oop - Transform an object to its subclass in Java -

this not regular question, please listen explain.

i have object of class animal. how can transform subclass cat? this:

animal = new animal(); cat c = (cat) a; 

of course, know it's not possible casting directly animal cat. don't want manually create new cat , copy fields animal it. because have lots of class hierarchies need transform. how can solve general way?

ignoring fact, in example want inverse as-is relation (as in "an apple is-a fruit" vs "a fruit is-an apple") - pointless degree.

but lets imagine case, have make potate look like apple. you'll carve out pulp , put potato apple (i use fruit example here because animals gets bit messy). either called proxying or wrapping.

both techniques can done in java either statically or dynamically.

statically: wrapper:

• create new class, i.e. veggiapplewrapper
• let extend apple (so wraps looks apple)
• define constructor accepts other type (i.e. potato)
• implement/override methods common , delegate calls wrapped object
• implement other methods throwing unsupportedoperationexception

dynamically: proxy:

• you have define interface @ least target type (i.e. interface apple)
• create proxy instance using proxy
• use apple interface in list of implemented interface
• implement methodinvocationhandler forwarding you, similar wrapper, using reflection
• cast proxy apple

alternatives using java's dynamic proxy code generation libraries such cglib or javaassist generate subclass of target type @ runtime, same wrapper implemented proxy.

in order copy values animal cat instance (as animal superclass, fields should common), use reflection api well

for(field f : animal.class.getdeclaredfields()) {    f.setaccessible(true)   f.set(cat, f.get(animal)); }  

this simple example, without exception handling , without considering fields of animal's superclass (getdeclaredfields retrieves field of this class).