Core Java 面试题与答案

问题 56. Explain the usage of the keyword transient?

This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
For example:
class T { transient int a; // will not persist int b; // will persist }

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Date;

public class Logon implements Serializable {
	private Date date = new Date();
	private String username;
	private transient String password;
	
	public Logon(String name, String pwd) {
		username = name;
		password = pwd;
	}
	public String toString() {
		String pwd = (password == null) ? "(n/a)" : password;
		return "logon info: n username: " + username + "n date: " + date
		+ "n password: " + pwd;
	}

	public static void main(String[] args) throws Exception {
		Logon a = new Logon("Hulk", "myLittlePony");
		System.out.println("logon a = " + a);
		ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream(
		"Logon.out"));
		o.writeObject(a);
		o.close();
		Thread.sleep(1000); // Delay for 1 second
		// Now get them back:
		ObjectInputStream in = new ObjectInputStream(new FileInputStream(
		"Logon.out"));
		System.out.println("Recovering object at " + new Date());
		a = (Logon) in.readObject();
		System.out.println("logon a = " + a);
	}
}