Check which type of object List<?> contains

问题

List contains the object type, but I need to check if that object is of type A or B:

A a = new A();
B b = new B();

List<A> aL = new ArrayList<A>();
List<B> bL = new ArrayList<B>(); 

How can I check whether List contains A objects or B objects?

Here is the code:

    SegmentDetailInfo segmentDetailInfo  = new SegmentDetailInfo();
    segmentDetailInfo.setSeg_Id("1");

    SegReqInfoBean segReqInfoBean  = new SegReqInfoBean();
    segReqInfoBean.setPageName("homepage");

    List<SegmentDetailInfo> rhsList1 = new ArrayList<SegmentDetailInfo>();
    rhsList1.add(segmentDetailInfo);

    List<SegReqInfoBean> rhsList2 = new ArrayList<SegReqInfoBean>();
    rhsList2.add(segReqInfoBean);

    doProspecListCompareCheck(rhsList1);
    doProspecListCompareCheck(rhsList2);

}

private static void doProspecListCompareCheck(Object rhsList) {
     if (rhsList instanceof List<SegmentDetailInfo>) //wrong Check
         //DoTHIS
     else if(rhsList instanceof List<SegReqInfoBean>)   //wrong Check       
         //Do THIS       

}

========================================================

    SegmentDetailInfo segmentDetailInfo  = new SegmentDetailInfo();
    segmentDetailInfo.setSeg_Id("1");

    SegReqInfoBean segReqInfoBean1  = new SegReqInfoBean();
    segReqInfoBean1.setPageName("Home");

    List<SegmentDetailInfo> rhsList1 = new ArrayList<SegmentDetailInfo>();
    rhsList1.add(segmentDetailInfo);

    List<SegReqInfoBean> rhsList2 = new ArrayList<SegReqInfoBean>();      
    rhsList2.add(segReqInfoBean1);

    String Homepage="homepage";

    doProspecListCompareCheck(Homepage);
    doProspecListCompareCheck(rhsList2);
    doProspecListCompareCheck(rhsList2);


private static void doProspecListCompareCheck(Object rhsListObj) {
         List<String> rhsStrList = new ArrayList<String>();
         List<SegReqInfoBean> segReqInfoBeanList = new ArrayList<SegReqInfoBean>();
         List<SegmentDetailInfo> segmentDetailInfoList = new ArrayList<SegmentDetailInfo>();


     if (rhsListObj != null && rhsListObj instanceof List) {

         if (((List<SegmentDetailInfo>) rhsListObj).get(0) instanceof SegmentDetailInfo){

                System.out.println("SegmentDetailInfo loading");
                segmentDetailInfoList = (List<SegmentDetailInfo>) rhsListObj;     
        }
         else if(((List<SegReqInfoBean>) rhsListObj).get(0) instanceof SegReqInfoBean){

                System.out.println("SegReqInfoBean loading");   
                segReqInfoBeanList = (List<SegReqInfoBean>) rhsListObj;
         }          

     }else if ( rhsListObj != null && rhsListObj instanceof String) {

         rhsStrList.add(rhsListObj.toString());

     }

}

回答1:

Generics only provide compile-time checks. At runtime, they are completely gone. This is known as type erasure. So at runtime, your code looks like this:

    List rhsList1 = new ArrayList();
    rhsList1.add(segmentDetailInfo);

    List rhsList2 = new ArrayList();
    rhsList2.add(segReqInfoBean);

    doProspecListCompareCheck(rhsList1);
    doProspecListCompareCheck(rhsList2);

}
private static void doProspecListCompareCheck(Object rhsList) {


     if (rhsList instanceof List) //wrong Check
         //DoTHIS
     else if(rhsList instanceof List)   //wrong Check       
         //Do THIS       

}

Distinguishing two generic objects by their generic parameter is simply not something you can do in Java.

回答2:

One way you can do it is by comparing first Object inside the List :

private static void doProspecListCompareCheck(List rhsList)
{
    if(rhsList != null && !rhsList.isEmpty())
    {
        if (rhsList.get(0) instanceof SegReqInfoBean)
        {

        }
        else if(rhsList.get(0) instanceof SegmentDetailInfo)    
        {

        }
    }
}

回答3:

If you know the list is non-empty you can do rhsList.get(0) instanceof SegReqInfobean

If the list may be empty you could start by inserting an object of the correct type and then remember that index 0 stores a dummy object, so remove it before processing (or just start processing the list at index 0). Generics are just a compile time convenience. You can't use the generic type at runtime as you discovered.

回答4:

for (Object aList : list) {
    Class cls = aList.getClass();
    System.out.println("The type of the object is: " + cls.getName());
}

This seems to work for me. This will not only detect if any index is of type A or B, but detects every type.

来源：https://stackoverflow.com/questions/16745355/check-which-type-of-object-list-contains