CODEWITHSUNDEEP
javagenericscasting
Display Name
Display Name

Reputation: 359

Java generics: Declare map value of generic type

I am new to generics.

Having a Map like 

private static Map<String, Object> map;

and a method like

public <T> T getObject(final Class<T> myClass) {
    return (T)map.get(myClass);
}

How to change the map declaration in order to not have to do the cast when returning from the method ?

Upvotes: 6

Views: 39329

Answers (4)

Dimas Lanjaka
Dimas Lanjaka

Reputation: 294

my working generic java map

public class JsonUtils {
    /** gson with disable html escaping */
    public static Gson gson = new GsonBuilder().disableHtmlEscaping().create();

    public static <K, V> String toJson(Map<K, V> dataMap) {
        return gson.toJson(dataMap);
    }
}

usage

java.util.Map<String, Object> dataMap = new java.util.HashMap<>();
dataMap.put("paymentchannel", "BALANCE");
dataMap.put("shortcode", "");
dataMap.put("transtype", "cvm");
System.out.println(JsonUtils.toJson(dataMap));

Upvotes: 0

yair
yair

Reputation: 9245

If you're willing to drop the static modifier of your map, than you can do like so:

public class MyClass<T> {

    private Map<String, T> map;

    public T getObject(final Class<T> myClass) {
        return map.get(myClass);
    }
}

Otherwise:

It is a compile-time error to refer to a type parameter of a generic class C anywhere in:

  • the declaration of a static member of C

(excerpt from the JLS), which prevents you from using parameterized class to achieve the above.

What you were trying to do, however, is to refer a parameterized method's type-parameter from another member (which happen to also be static), which also unreachable.

Upvotes: 0

Pablo
Pablo

Reputation: 3673

You would need to make a generic class, not a generic method:

public class MyClass<T> {
   private Map<String, T> map;

   public T getObject(final String key) {
    return map.get(key);
   }
}

Also, I changed the parameter from a Class to a String. It doesn't make sense to pass a Class if map.get() expects a String.

Edit: I didn't notice that map was static. If you can change it to non-static without it breaking other parts of your program, this could work. If you can't, then you cannot avoid a cast.

Upvotes: 10

Grimmy
Grimmy

Reputation: 826

You can't avoid the cast operation as the get() method returns Object see here for more info

Upvotes: 0

Related Questions