Use enum as generic type of my class but failed to create instance

Question

I have Enum:

public enum MyTypes {FOO, BAR}

I defined a class with generic types:

public MyClass<T extends MyTypes> {
  private T theType;
  ...
}

Above code compiles successfully.

Next, I try to create an instance of MyClass with type Foo as the generic parameter:

// Compilation error: Cannot resolve symbol FOO
MyClass<FOO> myInstanceWithFoo = new MyClass();

Is it because Java don't accept enum type as generic type? If so, is there a workaround to achieve what I want to achieve?

Andrew Tobilko · Answer 1 · 2019-10-02T21:02:03.207

Is it because Java don't accept enum type as generic type?

Any enum type is a valid type and can participate in both forming a type parameter (e.g. MyClass<T extends MyTypes>) and parameterizing a type (e.g. MyClass<MyTypes> myInstance).

What you did

MyClass<FOO> myInstanceWithFoo = new MyClass();

is never corrent since FOO is an instance (or object) of MyTypes. It's not a type, and can't be treated as such.

If so, is there a workaround to achieve what I want to achieve?

There is no need for MyClass to be generic. It declares a MyTypes field which could be either MyTypes.FOO or MyTypes.BAR (or null, of course).

class MyClass {
  private MyTypes myType;

  public MyClass(MyTypes myType) {
    this.myType = myType;
  }
}

Each MyTypes instance knows how to create a MyClass from itself.

enum MyTypes {
  FOO, BAR;

  public MyClass createMyClass() {
    return new MyClass(this);
  }
}

An example of how to create them would be

class Example {
  public static void main(String[] args) {
    MyClass myInstanceWithFoo = MyTypes.FOO.createMyClass();
    MyClass myInstanceWithBar = MyTypes.BAR.createMyClass();
  }
}

Thanks. On top of your answer, what if `MyTypes` is from 3rd party library that I can't modify to add the `createMyClass()` function there? — Leem.fin, Oct 02 '19 at 21:05
@Leem.fin then init them directly: `new MyClass(MyTypes.FOO)` — Andrew Tobilko, Oct 02 '19 at 21:08
@Leem.fin any reason why you unaccepted? is there anything I can improve? — Andrew Tobilko, Oct 10 '19 at 08:12

score 0 · Answer 2 · answered Oct 02 '19 at 21:28

It's my guess that you're confusing FOO as a subtype as MyClass when it is in fact an instance.

With 'normal' generics you specify the type the placeholder will be:

public class GenericClass <T extends Object> { // a bit contrived here
    private T generic;
}

which is instantiated

GenericClass<String> gen = new GenericClass<>(); // String is a subclass of Object

this is then compiled to

public class GenericClass {
   private String generic;
}

Notice there is no value for the String. There is no String instance. To do this we either need a setter or constructor:

public class GenericClass<T extends Object> {
    private T generic;

    public GenericClass(T generic) {
        this.generic = generic;
    }

}

GenericClass<String> gen = new GenericClass<>("Passing in a String because T was a String");

or

GenericClass<Integer> gen2 = new GenericClass<>(2); // T is integer so we can only pass integers - 2 will be the instance.

So for the Enum as a generic type we're stating the type which is MyType. The value would be FOO, so again we'd need either a setter or a constructor:

public class MyClass<T extends MyTypes> {
    private T theType;

    public MyClass(T theType) {
        this.theType = theType;
    }

}

and to make sure we're have a FOO instance we pass it in to the constructor

MyClass<MyTypes> myInstanceWithFoo = new MyClass<>(MyTypes.FOO);

Hope this makes sense.

Use enum as generic type of my class but failed to create instance

2 Answers2