How to check if a subclass is an instance of a cla

问题:

This question already has an answer here:

• Check if a class is subclass of another class in Java 6 answers

In an android app test suite I have a class like this where B is a view:

public class A extends B {
... etc...
}

now I have a list of view objects which may contain A objects but in this case I only care if they're subclasses or "instances of" B. I'd like to do something like:

ArrayList<View> viewList = getViews();
Iterator<View> iterator = viewList.iterator();
while (iterator.hasNext() && viewList != null) {
    View view = iterator.next();
    if (view.getClass().isInstance(B.class)) {
        // this is an instance of B
    }
}

The problem is that when the if encounters an A object it doesn't evaluate to an "instance of B". Is there a way to do isSubclassOf or something?

回答1:

You have to read the API carefully for this methods. Sometimes you can get confused very easily.

It is either:

if (B.class.isInstance(view))

API says: Determines if the specified Object (the parameter) is assignment-compatible with the object represented by this Class (The class object you are calling the method at)

or:

if (B.class.isAssignableFrom(view.getClass()))

API says: Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter

or (without reflection and the recommend one):

if (view instanceof B)

回答2:

if(view instanceof B)

This will return true if view is an instance of B or the subclass A (or any subclass of B for that matter).

回答3:

Maybe I'm missing something, but wouldn't this suffice:

if (view instanceof B) {
    // this view is an instance of B
}

回答4:

Class.isAssignableFrom() - works for interfaces as well. If you don't want that, you'll have to call getSuperclass() and test until you reach Object.

回答5:

It's the other way around: B.class.isInstance(view)

回答6:

If there is polymorphism such as checking SQLRecoverableException vs SQLException, it can be done like that.

try {
    // sth may throw exception
    ....
} catch (Exception e) {
    if(SQLException.class.isAssignableFrom(e.getCause().getClass()))
    {
        // do sth
        System.out.println("SQLException occurs!");
    }
}

Simply say,

ChildClass child= new ChildClass();
if(ParentClass.class.isAssignableFrom(child.getClass()))
{
    // do sth
    ...
}

回答7:

I've never actually used this, but try view.getClass().getGenericSuperclass()