How to solve this Java type safety warning?

Question

Map session = ActionContext.getContext().getSession();
session.put("user", user);

This code generates a warning: Type safety: The method put(Object, Object) belongs to the raw type Map. References to generic type Map<K,V> should be parameterized.

Map<String, Serializable> session = (Map<String, Serializable>)ActionContext.getContext().getSession();
session.put("user", user);

This code generates a warning: Type safety: Unchecked cast from Map to Map<String,Serializable>.

The getSession method belongs to Struts2 so I can't modify it. I would like to avoid using @SuppressWarnings because other warnings can be useful.

I guess all Struts2 users in the world faced the same problem... is there an elegant solution?

Answer 1

The safest, most efficient way to deal with this is probably:

Map<?, ?> session = ActionContext.getContext().getSession();

and then type cast the objects retrieved from the session map.

The @SuppressWarnings approach will actually result in compiled code that is identical. However the type cast will be implicit; i.e. it won't be easy to spot by looking at the source code. And the @SuppressWarnings annotation could (hypothetically) suppress some other warning in the same code block that represents a real error; i.e. one that will result in one of the hidden typecasts, etc failing at runtime.

Other more expensive alternatives include:

• an entry by entry copy from the Map<?, ?> to a new Map<String, Serializable> instance casting the keys and values to String and Serializable respectively, or

• a generic method like the following that performs the typecast safely.

---

@SuppressWarnings("unchecked")
public <K,V> Map<K,V> castMap(Map<?, ?> map, Class<K> kClass, Class<V> vClass) {
    for (Map.Entry<?, ?> entry : map.entrySet()) {
        kClass.cast(entry.getKey());
        vClass.cast(entry.getValue());
    }
    return (Map<K,V>) map;
}

Answer 2

I don't think there's any other way but @SuppressWarnings("unchecked"). I believe you can put it just above the line in question, and it will only suppress that line.

Edit: you can also do Map<?, ?> session = ActionContext.getContext().getSession(); but I'm not sure how willing you are to do that; you won't be able to put anything into the map that way (since the compiler can't check the type of what you're putting), only read from it.