Class as function argument

Question

I have a function that filtering list of some values and it use instanseof construction:

 public static List<View> getAllChildren(View v) {
    /* ... */
    if (v instanceof Button) {
        resultList.add(v);
    }
    /* ... */
 }

I want to make it more general and set Button as function parameter:

 public static List<View> getAllChildren(View v, ? myClass) {
    /* ... */
    if (v instanceof myClass) {
        resultList.add(v);
    }
    /* ... */
 }

But i don't know how to pass myClass to the function. Please, tell me how i can generalize this function?

Answer 1

You can pass a class type as a parameter using the Class class. Note that it is a generic type. Also, the instanceof operator works only on reference types, so you will have to flip it around to get this to work:

public static List<View> getAllChildren(View v, Class<?> myClass) {
    /* ... */
    if (myClass.isInstance(v)) {
        resultList.add(v);
    }
    /* ... */
}

In order to get the Class type to be passed in like this, you can simply use the name of the class you want, with ".class" appended. For example, if you wanted to call this method with the Button class, you would do it like so:

getAllChildren(view, Button.class);

Or if you had an instance of something that you wanted the class of, you would use the getClass() method:

Button b = new Button();
getAllChildren(view, b.getClass());

As Evan LaHurd mentioned in a comment, isInstance() will check if the two classes are assignment-compatible, so they may not be the exact same class. If you want to make sure they are exactly the same class, you can check them for equality like so:

myClass.equals(v.getClass());

or

myClass == v.getClass();

will also work in this case, as pointed out by bayou.io