Why does using a raw type with a generic method in Java lead to unexpected type behavior?

Combining Raw Types and Generic Methods: Unforeseen Consequences

When dealing with raw types and generic methods, unexpected behavior can arise. Consider the following code snippet:

ArrayList<String> a = new ArrayList<String>();
String[] s = a.toArray(new String[0]);

This code compiles successfully in both JDK 1.6 and 1.7. However, changing the ArrayList reference to a raw type results in a compiler error:

ArrayList a = new ArrayList();
String[] s = a.toArray(new String[0]); // Error: String[] expected, Object[] found

Surprisingly, the compiler interprets the generic method toArray as returning Object[] despite receiving a String[] as an argument. This is counterintuitive, as one would expect the method to conform to its type parameters.

Understanding the Behavior

The JLS 4.8 sheds light on this behavior:

"The type of a constructor, instance method, or non-static field M of a raw type C that is not inherited from its superclasses or superinterfaces is the raw type that corresponds to the erasure of its type in the generic declaration corresponding to C."

In simpler terms, when dealing with raw types, the compiler ignores any type parameters associated with methods or fields. Therefore, the type of the toArray method in the raw type ArrayList is Object[], regardless of the type parameter T.

This behavior extends beyond generic methods. Even if you use generics in other parts of the class, referring to it as a raw type effectively nullifies the use of generics altogether within that instance. For instance, this code will generate an unchecked warning:

public class MyContainer <T> {
    public List<String> strings() {
        return Arrays.asList("a", "b");
    }
}

MyContainer container = new MyContainer<>();
List<String> strings = container.strings(); // Unchecked warning

Conclusion

When dealing with raw types and generic methods, it's crucial to understand that using a raw type disables the use of generics for that particular instance. Ensure you use raw types judiciously, as they can lead to confusing and unexpected behavior.

The above is the detailed content of Why does using a raw type with a generic method in Java lead to unexpected type behavior?. For more information, please follow other related articles on the PHP Chinese website!