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How to access an abstract method

Posted on 2006-10-20
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Last Modified: 2010-03-31
Dear Expert:

I'm new to java.

I have an abstract class A, with an abstract method "calculate()".
I extend from A, classes B, C, and D.  The all implement "calculate()" differently.
I then put these classes as values into a MAP.
When I try to access the calculate() method using the .get method of the MAP,  the compiler says that there is not calculate() method.    "foo.get("add").calculate()"

How do I polymorphically get at the calculate method?

Any simple examples out there?

Thanks
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Question by:nathanChun
3 Comments
 
LVL 9

Accepted Solution

by:
shinobun earned 400 total points
ID: 17777414
By default, Map#get() will return Objects.

For java 5, you can use generics:

Map<String, A> foo = new HashMap<String, A>();
foo.put("add", new B());
foo.get("add").calculate();

Java 1.4 and before, you need to cast it to A:

((A) foo.get("add")).calculate();
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Author Comment

by:nathanChun
ID: 17777862
Casting it to the base class will get met acess to the abstract method calculate?
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LVL 2

Assisted Solution

by:harshgrover
harshgrover earned 100 total points
ID: 17777946
yes that would be the right way to do it. U can cast it to the parent class, and you would be able to access the method. this is one of the most oft used principles in factory loading...

Gary
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