理解serialVersionUID是什么？有什么用？如何生成？

import java.io.Serializable;
 
public class Address implements Serializable{
 
       private static final long serialVersionUID = 1L;
    
       String street;
       String country;
 
       public void setStreet(String street){
           this.street = street;
       }
 
       public void setCountry(String country){
           this.country = country;
       }
 
       public String getStreet(){
           return this.street;
       }
 
       public String getCountry(){
           return this.country;
       }
 
       @Override
       public String toString() {
           return new StringBuffer(" Street : ")
           .append(this.street)
           .append(" Country : ")
           .append(this.country).toString();
       }
}

import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
 
public class WriteObject{
 
    public static void main (String args[]) {
     
       Address address = new Address();
       address.setStreet("wall street");
       address.setCountry("united states");
 
       try{
 
        FileOutputStream fout = new FileOutputStream("c:address.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fout);   
        oos.writeObject(address);
        oos.close();
        System.out.println("Done");
 
       }catch(Exception ex){
           ex.printStackTrace();
       } 
    }
}

import java.io.FileInputStream;
import java.io.ObjectInputStream;
 
public class ReadObject{
 
   public static void main (String args[]) {
 
       Address address;
       
       try{
 
           FileInputStream fin = new FileInputStream("c:address.ser");
           ObjectInputStream ois = new ObjectInputStream(fin);
           address = (Address) ois.readObject();
           ois.close();
 
           System.out.println(address);
           
       }catch(Exception ex){
           ex.printStackTrace(); 
       } 
   }
}