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What is downcasting in Java?

My textbook says "To access a specific method of the object, we must downcast to the correct type. (Of course, in this example we do not need the downcast, since we are simply invoking the toString method at line 9, and this can be done for any object). A second important detail is that primitive types cannot be used. Only reference types are compatible with Object"

I typed the body of the main method below, and I put the MemoryCell class in the code section. First of all, what does downcast mean? Second, I understand why primitive types like int and float can't be used, but why does my book say that the toString method is automatically invoked? Thirdly, there are some situations in which primitive types can be passed as arguments into methods of objects?

MemoryCell m = new MemoryCell();

m.write("37");
String val = (String) m.read();
System.out.println("Contents are: " + val);

//
//
//

public class MemoryCell
{
     //Public methods
     public Object read() {return storeValue;}
     public void write(Object x) {storedValue = x;}

     private Object storedValue;
}

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for_yan

downcasting:

http://stackoverflow.com/questions/380813/downcasting-in-java

shampouya

ASKER

that page doesn't really define it clearly

for_yan

String extends Object so String s = (String) o; - is example of downcasting

if you want to use methods of String you need to do downcasting, otherwise compiler wuill complain if
you apply those methods to Object instance

Downcasting is allowed when there is a possibility that it suceeds at run time:

Object o = getSomeObject(),
String s = (String) o;

for_yan

Dowbncasting mans casting to a class which inherrits from the current typ
So if o was odeclared of type Object - you cast it tio String which inherist from Ovbject

for_yan

Some methods lilke

write(object) they inside their body do
this

write(Object o){

write(o.toString());

}

so you don't need
to imnvoke them in this way:

write(o.toString())

because this method will take care of getting string fro your object by itself in its body

for_yan

theer fore you can jaust say:

ArrayList ar = ...
System.out.println(ar);

and ptrintln() method will in fact involde toString() method on the argument within its own body - you don't need to worrty about it

evedder

here is another link with info about downcasting, and example and a brief explanation

http://www.programmerinterview.com/index.php/java-questions/downcasting-in-java/

for_yan

Of course primiteives are being passed as argiuments of the methods very often - why not?

say

Struing s = "1234";

String s2 = s.substring(2);

2 is int and it is passwd as arguiment to String;'s metghod substring() - plenty of times happens

for_yan

Important thing about downcasting is that if you use downcasting in your program
you need to do it explicitly

like

Object o;
String s = (String) o;

If you do upcasting you don't need to mention it expicilty

like

Strung s;

Object o = s;

(no parenthesses with the type is necessary).

Why?

because every instance of String is ALQWAYS
inastance of Object but not vice versa

So compiler needs programmes confirmatuion that he/sshe (programmer)beleives that
this particular object
in fact can be downcast

for upcatsing compiler odes not need it

for_yan

So if you do downcasting without explicit type in parenthese compiler raises an error - in order to
insure you form doing a mistake

Upxcasting is bnever a problem - compiler does not need to worruy that this is a mistake on your part

shampouya

ASKER

So the easy-to-understand definition of downcasting is taking an object of a parent class and then trying to convert into the type of one of the child classes.

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for_yan

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shampouya

ASKER

thanks