What is the use of flush method in serializing an object after writing the object in to the stream?

Question

I'm trying to do a simple serialization of an object. i have an entity called Student as follows:

package serialization;

import java.io.Serializable;

public class Student  implements Serializable{ 

/**
 * 
 */
private static final long serialVersionUID = 1L;
@Override
public String toString() {
    return "Student [id=" + id + ", name=" + name + "]";
}
public Student(int id, String name) {
    super();
    this.id = id;
    this.name = name;
}
public Student() {
    super();
    // TODO Auto-generated constructor stub
}
public int getId() {
    return id;
}
public void setId(int id) {
    this.id = id;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public int id;
public String name;

}

and i have my main method as follows ,

package serialization;

import java.io.File;
import java.io.FileWriter;

public class Serialization {

public static void main(String args[])throws Exception{  
      Student s1 =new Student();  
      s1.setId(2);
      s1.setName("hihihihihihi");
     File f = new File("D://abc.txt");
     FileWriter fw = new FileWriter(f);
      fw.write(s1.getName().toString());

     // fw.flush();
     // fw.close();


      System.out.println("success");  
     }  

}

Once after writing the object in to the file in the prescribed location, i cannot see any content on the file. If i use a flush or close method , i'm able to see the name printed as "hihhihihiiii" etc.

the write method will write the object in to the stream , so why is it not present in the file.Why it is writing the data Only after closing or flushing the file Writer ??