

		} catch (FileNotFoundException e) {
			check = false;
		} catch (IOException e) {
			check = false;
		} catch (ClassNotFoundException e) {
			check = false;
			e.printStackTrace();
		}
		return check;
	}

	public static void write(MyStudent s) {
		try {

			File f = new File("student.dat");
			FileOutputStream fo;
			ObjectOutputStream oStream = null;

			// neu file chu ton tai thi tao file va ghi binh thuong
			if (!f.exists()) {
				fo = new FileOutputStream(f);
				oStream = new ObjectOutputStream(fo);
			} else { // neu file ton tai

				// neu chua co thi ghi binh thuong
				if (!hasObject(f)) {
					fo = new FileOutputStream(f);
					oStream = new ObjectOutputStream(fo);
				} else { // neu co roi thi ghi them vao

					fo = new FileOutputStream(f, true);

					oStream = new ObjectOutputStream(fo) {
						protected void writeStreamHeader() throws IOException {
							reset();
						}
					};
				}
			}

			oStream.writeObject(s);
			oStream.close();

		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	public static void read() {
		try {
			File f = new File("student.dat");
			FileInputStream fis = new FileInputStream(f);
			ObjectInputStream inStream = new ObjectInputStream(fis);
			Object s;
			int i = 0;
			while (true) {
				s = inStream.readObject();
				System.out.println(++i + ":" + s.toString());
			}
		} catch (ClassNotFoundException e) {
		} catch (IOException e) {
		}
	}

	public static void main(String[] args) {

		// ghi sinh vien 1
		write(new MyStudent("nguyenvanquan7826", 22));

		// ghi tiep sinh vien 2
		write(new MyStudent("phamha", 22));

		// doc file
		read();

	}
}

class MyStudent implements Serializable {
	String name;
	int age;

	public MyStudent(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}

	public String toString() {
		return "MyStudent [name=" + name + ", age=" + age + "]";
	}
}