Java | 泛型实现机制

public <T> void genericMethod(T t){
  T newInstance = new T(); //Error
  Class c = T.class;    //Error
    Class c = T.class;    //Error
    List<T> list = new ArrayList<T>(); //Ok，用于其他的泛型类型可以
    if(list instanceof List<String>) //Error，非法的类型判断，因为 List<String> 不是一个真实的类型，真实的类型就是 List
  ....
}

public <T> T fromJson(Reader json, Class<T> classOfT) throws JsonSyntaxException, JsonIOException {
  JsonReader jsonReader = newJsonReader(json);
  Object object = fromJson(jsonReader, classOfT);
  assertFullConsumption(object, jsonReader);
  return Primitives.wrap(classOfT).cast(object);
}

class Demo<T>{
  public static T test(T t){}
}

List<String> strs = new ArrayList();
strs.add("hello");
String value = strs.get(0);

class Test<T> {
    public static void main(String[] args) {
        try {
            //凡是看到 Generic..Type 都是获取的泛型类型，例如 getGenericReturnType 就是获取返回泛型类型
            Type type = Test2.class.getField("list").getGenericType();
            System.out.println(type);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }
}
class Test2 extends Test<String> {
    public List<String> list = new ArrayList<>();
}

Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();
Collection<Ingeger> ints = gson.fromJson(json,collectionType);

Type type = new TypeToken<Collection<Integer>>() {
}.getType();
System.out.println(type);

java.util.Collection<java.lang.Integer>
Retrofit
interface Api{
    @GET("users/${name}")
  Call<User> getUser(@Path("user")String name)
}