Question

look at this:

package test;

public abstract class BaseClass {
    public abstract void doSomething();
}

package test;

public class A extends BaseClass {

    @Override
    public void doSomething() {
        // TODO Auto-generated method stub
        System.out.println("A");
    }

}

package test;

public class GenericFoo<T extends BaseClass>{

    public GenericFoo(){

    }

    private T instance;

    public void  setType(String type){
        // client code  call this to set concrete type of T
    }

    public void doSomething(){
        instance.doSomething();
    }

    public static void main(String s) throws InstantiationException, IllegalAccessException{
        GenericFoo<?> obj = GenericFoo.class.newInstance();
        obj.setType("A");
        // I want  it to print "A" on console
        obj.doSomething();

    }
}

how to implement this, my class is only allowed to create instance with calling class#newInstance() , but it’s typed.

Answer 1

You cannot set the type of an object at run time; you can only create instances of specific types. Once an instance of GenericFoo has been created, there’s no way to set it’s type parameter. (Due to type erasure, that doesn’t even make sense at run time anyway.)

The best you can do is something like this:

private static class GenericFoo<T extends BaseClass>
{
    T createInstance(Class<T> clazz)
    {
        return clazz.newInstance();
    }
}