我有;

List<String> stringList = new ArrayList<String>();
List<Integer> integerList = new ArrayList<Integer>();

是否有(简单)方法检索列表的泛型类型?


当前回答

通常不可能,因为List<String>和List<Integer>共享相同的运行时类。

不过,您可能能够反映包含列表的字段的声明类型(如果声明的类型本身并不引用您不知道其值的类型参数)。

其他回答

扩展Steve K的回答:

/** 
* Performs a forced cast.  
* Returns null if the collection type does not match the items in the list.
* @param data The list to cast.
* @param listType The type of list to cast to.
*/
static <T> List<? super T> castListSafe(List<?> data, Class<T> listType){
    List<T> retval = null;
    //This test could be skipped if you trust the callers, but it wouldn't be safe then.
    if(data!=null && !data.isEmpty() && listType.isInstance(data.iterator().next().getClass())) {
        @SuppressWarnings("unchecked")//It's OK, we know List<T> contains the expected type.
        List<T> foo = (List<T>)data;
        retval=foo;
    }
    return retval;
}
Usage:

protected WhateverClass add(List<?> data) {//For fluant useage
    if(data==null) || data.isEmpty(){
       throw new IllegalArgumentException("add() " + data==null?"null":"empty" 
       + " collection");
    }
    Class<?> colType = data.iterator().next().getClass();//Something
    aMethod(castListSafe(data, colType));
}

aMethod(List<Foo> foo){
   for(Foo foo: List){
      System.out.println(Foo);
   }
}

aMethod(List<Bar> bar){
   for(Bar bar: List){
      System.out.println(Bar);
   }
}

查找一个字段的泛型类型:

((Class)((ParameterizedType)field.getGenericType()).getActualTypeArguments()[0]).getSimpleName()
import org.junit.Assert;
import org.junit.Test;

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

public class GenericTypeOfCollectionTest {
    public class FormBean {
    }

    public class MyClazz {
        private List<FormBean> list = new ArrayList<FormBean>();
    }

    @Test
    public void testName() throws Exception {
        Field[] fields = MyClazz.class.getFields();
        for (Field field : fields) {
            //1. Check if field is of Collection Type
            if (Collection.class.isAssignableFrom(field.getType())) {
                //2. Get Generic type of your field
                Class fieldGenericType = getFieldGenericType(field);
                //3. Compare with <FromBean>
                Assert.assertTrue("List<FormBean>",
                  FormBean.class.isAssignableFrom(fieldGenericType));
            }
        }
    }

    //Returns generic type of any field
    public Class getFieldGenericType(Field field) {
        if (ParameterizedType.class.isAssignableFrom(field.getGenericType().getClass())) {
            ParameterizedType genericType =
             (ParameterizedType) field.getGenericType();
            return ((Class)
              (genericType.getActualTypeArguments()[0])).getSuperclass();
        }
        //Returns dummy Boolean Class to compare with ValueObject & FormBean
        return new Boolean(false).getClass();
    }
}

一个小的帮助方法:

/**
 * Get type of collection field.
 *
 * @param aClass A class containing collection.
 * @param collectionName A collection field name.
 */
@SneakyThrows
public static Class<?> getCollectionType(Class<?> aClass, String collectionName) {
  Field field = aClass.getDeclaredField(collectionName);
  ParameterizedType genericType = (ParameterizedType) field.getGenericType();
  return (Class<?>) genericType.getActualTypeArguments()[0];
}

使用反射来获取这些字段,然后你可以做:Field。genericType来获取包含generic信息的类型。