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

class ValueOfObject {
    public static void main(String[] args) {
        Date d = new Date(2013, 8,11);
        
        //原型：public static String valueOf(Object obj)
        // 3、父类的引用指向子类的对象
        String str = String.valueOf(d); 
        System.out.println("value of date="+str);
        
        System.out.println("int 198 value of="+String.valueOf(198));
        System.out.println("double 198.98 value of="+String.valueOf(198.98));
        
        // 求整形数的长度（转换为String)
        int i = 123456789;
        String sNumber = String.valueOf(i);
        System.out.println("the length of i is:" + sNumber.length());
    }
}

class Date {
    int year;
    int month;
    int date;
    
    Date(int year, int month, int date) {
        this.year = year;
        this.month = month;
        this.date = date;
    }
    
    // override from the Object class
    // 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）
    public String toString() {
        return "year:month:date--"+year+"-"+month+"-"+date;
    }
    
}

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