Java HashMap 用于类，而不是对象
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HashMap for classes, not objects
提问by kerrl
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 ;-)
我想将 ui-Classes 分配给每个模型类。通过这个，我想从用户界面找到存储日期的类。请不要参考设计，而是参考我关于 aHashMap用法的问题；-)
I am aware of the class HashMapbut only used it to assign objects to other objects.
我知道这个类，HashMap但只用它来将对象分配给其他对象。
How can I manage to link always two CLASSES with each other?
我怎样才能设法将两个类始终相互链接？
public static final HashMap<class,class> componentMap=new HashMap<class, class>();
componentMap.put(ToolPanel.class, ToolComponent.class);
The code above does not work...
上面的代码不起作用...
回答by Eyal Schneider
It should have been:
本来应该是：
HashMap<Class,Class>
(capital C)
（大写 C）
or better:
或更好：
HashMap<Class<?>,Class<?>>
回答by polygenelubricants
You want a Map<Class<?>, Class<?>>.
你想要一个Map<Class<?>, Class<?>>.
Classhere 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)
Class这里指的java.lang.Class是泛型类型。除非您有更具体的界限，否则<?>可以使用无界通配符（参见Effective Java 2nd Edition，Item 23：Don't use raw types in new code）
Note that the interface Mapis used here instead of a specific implementation HashMap(see Effective Java 2nd Edition, Item 52: Refer to objects by their interfaces).
请注意，interface Map此处使用的是 而不是特定的实现HashMap（参见Effective Java 2nd Edition，Item 52：通过其接口引用对象）。
Note that Map<Class<?>, Class<?>>still maps objects, but the type of those objects are now Class<?>. They are still objects nonetheless.
请注意，Map<Class<?>, Class<?>>仍然映射对象，但这些对象的类型现在是Class<?>. 尽管如此，它们仍然是对象。
See also
也可以看看
- 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 tokenclass. The type of a class literal,C.class, whereCis the name of aclass[...] isClass<C>.
- Java 教程/泛型（来自 Archive.org 的原始死链接）
- Angelika Langer 的 Java 泛型常见问题解答
- JLS 15.8.2 类文字
类文字是由
class[...]的名称后跟 a.和标记组成的表达式class。类文字的类型，C.class，其中[...]C的名称class是Class<C>。
Related questions
相关问题
Imposing restrictions with bounded wilcards
使用有界通配符施加限制
Here's an example of imposing bounded wildcards to have a Mapwhose keys must be Class<? extends Number>, and values can be any Class<?>.
这是一个强加有界通配符的示例，Map其键必须是Class<? extends Number>，值可以是 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.classfrom being used as a key, since Stringdoes not extends Number.
正如你所看到的，通用的编译时类型安全机制将阻止String.class被用作一个关键，因为String没有extends Number。
See also
也可以看看
回答by Sergii Pozharov
The declaration should be:
声明应为：
public static final HashMap<Class<?>, Class<?>> componentMap = new HashMap<Class<?>, Class<?>>();
