返回通用类类型

Question

I have a method that might return Class A or Class B. How can I define its return type to be generic, independent of what class type is returned. For eg

public <Generic_Class_Return_Type> showForm() {
   if (true)
      return new ClassA();
   else 
      return new ClassB();
}

Answer 1

Not really sure if you need generics in this case, however you can parameterize either the whole class or just the method and then use reflection like this:

public <T> T getForm() {
  Class<T> clazz = (Class<T>) ((true) ? Foo.class : Bar.class);
  Constructor<T> ctor = clazz.getConstructor();
  return ctor.newInstance();
}

However if you specify your use case, we can further suggest if going generics is the way, or if you'd better use standard polymorphism.

Answer 2

You could use an interface both classes implement like this:

public SomeInterface showForm() {
   if (true)
      return new ClassA();
   else 
      return new ClassB();
}

class ClassA implements SomeInterface{}
class ClassB implements SomeInterface{}

Answer 3

public object showForm()
{
   if (true)
      return new ClassA();
   else
      return new ClassB(); 
}

or

public superClassName showForm()
{
   if (true)
      return new ClassA();
   else
      return new ClassB();
}

Answer 4

The simplest way is to return them as an Object.

public Object showForm() {
   if (true)
      return new ClassA();
   else 
      return new ClassB();
}

It's not so useful though, a far more useful solution would be to have them either extend a common class or implement a common interface.

public CommonInterface showForm() {
   if (true)
      return new ClassA();
   else 
      return new ClassB();
}

class ClassA implements CommonInterface { }
class ClassB implements CommonInterface { }
interface CommonInterface { }

Or

public CommonClass showForm() {
   if (true)
      return new ClassA();
   else 
      return new ClassB();
}

class ClassA extends CommonClass { }
class ClassB extends CommonClass { }
class CommonClass { }

Generics

If you want to use generics then ClassA and ClassB would need to be the same Class, modified by some generic type eg. Class<T> . Whether generics are relevant all depends on the implementation of your classes. You're probably best to go with an interface or base class.