如何从Java中的泛型列表中获取参数类型？

Question

I have code like this :

List<TRt> listTrt;
List<TRtKuesioner> listTrtKuesioner;
List<TArtKuesioner> listTArtKuesioner;
Object[] objects = new Object[] { 
    listTrt, listTrtKuesioner,listTArtKuesioner 
};

How can I do function like my wish below :

for(Object o :objects){
    if(o instanceof List<TRt>){

    }else if(o instanceof List<TRtKuesioner>){


    }else if(o instanceof List<TArtKuesioner>){


    }
}

How I can accomplish this ?

Answer 1

The type arguments of a generic type are not available at runtime due to a process known as type erasure. In a nutshell, type erasure is the process by which the compiler removes type arguments and replaces the with casts where appropriate.

See: http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

Answer 2

Provided that the lists are non-empty, you could simulate it with

if(((List)o).get(0) instanceof TRt)

but a better idea would be to try to avoid the Object array completely. What are you trying to do?

Answer 3

you could simulate with isAssignableFrom , to check first element and then cast the whole List.

ex :

public static void main(String[] args) throws Exception {
        List<String> strings = new ArrayList<String>();
        strings.add("jora string");
        List<Integer> ints = new ArrayList<Integer>();
        ints.add(345);
        Object[] objs = new Object[]{strings,ints};
        for (Object obj : objs){
            if (isListInstanceOf(obj, String.class)){
                List<String> strs = castListTo(obj, String.class);
                for (String str : strs){
                    System.out.println(str);
                }
            }else if (isListInstanceOf(obj, Integer.class)){
                List<Integer> inList = castListTo(obj, Integer.class);
                for (Integer integ : inList){
                    System.out.println("Int: "+integ);
                }
            }
        }
    }


    public static boolean isListInstanceOf(Object list, Class clazz){
        return (list instanceof List && ((List)list).size() > 0) ? ((List)list).get(0).getClass().isAssignableFrom(clazz) : false;
    }

    public static <T> List<T> castListTo(Object list, Class<T> clazz){
        try{
            return (List<T>)list;
        }catch(ClassCastException exc){
            System.out.println("can't cast to that type list");
            return null;
        }
    }