//【例3.1】  聲明日期類。

public class Date1                     //類聲明
{
    int year,month,day;                //成員變量，表示年、月、日
    
    void setDate(int y,int m,int d)    //成員方法，設置日期值
    {                                  //無返回值，有3個參數(shù) 
        year = y;
        month = m;
        day = d;
    } 
    
    boolean isLeapYear()               //判斷年份是否為閏年
    {                                  //布爾型返回值，無參數(shù)
        return (year%400==0) || (year%100!=0) && (year%4==0);
    }
    
    void print()                       //輸出日期值，無返回值,無參數(shù)
    {
        System.out.println("date is "+year+'-'+month+'-'+day);
    } 
    
    
    public static void main(String args[])
    {
        Date1 aday = new Date1() ;     //創(chuàng)建對象
        aday.setDate(2006,1,28);       //調(diào)用類方法
        aday.print();
        
        Date1 aday2 = aday;            //對象引用賦值
        aday2.year = 2007;
        System.out.println(aday.year+" is a leap year,"+aday.isLeapYear());
    } 
}

/* 
程序運行結(jié)果如下：
date is 2006-1-28
2007 is a leap year,false


*/
/*
程序正確:引用模型

        Date1 aday,b;
        aday = new Date1() ;     //創(chuàng)建對象
        System.out.println("==null"+aday.equals(null));
        
        b=aday;
        
        
        aday.setdate(2006,1,28);       //調(diào)用類方法
        
        b.print();
        aday.print();
        System.out.println(aday.year+" is a leap year,"+aday.isleapyear());

程序錯誤：
        Date1 aday;
        System.out.println("==null"+aday.equals(null));//編譯錯，變量未初始化variable aday might not have been initialized
        
*/