如何从擦除类型转换类型

我有一个父班，Parent有两个孩子班，A和B。我有一个接口，Function<Type1 extends Parent,Type2 extends Parent>它允许程序员编写一个Type2 of(Type1 t)将Type1转换为Type2的特定函数。该接口包装在一个类中，该类Wrapper<Type1 extends Parent,Type2 extends Parent>包含有用的信息，例如Class<Type1> type1Class等等。

我的问题出现了，当我尝试实施add该方法的Wrapper类Wrapper<Type1,Type2> add(Wrapper<Type1,Type2> additionalWrapper)。我正在尝试将两个Functions相加，但是由于擦除，我很难Type2输出aParent而不是a 。

如何使add方法输出aType2而不是a Parent？

public class Parent {
    protected int value;
    public void setValue(int x){ value = x; }
    public int getValue(){ return value; }
    public Parent(){}
    public Parent(int x){setValue(x);}
    public Parent add(Parent p){return null;}
}


public class A extends Parent{
    public A(){ setValue(1); }
    public A(int x){ setValue(x); }
    public A(B b){ setValue( b.getValue()); }
    public A add(A a){ return new A( getValue()+a.getValue()); }
    public A add(B b){ return new A( getValue()*b.getValue()); }
}

public class B extends Parent{
    public B(){ setValue(2); }
    public B(int x){ setValue(x); }
    public B(A a){ setValue(a.getValue()); }
    public B add(B b){ return new B(getValue() + b.getValue()); }
    public B add(A a){ return new B(getValue() * a.getValue()); }
}

public interface Function <Type1 extends Parent, Type2 extends Parent> {
    public Type2 of(Type1 t);
}

public class Wrapper<Type1 extends Parent, Type2 extends Parent> {

    protected Function<Type1,Type2> function;
    protected Class<Type1> type1Class;
    protected Class<Type2> type2Class;
    public Wrapper(final Class<Type1> t1, final Class<Type2> t2, Function<Type1,Type2> x) {
        type1Class = t1;
        type2Class = t2;
        function = x;
    }

    public Type2 of(Type1 t){
        return function.of(t);
    }
    public Wrapper<Type1,Type2> add(final Wrapper<Type1,Type2> additionalWrapper){
        return new Wrapper<Type1,Type2>( type1Class, type2Class, new Function<Type1,Type2>(){
            public Type2 of(Type1 t){
                try{
                    Type2 term = function.of(t);
                    Type2 summand = additionalWrapper.of(t);
                    Type2 sum = (Type2) term.add(summand); ///Problem happens here. term and summand are both interpreted as Parent by the erasure, I think, and therefore add outputs null, setting sum=null.
                    return sum;
                } catch(Exception e){
                    e.printStackTrace();
                    return null;
                }
            }
        });
    }
}

public class Main {
    public static void main(String[] args){
        Wrapper<A, B> wrapper1 = new Wrapper<A, B>(A.class,B.class,new Function<A, B>() {
            @Override
            public B of(A a) {
                return new B(a);
            }
        });
        Wrapper<A, B> wrapper2 = new Wrapper<A, B>(A.class,B.class,new Function<A, B>() {
            @Override
            public B of(A a) {
                B b = new B();
                return b.add(a);
            }
        });
        Wrapper<A,B> wrapper3 = wrapper1.add(wrapper2);

        A a = new A(3);
        B b = wrapper3.of(a);

        System.out.println(b.getValue()); ///Error happens here because b was set to null and therefore doesn't contain int value.

    }
}