Are the both same?
ParentClass object=new ChildClass();
ParentClass object = (ParentClass) new ChildClass();
If I want to create a map which maps a string to Children Objects like
"dog"->childA;
"cat"->childB
How should I declare the Map?
Map<String,Parent>
or
Mpa<String,T extends Parent>
I am not sure if the second one is right or not but I read it somewhere
Are the both same?
Casting is unnecessary in the 2nd assignment. The first one is valid assignment. You can assign a child class object to super class reference.
How should I declare the Map?
You should declare it the first way. A Map<String, Parent> will be capable of holding instances of Parent or any of it's subclasses.
As for your 2nd declaration:
Map<String, T extends Parent> // This won't compile
this is not the correct way to specify bound in type parameter while declaring map. You can't use bounded type parameter there. You should either use a wildcard bound, or just a type parameter - Map<String, T>, with bounds for T being declared where you declare that type (perhaps in the generic class where you declare the map). So, you can use <? extends Parent>. The issue with declaring map this way is that, you won't be able to add anything into it, accept null.
Related Post:
Reference:
The cast and non-cast are indeed the same.
For the generic, if you declare it with <String, ? extends Parent>, you won;t be able to add things to the list due to the fact that if it was declared <String, Child> parents couldn't be added. If NewChild was created by another dev, then we can;t know if Parent, Child, or NewChild is used and writes would not be allowed.
The first two are identical in effect. Casting is unnecessary.
If you want the map to contain a mixture of child classes, you need to declare
Map<String,Parent>
If you declare
Map<String,T extends Parent>
it declares a map from String to one particular child class of type T. The type parameter will need to be specified elsewhere (as a type parameter to the containing class or to an enclosing method). Note that you cannot instantiate a Map of an unbound type. Thus, for instance:
Map<String,? extends Parent> = new HashMap<String, ? extends Parent>();
generates a compiler error. The best you could do is:
Map<String,? extends Parent> = new HashMap<String, Parent>();
in which case you might as well stick with
Map<String,Parent> = new HashMap<String, Parent>();
