Object obj = new Integer(100);

String  strVal = (String)obj;

Integer obj = new Integer(100);

String  strVal = (String)obj;

/**

* Returns the string representation 
of the Object argument.

*
* @param   obj   an Object.

* @return  if the argument is null,
then a string equal to
*"null"; otherwise, the value of
*  obj.toString() is returned.
* @see java.lang.Object#toString()
*/
public static String 
valueOf(Object obj) {
return (obj == null) ? "null" 
: obj.toString();

}

if(String.valueOf（object）==null)
{
System.out.println(“传入的值是null！”);
}

System.out.println(String.valueOf(null));

System.out.println(null);