消除未经检查的警告：将String强制转换为T

Question

Following method which returns a list with a dynamic type parameter:

public List<T> getDataList() throws SQLException {
  List<T> l = new ArrayList<T>();
  l.add((T) "Test");
  return l;
}

This gives me an unchecked cast warning.

If I change the code to:

public List<T> getDataList() throws SQLException {
  List<String> l = new ArrayList<String>();
  l.add("Test");
  (List<T>) return l;
}

it is nearly the same. I get a unchecked cast warning.

Question:

Is it possible to eliminate this unchecked warning without loosing flexibility of getDataList method?

Answer 1

public MyClass implements DataListInterface<String>

Answer 2

I think the warning is very appropriate in this situation.

Consider your method containing the generic type, its really not so generic since it would only work for a type argument of String .

public class Generic<T> {

    public List<T> getDataList() throws SQLException {
          List<T> l = new ArrayList<T>();
          l.add((T) "Test");
          return l;
        }
}

If I were to execute:

   Generic<Integer> generic = new Generic<Integer>();

A ClassCastException will be thrown appropriately since the code will attempt to cast an Integer to a String .

Answer 3

Firstly it is highly risky doing any of the above.

Doing l.add((T) "Test"); will not throw ClassCastException iff the type T is String . In that case, once could directly just return List<String> because for anything else an Exception will be thrown.

And if you are trying to protect the warning by @SuppressWarning then it is only a time-off for a bomb that will occur later. Warnings are thrown for a reason.

You can solve it by

class Whatever implements SomeInterface<String>

Answer 4

Try using it like this. (Modified your second approach)

public  <T> List<T> getDataList() throws SQLException {
          List<String> l = new ArrayList<String>();
          l.add("Test");
           return (List<T>)l;
        }