答案可能很无聊：出现警告时，它不是类型安全的。而已。
__在此示例中可以看到 为什么 它不是类型安全的：
import java.util.HashMap;import java.util.Map;class SomeType {}class SomeSubType extends SomeType {}class SomeOtherType {}public class CastWarning{ public static void main(String[] args) { Map<SomeSubType, SomeOtherType> originalMap = new HashMap<SomeSubType, SomeOtherType>(); Map<? extends SomeType, SomeOtherType> map = originalMap; Map<SomeType, SomeOtherType> castedMap = (Map<SomeType, SomeOtherType>) map; // Valid because of the cast: The information that the // key of the map is not "SomeType" but "SomeSubType" // has been cast away... SomeType someType = new SomeType(); SomeOtherType someOtherType = new SomeOtherType(); castedMap.put(someType, someOtherType); // Valid for itself, but causes a ClassCastException // due to the unchecked cast of the map SomeSubType someSubType = originalMap.keySet().iterator().next(); }}
