how to instanceof List MyType

Question

How can I get this sort of thing to work   I can check if  obj instanceof List lt   gt   but not if  obj instanceof List lt MyType gt     Is there a way this can be done

User · Answer

The major concern here is that the collections don't keep the type in the definition. The types are only available in runtime. I came up with a function to test complex collections (it has one constraint though).

Check if the object is an instance of a generic collection. In order to represent a collection,

No classes, always false
One class, it is not a collection and returns the result of instanceof evaluation
To represent a List or Set, the type of the list comes next e.g. {List, Integer} for List<Integer>
To represent a Map, the key and value types come next e.g. {Map, String, Integer} for Map<String, Integer>

More complex use cases could be generated using the same rules. For example in order to represent List<Map<String, GenericRecord>>, it can be called as

    Map<String, Integer> map = new HashMap<>();
    map.put("S1", 1);
    map.put("S2", 2);
    List<Map<String, Integer> obj = new ArrayList<>();
    obj.add(map);
    isInstanceOfGenericCollection(obj, List.class, List.class, Map.class, String.class, GenericRecord.class);

Note that this implementation doesn't support nested types in the Map. Hence, the type of key and value should be a class and not a collection. But it shouldn't be hard to add it.

    public static boolean isInstanceOfGenericCollection(Object object, Class<?>... classes) {
        if (classes.length == 0) return false;
        if (classes.length == 1) return classes[0].isInstance(object);
        if (classes[0].equals(List.class))
            return object instanceof List && ((List<?>) object).stream().allMatch(item -> isInstanceOfGenericCollection(item, Arrays.copyOfRange(classes, 1, classes.length)));
        if (classes[0].equals(Set.class))
            return object instanceof Set && ((Set<?>) object).stream().allMatch(item -> isInstanceOfGenericCollection(item, Arrays.copyOfRange(classes, 1, classes.length)));
        if (classes[0].equals(Map.class))
            return object instanceof Map &&
                    ((Map<?, ?>) object).keySet().stream().allMatch(classes[classes.length - 2]::isInstance) &&
                    ((Map<?, ?>) object).values().stream().allMatch(classes[classes.length - 1]::isInstance);
        return false;
    }

User · Answer

You can use a fake factory to include many methods instead of using instanceof   public class Message1 implements YourInterface      List lt YourObject1 gt  list     Message1 List lt YourObject1 gt  l           list   l          public class Message2 implements YourInterface      List lt YourObject2 gt  list     Message2 List lt YourObject2 gt  l           list   l          public class FactoryMessage       public static List lt YourInterface gt  getMessage List lt YourObject1 gt  list            return  List lt YourInterface gt   new Message1 list             public static List lt YourInterface gt  getMessage List lt YourObject2 gt  list            return  List lt YourInterface gt   new Message2 list

User · Answer

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 lt Object gt  setOfIntegers   new HashSet Arrays asList 2  4  5    assetThat setOfIntegers  instanceOf new ArrayList lt Integer gt    getClass      Set lt Object gt  setOfStrings   new HashSet Arrays asList  my    name    is     assetThat setOfStrings  instanceOf new ArrayList lt String gt    getClass

User · Answer

if  list instanceof List  amp  amp    List  list  stream                                                 noneMatch  o - gt    o instanceof MyType

User · Answer

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

User · Answer

if  myList isEmpty    amp  amp  myList get 0  instanceof MyType          MyType object

User · Answer

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  lt T gt  extends ArrayList lt T gt         private Class lt T gt  genericType        public GenericList Class lt T gt  c                   this genericType   c               public Class lt T gt  getGenericType                    return genericType

User · Answer

This could be used if you want to check that object is instance of List lt T gt   which is not empty   if object instanceof List       if   List object  size   gt 0  amp  amp     List object  get 0  instanceof MyObject               The object is of List lt MyObject gt  and is not empty  Do something with it

User · Answer

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 lt Object gt  ls   new ArrayList lt  gt     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

[java] how to instanceof List<MyType>?

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