How to go from wildcard to generic return type without unchecked cast warning

Question

The following code is producing an Unchecked cast warning because of the cast of (GenericKeyedObjectPool<String, T>) .

As can be seen in the code, the wildcard ( ? extends ConnectionBase ) and generic type ( T extends ConnectionBase ) are the same.

How can I get rid of the Unchecked cast warning and convert without warnings from the wildcard to the generic type?

private Map<String, GenericKeyedObjectPool<String, ? extends ConnectionBase>> poolMap 
     = new HashMap<>();

public <T extends ConnectionBase> T borrowObject(
        String guid, 
        Class<T> connClass) 
        throws Exception {
    GenericKeyedObjectPool<String, T> pool 
        = (GenericKeyedObjectPool<String, T>) poolMap.get(connClass.getName());
    return pool.borrowObject(guid);
}

Answer 1

The cast is unsafe.

if A isConnectionBase

B extends A

C extends A

B and C are not safe to cast between.

To make the cast safe, you need to make it redundant by specifying the type of T at the class level to enforce it is same class throughout the class.