声明存储在集合中的类型（文字）的变量

Question

Is it possible to declare variable of a type (literal) stored in a collection? Or that might be more of a feature/task for an interpreted, not a compiled language like Java?

For example:

import java.util.*;

class Base {}
class A1 extends Base{}
class A2 extends Base{}

public class Test{

    public static void main(String[] args){

        ArrayList<Class<? extends Base>> typeArrayList=new ArrayList<Class<? extends Base>>();
        typeArrayList.add(A1.class);
        typeArrayList.add(A2.class);
        typeArrayList.add(A1.class);
        typeArrayList.add(A1.class);
        //etc. etc.

        //now that all the types are nicely stored in a collection,
        //could we somehow use them to declare variables of those types?
        //e.g. declare something like:
        //typeArrayList.get(1) someVariable;

    }//main
}//Testlass

Answer 1

You can use:

Object o = typeVarArrayList.get(1).newInstance();

or

Type o = (Type) typeVarArrayList.get(1).newInstance();

But you need that the class have one constructor without parameters.

If you need a more specific constructor, please see here

Answer 2

You can use newInstance() if there is a default constructor available, if not then you will need to have people supply you with factory objects and then call a method in the factory to build your objects.

The Factory pattern is generally neater too for most implementations unless the objects being created really are just plain beans with nothing but default values as the factory allows you to properly set up the object.

Answer 3

    //e.g. declare something like:
    //typeArrayList.get(1) someVariable;

In short, no, type declarations must be explicit.

For example, the Java language defines these rules for local variable declarations:

LocalVariableDeclaration:
 VariableModifiersopt Type VariableDeclarators

Type:
 PrimitiveType
 ReferenceType

ReferenceType:
 ClassOrInterfaceType
 TypeVariable
 ArrayType

Drilling down to TypeVariable (because I'm not copying the entire language grammar) we get:

TypeVariable:
 Identifier

An identifier is restricted to the things you'd expect in a class name.