Is the following OK? 


Object obj;
String        str = "Yertle" ;

obj = str;
((YouthBirthday)obj).greeting();





A good answer might be:



No.









The   instanceof   Operator




A typecast is used to tell the compiler what is "really"
in a variable that itself is not specific enough.
You have to tell the truth.
In a complicated program,
a reference variable might end up with any of several
different objects, depending on the input data or other
unpredictable conditions.
To deal with this, the instanceof operator is used.



variable instanceof Class




This operator evaluates to true or false
depending on whether the variable refers to an object of
type Class.
For example in the following fragment,
instanceof is used to ensure that the object
referenced by obj is used correctly:






Object obj;
YouthBirthday ybd = new YouthBirthday( "Ian", 4 );
String        str = "Yertle" ;

obj = ybd;

if ( obj instanceof YouthBirthday )
  ((YouthBirthday)obj).greeting();

if ( obj instanceof String )
  System.out.print( (String)obj );











instanceof will also return true if the
class of the object on the left is a child 
(or grandchild or greatgrandchild or ...)
of the class on the right.









QUESTION 17:



If you don't know in advance what classes a variable might hold,
will instanceof help?


Click Here after you have answered the question