Classes (not objects) in a HashMap

Question

I am storing Classes (not objects) in a HashMap...

But I don't understand if it will work correctly... because I can't override equals() or hashCode(), right? I mean, the HashMap must compare the 2 Classes, not the 2 Objects...

So, will it work simply putting Classes into a HashMap?

or will it cause problems?

protected HashMap<Class<? extends MyBaseClass>, int> someTable = new HashMap<>();

What have you tried and what was the result? As you did in school... please show your work. :) It's part of the process of getting questions answered on SO. It's helpful to you because it forces you to investigate your own problem and think it through. It also proves to readers that you did your homework and made a reasonable attempt to answer your own question. Thirdly, it helps readers find and diagnose the problem resulting in a better answer for you and less time wasted for us. — JeffC
– JeffC, Commented Sep 20, 2015 at 2:52
I need the HashMap to keep track of a bunch of Classes, which I later use to figure out what objects to create from these Classes. The number of objects of each type of class that will be created dynamically, varies - so that's what the HashMap is for - to track how many I will need of each Class. Also, the Classes can be different but all descend from the same "root" Class. Think of it like the HashMap stores only Cars, but they can be BMW, Ford, Toyota models etc.... and this is a template for production... that we are storing say, 3 BMW, 5 Ford and 10 Toyotas to dynamically create later. — ycomp
– ycomp, Commented Sep 20, 2015 at 2:54
It will work. You can safely use Classes as keys in a HashMap. — Mick Mnemonic
– Mick Mnemonic, Commented Sep 20, 2015 at 2:57

Community · Accepted Answer · 2017-05-23 11:58:09Z

1

It will work; you can safely use Class objects as keys in a HashMap. The values returned by the getClass() method for different instances of a specific class will refer to the same Class instance. The equals and hashCode methods of Class are inherited directly from Object (== equality and a native hash code implementation).

One thing to note is that depending on the lifetime of the Map, there's a chance for a memory leak due to the way that classes contain a reference to their class loaders. Usually, you don't need to worry about this, however.

edited May 23, 2017 at 11:58

CommunityBot

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answered Sep 20, 2015 at 3:07

Mick Mnemonic

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2 Comments

yas Over a year ago

would it work better to store the name of the class as a String, e.g. "com.example.MyClass"? You can obtain the Class object with Class.forName

Stephen C Over a year ago

@dave - It depends. If you have a complicated scenario with multiple classloaders, class names may not be unique. Class objects will work properly as keys in that scenario too.

yelliver · Accepted Answer · 2015-09-20 03:46:13Z

0

You said you did not understand. Because your question is WRONG

I am storing Classes (not objects) in a HashMap...

Classes which you store are definitely Objects. They are instances of java.lang.Class class

answered Sep 20, 2015 at 3:46

yelliver

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