Question

I am learning java, and studying the following example from sun.com.

import java.io.*;
import java.util.*;

public class UserInfo implements Serializable {
   String name = null;

   public UserInfo(String name) {
   this.name = name;
 }

 public void printInfo() {
    System.out.println("The name is: "+name);
 }
}




import java.io.*;
import java.util.Date;

public class ReadInfo {

public static void main(String argv[]) throws Exception {
    FileInputStream fis = new FileInputStream("name.out");
    ObjectInputStream ois = new ObjectInputStream(fis);

    UserInfo user1 = (UserInfo) ois.readObject();
    UserInfo user2 = (UserInfo) ois.readObject();

    user1.printInfo();
    user2.printInfo();
    ois.close();
    fis.close();
 }
 }

I am having question regarding the ReadInfo.java. In specific, I do not know how to understand the code of line UserInfo user1 = (UserInfo) ois.readObject(); Especially, what is the functionality of “UserInfo” in the parenthesis. What is the relationship between (UserInfo) and ols.readObject( ).

Answer 1

The Object is being read and the code is casting it to a UserInfo Object, as there is some knowledge by the code author that “name.out” contains serialized Objects of type UserInfo.

The API makes this very clear:

The method readObject is used to read
an object from the stream. Java’s safe
casting should be used to get the
desired type. In Java, strings and
arrays are objects and are treated as
objects during serialization. When
read they need to be cast to the
expected type.

Additionally you might want to read up on the Java tutorial’s section re: Casting Objects.