String类的valueOf以及动态绑定（多态）

1、String类的valueOf

Class String

public static String valueOf(Object obj)

Returns the string representation of the Object argument.

Parameters:: obj - an Object.
Returns:: if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.
See Also:: Object.toString()

2、动态绑定（多态）的3必要个条件：

1）、继承（Date继承Object），2）、重写（toString），3）、父类的引用指向子类的对象（见ValueOfObject类

3、求整形数的大小

//ValueOfObject.java

 1 class ValueOfObject {
 2     public static void main(String[] args) {
 3         Date d = new Date(2013, 8,11);
 4         
 5         //原型：public static String valueOf(Object obj)
 6         // 3、父类的引用指向子类的对象
 7         String str = String.valueOf(d); 
 8         System.out.println("value of date="+str);
 9         
10         System.out.println("int 198 value of="+String.valueOf(198));
11         System.out.println("double 198.98 value of="+String.valueOf(198.98));
12         
13         // 求整形数的长度（转换为String)
14         int i = 123456789;
15         String sNumber = String.valueOf(i);
16         System.out.println("the length of i is:" + sNumber.length());
17     }
18 }
19 
20 class Date {
21     int year;
22     int month;
23     int date;
24     
25     Date(int year, int month, int date) {
26         this.year = year;
27         this.month = month;
28         this.date = date;
29     }
30     
31     // override from the Object class
32     // 动态绑定（多态）3个必要条件：1、继承（Date继承Object），2、重写（toString），3、父类的引用指向子类的对象 （见ValueOfObject类）
33     public String toString() {
34         return "year:month:date--"+year+"-"+month+"-"+date;
35     }
36     
37 }

补充：

Class Object

public String toString()

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())

Returns:: a string representation of the object.

结果：

E:学习Java编程练习TestString>java ValueOfObject
value of date=year:month:date--2013-8-11
int 198 value of=198
double 198.98 value of=198.98

the length of i is:9