How to declare raw types in Kotlin?

Question

In Java I can declare this

private List a;

//onCreate
a = new ArrayList()

But in Kotlin it shows error, it forces me to specific a type

private List<String> a

Sometimes I do not want to provide a type ( I do not need to) but it shows error in Kotlin For example In Java

public abstract class BaseActivity<T extends ViewDataBinding> extends AppCompatActivity {
 //something
}

public abstract class BaseFragment {
 private BaseActivity activity;
 //something
}

//in kotkin I can't write
lateinit var activity: BaseAtivity //show error here (I have to specific a type but this is the base class and I do not want to specific a type here). I just want a reference of BaseActivity

@Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof BaseActivity) { //good in java but show error 
                                              //in kotlin because I have to //specific a type like BaseAtivity<something>
            BaseActivity activity = (BaseActivity) context;
            this.mActivity = activity;
            activity.onFragmentAttached();
        }
    }

What can I write in Kotlin to achieve the same code in java

Answer 1

You can use star projections

e.g.

private List<*> a

Answer 2

Well, technically the star projection isn't the raw type. The raw types are not supported by Kotlin. You cannot define a variable of raw type, nor can you define a method param of raw type. It won't compile.

And if you use the star projection, there's little help i it since there's not much you can do with it. So you will end up casting it to a type you need, cross your fingers and hope for the best. Best is to avoid java API with raw types.

Or create your own variable of that generic class and safely transfer all the information to it. If it's a Collection, then iterate through it and put elements into new Collection. it will save you from null pointer exception too since the elements or the collection itself can be null.

By the way, in your example there are some serious design flaws.

1. First and foremost you should not have a field of your activity in your fragment. Each time you need your activity, call (BaseActivity<*>getActivity()) or you create a memory leak.
2. Next, you made your Activity generic and then use it as a raw type. This won't do. If you don't care for a concrete type in triangle brackets, then use a wildcard <*> for Kotlin, <?> for Java.