how to instanceof List?

问题

How can I get this sort of thing to work? I can check if (obj instanceof List<?>) but not if (obj instanceof List<MyType>). Is there a way this can be done?

回答1:

That is not possible because the datatype erasure at compile time of generics. Only possible way of doing this is to write some kind of wrapper that holds which type the list holds:

public class GenericList <T> extends ArrayList<T>
{
     private Class<T> genericType;

     public GenericList(Class<T> c)
     {
          this.genericType = c;
     }

     public Class<T> getGenericType()
     {
          return genericType;
     }
}

回答2:

if(!myList.isEmpty() && myList.get(0) instanceof MyType){
    // MyType object
}

回答3:

You probably need to use reflection to get the types of them to check. To get the type of the List: Get generic type of java.util.List

回答4:

This could be used if you want to check that object is instance of List<T>, which is not empty:

if(object instanceof List){
    if(((List)object).size()>0 && (((List)object).get(0) instanceof MyObject)){
        // The object is of List<MyObject> and is not empty. Do something with it.
    }
}

回答5:

If you are verifying if a reference of a List or Map value of Object is an instance of a Collection, just create an instance of required List and get its class...

Set<Object> setOfIntegers = new HashSet(Arrays.asList(2, 4, 5));
assetThat(setOfIntegers).instanceOf(new ArrayList<Integer>().getClass());

Set<Object> setOfStrings = new HashSet(Arrays.asList("my", "name", "is"));
assetThat(setOfStrings).instanceOf(new ArrayList<String>().getClass());

回答6:

If this can't be wrapped with generics (@Martijn's answer) it's better to pass it without casting to avoid redundant list iteration (checking the first element's type guarantees nothing). We can cast each element in the piece of code where we iterate the list.

Object attVal = jsonMap.get("attName");
List<Object> ls = new ArrayList<>();
if (attVal instanceof List) {
    ls.addAll((List) attVal);
} else {
    ls.add(attVal);
}

// far, far away ;)
for (Object item : ls) {
    if (item instanceof String) {
        System.out.println(item);
    } else {
        throw new RuntimeException("Wrong class ("+item .getClass()+") of "+item );
    }
}

回答7:

You can use a fake factory to include many methods instead of using instanceof:

public class Message1 implements YourInterface {
   List<YourObject1> list;
   Message1(List<YourObject1> l) {
       list = l;
   }
}

public class Message2 implements YourInterface {
   List<YourObject2> list;
   Message2(List<YourObject2> l) {
       list = l;
   }
}

public class FactoryMessage {
    public static List<YourInterface> getMessage(List<YourObject1> list) {
        return (List<YourInterface>) new Message1(list);
    }
    public static List<YourInterface> getMessage(List<YourObject2> list) {
        return (List<YourInterface>) new Message2(list);
    }
}

回答8:

    if (list instanceof List && ((List) list).stream()
                                             .noneMatch((o -> !(o instanceof MyType)))) {}

来源：https://stackoverflow.com/questions/10108122/how-to-instanceof-listmytype

标签

java

generics

how to instanceof List<MyType>?

问题