原文链接:https://www.cnblogs.com/yuyutianxia/p/6051682.html
1. 泛型通常用于集合,如Set和Map等。这样的用法也就限制了每个容器只能有固定数目的类型参数,一般来说,这也确实是我们想要的。
然而有的时候我们需要更多的灵活性,如数据库可以用任意多的Column,如果能以类型安全的方式访问所有Columns就好了,幸运的是
有一种方法可以很容易的做到这一点,就是将key进行参数化,见以下代码
public class Favorites {
private Map<Class<?>, Object> favorites = new HashMap<Class<?>, Object>();
public <T> void setFavorite(Class<T> klass, T thing) {
favorites.put(klass, thing);
}
public <T> T getFavorite(Class<T> klass) {
return klass.cast(favorites.get(klass));
}
public static void main(String[] args) {
Favorites f = new Favorites();
f.setFavorite(String.class, "Java");
f.setFavorite(Integer.class, 0xcafebabe);
String s = f.getFavorite(String.class);
int i = f.getFavorite(Integer.class);
}
}
2.不足之处
There is a limitation to this pattern.
//You can't add your favorite List<String> to a Favorites because you simply can't make a type token for a generic type.
f.setFavorite(List<String>.class, Collections.emptyList());
3.改进
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;
/**
* References a generic type.
*
* @author [email protected] (Bob Lee)
*/
public abstract class TypeReference<T> {
private final Type type;
private volatile Constructor<?> constructor;
protected TypeReference() {
Type superclass = getClass().getGenericSuperclass();
if (superclass instanceof Class) {
throw new RuntimeException("Missing type parameter.");
}
this.type = ((ParameterizedType) superclass).getActualTypeArguments()[0];
}
/**
* Instantiates a new instance of {@code T} using the default, no-arg
* constructor.
*/
@SuppressWarnings("unchecked")
public T newInstance()
throws NoSuchMethodException, IllegalAccessException,
InvocationTargetException, InstantiationException {
if (constructor == null) {
Class<?> rawType = type instanceof Class<?>
? (Class<?>) type
: (Class<?>) ((ParameterizedType) type).getRawType();
constructor = rawType.getConstructor();
}
return (T) constructor.newInstance();
}
/**
* Gets the referenced type.
*/
public Type getType() {
return this.type;
}
public static void main(String[] args) throws Exception {
List<String> l1 = new TypeReference<ArrayList<String>>() {}.newInstance();
List l2 = new TypeReference<ArrayList>() {}.newInstance();
}
}
参考:
- com.google.common.reflect.TypeToken<T>
- com.fasterxml.jackson.core.type.TypeReference
扫描二维码关注公众号,回复:
4129912 查看本文章