Solved

What is downcasting in Java?

Posted on 2011-09-09
13
604 Views
Last Modified: 2012-05-12
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;
}

Open in new window

0
Comment
Question by:shampouya
  • 9
  • 3
13 Comments
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
0
 

Author Comment

by:shampouya
Comment Utility
that page doesn't really define it clearly
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility



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;
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
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
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
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

0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
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
0
6 Surprising Benefits of Threat Intelligence

All sorts of threat intelligence is available on the web. Intelligence you can learn from, and use to anticipate and prepare for future attacks.

 
LVL 3

Expert Comment

by:evedder
Comment Utility
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/
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
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
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility


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
0
 
LVL 47

Expert Comment

by:for_yan
Comment Utility
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
0
 

Author Comment

by:shampouya
Comment Utility
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.
0
 
LVL 47

Accepted Solution

by:
for_yan earned 500 total points
Comment Utility
yes - you are right
0
 

Author Closing Comment

by:shampouya
Comment Utility
thanks
0

Featured Post

Top 6 Sources for Identifying Threat Actor TTPs

Understanding your enemy is essential. These six sources will help you identify the most popular threat actor tactics, techniques, and procedures (TTPs).

Join & Write a Comment

Suggested Solutions

After being asked a question last year, I went into one of my moods where I did some research and code just for the fun and learning of it all.  Subsequently, from this journey, I put together this article on "Range Searching Using Visual Basic.NET …
Introduction Java can be integrated with native programs using an interface called JNI(Java Native Interface). Native programs are programs which can directly run on the processor. JNI is simply a naming and calling convention so that the JVM (Java…
Viewers learn about the “for” loop and how it works in Java. By comparing it to the while loop learned before, viewers can make the transition easily. You will learn about the formatting of the for loop as we write a program that prints even numbers…
Viewers will learn one way to get user input in Java. Introduce the Scanner object: Declare the variable that stores the user input: An example prompting the user for input: Methods you need to invoke in order to properly get  user input:

771 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

12 Experts available now in Live!

Get 1:1 Help Now