Compare object with class stored in field

Question

I am trying to compare part with the c class given from the constructor but for some reason my IDE pops a warning that it will always be false.

Class<? extends Part> c;

public Slot(Class<? extends Part> c){
    this.c = c;
}

public boolean addItem(Part part){
    return part instanceof c;
}

Why is this instanceof statement always false and what is the correct syntax to compare part with c?

@gtgaxiola and what would be the correct syntax to compare `part` with a class saved in a field? — user36976, Jul 10 '14 at 13:02
@Nick - now your question does not make sense! You changed the code so it does not match your text below. Please return it back so the question remains consistent. — Honza Zidek, Jul 10 '14 at 13:18
@HonzaZidek oops sorry updated the code to fix the `extends Part` part and forgot I was trying out the code from one from the answers, fixed. — user36976, Jul 10 '14 at 13:23

Fabien Fleureau · Answer 1 · 2014-07-10T13:42:01.247

8

You should call the following Class method:

Class.isInstance(Object obj)

When you call "instanceof c", "c" should be a Class, not a variable. In your example, c is a variable whose type is Class.

edited Jul 10 '14 at 13:42

answered Jul 10 '14 at 13:02

Fabien Fleureau

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score 1 · Accepted Answer · edited May 23 '17 at 12:16

1

Instead of this

if(part instanceof c) return true;

Try this

if(c.isAssignableFrom(part.getClass())) return true;

This is different than Class.isInstance(Object), because isAssignableFrom also returns true when part is the same type as or a sub-class of c.

So if you need to know if this is the exact same type use isInstance, if it could also be a sub-class, then use isAssignableFrom.

edited May 23 '17 at 12:16

Community

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answered Jul 10 '14 at 13:03

aliteralmind

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Wont work I need to supply a subclass of `Part` in the constructor as you see in the code and this if statement to check if its the same subclass. – user36976 Jul 10 '14 at 13:04
Oh. c is a *class*. I didn't catch that. – aliteralmind Jul 10 '14 at 13:05
its `classObject.isAssignableFrom` not `object`. so it would be `c.isAssignableFrom(part.getClass())` if I'm not mistaken. – user36976 Jul 10 '14 at 13:10

Honza Zidek · Answer 3 · 2014-07-10T14:08:58.793

It's about understanding the operator instanceof and the Class class.

Type Comparison Operator instanceof:

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

This means that your code if (part instanceof c) ... will always return false, as part can never be cast to Class.

If you have a look at Class.inInstance(Object obj), you find that

This method is the dynamic equivalent of the Java language instanceof operator.

In other words:

If you know the checked type in compile-time, you use instanceof.
If you do not know the checked type in compile-time, but in run-time (so you have it stored in a variable of the Class type), you use Class.inInstance(Object obj).

Compare object with class stored in field

3 Answers3