如何在没有未经检查的强制转换警告的情况下从通配符转换为通用返回类型

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.