如何在Java中将泛型列表类作为参数传递给泛型方法？

Question

I want to have a method which would return required type of objects extracted from a map. Here is an example

Map<String,Object> map = new HashMap<>();
map.put("ids", Arrays.asList(5,345,45645)); 
map.put("id", 325); 

<T> T get(String arg, Class<T> clazz) {
    return (T) map.get(arg);
}

Then, for example, Integer can be gotten with this code

Integer id = get("id", Integer.class);

But when I'm trying to get a list of objects in these ways

List<Integer> ids = get("ids", new ArrayList<Integer>().getClass());
List<Integer> ids = get("id", List.class);

Intellij IDEA warns: Unchecked assignment: 'java.util.ArrayList' to 'java.util.List<java.lang.Integer>'

I expected something like List<Integer>.class (by analogy to Integer.class , Boolean.class etc...) what would be used as

List<Integer> ids = get("ids", List<Integer>.class`)

but this is not compiled.

So is there a solution?

Answer 1

You don't actually need the Class parameter.

<T> T unsafeGet(String arg) {
    return (T) map.get(arg);
}

I renamed the method, because you're throwing away all compiler type-checking .

These two lines compile, but fail at runtime:

Integer ids = unsafeGet("ids"); // java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.lang.Integer
Element id = unsafeGet("id");   // java.lang.ClassCastException: java.lang.Integer cannot be cast to org.w3c.dom.Element