HashMap for classes, not objects

Question

I want to assign ui-Classes to a model-class each. By this I want to find the class where to store the date from the user interface. Please don't refer to the design but to my question on a HashMap's usage ;-)

I am aware of the class HashMap but only used it to assign objects to other objects.

How can I manage to link always two CLASSES with each other?

public static final HashMap<class,class> componentMap=new HashMap<class, class>();
componentMap.put(ToolPanel.class, ToolComponent.class);

The code above does not work...

Answer 1

You want a Map<Class<?>, Class<?>>.

Class here refers to java.lang.Class, which is a generified type. Unless you have more specific bounds, the unbounded wildcard <?> can be used (see Effective Java 2nd Edition, Item 23: Don't use raw types in new code)

Note that the interface Map is used here instead of a specific implementation HashMap (see Effective Java 2nd Edition, Item 52: Refer to objects by their interfaces).

Note that Map<Class<?>, Class<?>> still maps objects, but the type of those objects are now Class<?>. They are still objects nonetheless.

See also

• Java Tutorial/Generics (original dead link from Archive.org)
• Angelika Langer's Java Generics FAQ
• JLS 15.8.2 Class Literals

  A class literal is an expression consisting of the name of a class [...] followed by a . and the token class. The type of a class literal, C.class, where C is the name of a class [...] is Class<C>.

Related questions

• What is a raw type and why shouldn't we use it?

---

Imposing restrictions with bounded wilcards

Here's an example of imposing bounded wildcards to have a Map whose keys must be Class<? extends Number>, and values can be any Class<?>.

    Map<Class<? extends Number>, Class<?>> map
        = new HashMap<Class<? extends Number>, Class<?>>();

    map.put(Integer.class, String.class);        // OK!
    map.put(Long.class, StringBuilder.class);    // OK!

    map.put(String.class, Boolean.class);        // NOT OK!
    // Compilation error:
    //     The method put(Class<? extends Number>, Class<?>)
    //     in the type Map<Class<? extends Number>,Class<?>>
    //     is not applicable for the arguments (Class<String>, Class<Boolean>)

As you can see, the generic compile-time typesafety mechanism will prevent String.class from being used as a key, since String does not extends Number.

See also

• Java Tutorials/Generics/More Fun with Wildcards
• Angelika Langer's Java Generics FAQ/What is a bounded wildcard?

Answer 2

It should have been:

HashMap<Class,Class>

(capital C)

or better:

HashMap<Class<?>,Class<?>>

Answer 3

The declaration should be:

public static final HashMap<Class<?>, Class<?>> componentMap = new HashMap<Class<?>, Class<?>>();