Static

1.统计产生对象的数量。

　　class Book{
    private String title ;
    private static int num = 0 ;

   public Book (String book){
    num ++ ;
    System.out.println("This is the num" + num + "book" ) ;
    }
}
public class Static{
    public static void main(String args[]){
        Book b1 = new Book ("..") ;
        Book b2 = new Book ("..") ;
        Book b3 = new Book ("..") ;
        Book b4 = new Book ("..") ;
        Book b5 = new Book ("..") ;
    }   
}

2.自动获得编号

class Book{
    private String title ;
    private static int num = 0 ;
    
    public Book(){
        this("NOTITL" + num++ ) ;
    }
    public Book(String title){
        this.title = title ;
    }
    public String getTitle(){
        return this.title ;
    }
     
}
public class Static{
    public static void main(String args[]){
        System.out.println(new Book().getTitle()) ;
        System.out.println(new Book().getTitle()) ;
        System.out.println(new Book().getTitle()) ;
        System.out.println(new Book().getTitle()) ;
        System.out.println(new Book().getTitle()) ;
    }   
}