Java（14 及以上）instanceof 转换为 generics（例如 List<string> )</string>

Question

I'm deserializing JSON payloads using Jackson. I have some fields that can be either Objects or Arrays of Objects. So In my Java code I'm declaring

class Outer{
    private Object inner;
}

I know that inner can be either Map<String,String> or List<Map<String,String>>

since I need to do some work with this inner object I have to reside to if - instance of

if (inner instanceof List){
    List<Map<String,String>> innerAsList = (List<Map<String,String>>)inner;
    ...
}else if (inner instanceof Map){
    Map<String,String> innerAsMap = (Map<String,String>)inner;
    ...
}else{
    throw new IllegalArgumentException("List or Map only");
}

Now trying to use the "new" pattern matching for instanceof fails to compile (I'm using JDK 17 with previews enabled)

if (inner instanceof List<Map<String,String>> innerAsList){
    ...
}else if (inner instanceof Map<String,String> innerAsMap){
    ...
}else{
    throw new IllegalArgumentException("List or Map only");
}

fails to compile

'Object' cannot be safely cast to 'List<String>'

Using wildcard cast compiles

if (inner instanceof List<?> innerAsList){
   ...
}else if (inner instanceof Map<?,?> innerAsMap){
  ...
}else{
   throw new IllegalArgumentException("List or Map only");
}

but I then I still need to cast to the correct generic.

Is this solvable somehow? Is it missing functionality?If so are there plans to have it in a future release?

Answer 1

instanceof is runtime type checking but generics uses type erasure at compile time. That's why instanceof is not allowed to use for < T > but allowed to use for the raw type List. You may check out this tutorial about generics cannot use instanceof: https://www.tutorialspoint.com/java_generics/java_generics_no_instanceof.htm

Alternatively, there are some solutions I can think of.

Approach 1: Check the type of the first list item.

if (inner instanceof List && !((List) inner).isEmpty() && ((List) inner).get(0) instanceof Map) {
    // You may continue to check map key and value if necessary.
} else if (inner instanceof Map) {
    // You may continue to check map key and value if necessary.
} else {
    throw new IllegalArgumentException("List or Map only");
}

Approach 2: Use type casting in try catch block.

try {
    Map<String, String> map = (Map<String, String>) inner;
    Map.Entry<String, String> entry = map.entrySet().iterator().next();
    String key = entry.getKey();
    String value = entry.getValue();
    System.out.println(key + " " + value);
} catch (ClassCastException e) {
    System.out.println("Object is not Map<String, String>");
}

You can use Approach 2 with Approach 1 to continue to check after instanceof Map.

Approach 3: Use a wrapper class for instanceof.

class ListWrapper extends ArrayList<Map<String, String>> { 
    //...
}

This way takes you more efforts to implement CRUD operations.