private的理解：private是对类而不是对对象而言！！！

[leizhengdeng]

class MyPoint
{
private int x, y;
private String pointName;

public MyPoint()
{
x = 0;
y = 0;
pointName = "";
}

public MyPoint(int x, int y, String pointName)
{
this.x = x;
this.y = y;
this.pointName = pointName;
}

public void printOther(MyPoint other)
{
//This is valid, for 'this' and 'other' object
//share the same class 'MyPoint'.
other.printPoint();
}

private void printPoint()
{
System.out.println("In object " + pointName);
System.out.println("x = " + x +"\ty = " +y);
}
}

public class UsePoint
{
public static void main(String[] args)
{
MyPoint start = new MyPoint(0, 0, "START");
MyPoint end = new MyPoint(20, 20, "END");
start.printOther(end);
// The following is an invalid statement, because
// UsePoint and MyPoint are two different classes.
// end.printPoint();
}
}

//The result is:
//In the object END
//x = 20 y = 20

[leizhengdeng]

这样的意思就是在同一类中的对象可以互相通讯，就是封装嘛

[hailong326]

gz

[xuxk]

gz

[hyhong_h]

什么跟什么啊！

[skyyoung]

??

[leizhengdeng]

同一类的对象可以访问同一类的另一个对象的私有成元

[虎叔]

确实如此！不知这个特性有什么作用，在编程上？

[Necromancer]

实例之间相互访问？私有成员？我的程序只有一个实例在跑的，不知道多实例的应用领域是什么。。赫赫。。。

[Necromancer]

哎呀说错了。。主体程序是只有一个实例，但helper类可能会有多个实例

[freezingfire]

这样真的可以吗？

[tanghuan]

方法private是对类而言，属性就不同了，他是对对象而言

如果你把public void printOther(MyPoint other)
改为如下试一试看看
public void printOther(MyPoint other)

{
//This is valid, for 'this' and 'other' object
//share the same class 'MyPoint'.
System.out.println("In object " + other.pointName);
System.out.println("x = " + other.x +"\ty = " +other.y);

}

[leizhengdeng]

to:tanghuan()　
我试过了，编译运行都可以啊

public void printOther(MyPoint other)
{
//This is valid, for 'this' and 'other' object
//share the same class 'MyPoint'.
System.out.println("In object " + other.pointName);
System.out.println("x = " + other.x +"\ty = " +other.y);
}

[leizhengdeng]

一个非常有用的例子：（计算任何两天的相差天数）
注意：方法DaysSpan用到了这个特性。
//////////////////////////////////////
//以前我写的C++类使用操作符重载的：
int CDate::operator-( CDate& date)
{
return this->DaysFromJesus() - date.DaysFromJesus();
}
//操作符重载在c++中的使用还是比较方便的
CDate birthday = new CDate(2000, 1, 1);
CDate today = new CDate(2001, 1, 1);
int span =today - birthday;
/////////////////////////////////////
现在用java重写了，由于java不支持操作符重载，所以改写成函数，刚好用到了这个特性：




//*************************************************
//Author: leizhengdeng(Arden)
//http://arden.3322.net [e-mail]
//Module: class CDate
//function: to calculate the span of two days
//**************************************************

class CDate
{
static final int MONTH[ ]={0,31,28,31,30,31,30,31,31,30,31,30,31};
private int m_year;
private int m_month;
private int m_day;

public CDate()
{
m_year = 0;
m_month = 0;
m_day = 0;
}

public CDate(int year, int month, int day)
{
m_year = year;
m_month = month;
m_day = day;
}

private int DaysFromJesus()
{
int totalDays;
totalDays = 365*m_year + m_year/4 - m_year/100 + m_year/400 + 1;
if (m_year%400 == 0) totalDays--;
for(int i = 0; i < m_month; i++)
totalDays += MONTH[i];
totalDays += m_day;
if (IsLeapYear() && m_month >= 3) totalDays++;
return totalDays;
}


private boolean IsLeapYear()
{
if ((m_year%4 == 0 && m_year%100 != 0) || (m_year%400 == 0))
return true;
else
return false;
}

public int DaysSpan(CDate date)
{
return this.DaysFromJesus() - date.DaysFromJesus();
}
}

public class TestDate
{
public static void main(String[] args)
{
CDate birthday = new CDate(2000, 1, 1);
CDate today = new CDate(2001, 1, 1);
int span =today.DaysSpan(birthday);

System.out.println("span is " + span);
}
}

[leizhengdeng]

这下大家满足了吧