类型安全的将对象强制转换为Class的方法

Question

I want to remove the suppression of the unchecked conversion warning. How can I cast o to T with complete surety?

Of note: the ResultSet in this snippet is a proprietary wrapper that is source-agnostic. It's very similar to java.sql.ResultSet, but it is not the same. Also, using Eclipse Mars.1 with Java 8 u45, and both the IDE and javac issue the warning. I realize that since it's wrapped in that if statement, that it's technically not an issue, but I absolutely hate to suppress warnings. And I feel like there's gotta be a completely type-safe way to perform that conversion.

public class ResultSetQuery {
    public static <T> List<T> collectValues(ResultSet rs, String keyName, Class<T> tclass) {

        List<T> result = new LinkedList<>();

        while(rs.next()) {
            Object o = rs.getData(keyName);

            if (tclass.isAssignableFrom(o.getClass())) {

                @SuppressWarnings("unchecked")
                T v = (T)o;

                result.add(v);
            } else {
                result.add(null);
            }
        }

        return result;
    }
}

Answer 1

Your intention to not blindly silence compiler warnings is honorable but your code example is a good example for a justified use of @SupressWarnings . Your code cannot fail, and still the compiler complains, therefore @SupressWarnings is the way to go.

Take a look at java.lang.Class#cast which has the same pattern as your example. It also suppresses the warning:

@SuppressWarnings("unchecked")
public T cast(Object obj) {
    if (obj != null && !isInstance(obj))
        throw new ClassCastException(cannotCastMsg(obj));
    return (T) obj;
}

Answer 2

You can write:

            if (tclass.isAssignableFrom(o.getClass())) {
                result.add(tclass.cast(o));
            }

(See https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#cast(java.lang.Object) .)

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Edited to add: Incidentally, as erickson points out in a comment, you should probably be writing tclass.isInstance(o) instead of tclass.isAssignableFrom(o.getClass()) : it's simpler and clearer, and it avoids a NullPointerException when o is null.