CODEWITHSUNDEEP
javaclasscastexception
Chintan
Chintan

Reputation: 545

What is difference between .toString() and (String) cast in Java

I want to read a value from the session object which returns Object type. I know that the object has either true/false value. I would like to convert that value into boolean type. So I have the following code.

session.getAttribute("MyKeyValue"); // It returns Object type

Below throws an Exception.

boolean myBoolVal = Boolean.parseBoolean((String) session.getAttribute("MyKeyValue"));

Below works properly.

boolean myBoolVal = Boolean.parseBoolean(session.getAttribute("MyKeyValue").toString());

Actually, I don't understand why option 1 is not working ?

Upvotes: 4

Views: 1098

Answers (2)

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36401

You can set in the session object any object you want and associate to it a key. Any object means any object of any class. As any class is a possibly indirect sub class of Object then type is Object in set and get.

When you retrieve an object via get it is simply typed as Object but is certainly of some subclass. (Down)casting it to String may fails (an exception is thrown saying that the cast/conversion can't be realized) if the original class is not String. Seems to be the case for your option 1.

Option 2 works because any Object instance has a method toString() that can be called to get a String representation of the object (mainly useful for onscreen presentation of it). Then you parse that representation (probably "true" or "false") to obtain a boolean of value true or false.

So if you want your option 1 to work, as your object is probably a Boolean instance, you can use:

boolean myBoolVal = (Boolean)session.getAttribute("MyKeyValue");

Upvotes: 0

Eran
Eran

Reputation: 393846

When the runtime type of the instance returned by session.getAttribute("MyKeyValue") is not a String, casting it to String throws a ClassCastException.

On the other hand, session.getAttribute("MyKeyValue").toString() always works (assuming session.getAttribute("MyKeyValue") is not null), since all Objects have an implementation of the toString() method.

BTW, since session.getAttribute("MyKeyValue") doesn't return a String, it is likely that it returns a Boolean (since you expect Boolean.parseBoolean() to work), so if that is the case, instead of converting it to String and then to Boolean, you can just cast it to Boolean :

Boolean myBoolVal = (Boolean) session.getAttribute("MyKeyValue");

Upvotes: 8

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