What is a raw type and why shouldn't we use it?

What is a raw type?

The Java Language Specification defines a raw type as follows:

JLS 4.8 Raw Types

A raw type is defined to be one of: The reference type that is formed by taking the name of a generic type declaration without an accompanying type argument list. An array type whose element type is a raw type. A non-static member type of a raw type R that is not inherited from a superclass or superinterface of R.

Here's an example to illustrate:

public class MyType<E> {
    class Inner { }
    static class Nested { }
    
    public static void main(String[] args) {
        MyType mt;          // warning: MyType is a raw type
        MyType.Inner inn;   // warning: MyType.Inner is a raw type

        MyType.Nested nest; // no warning: not parameterized type
        MyType<Object> mt1; // no warning: type parameter given
        MyType<?> mt2;      // no warning: type parameter given (wildcard OK!)
    }
}

Here, MyType<E> is a parameterized type (JLS 4.5). It is common to colloquially refer to this type as simply MyType for short, but technically the name is MyType<E>.

mt has a raw type (and generates a compilation warning) by the first bullet point in the above definition; inn also has a raw type by the third bullet point.

MyType.Nested is not a parameterized type, even though it's a member type of a parameterized type MyType<E>, because it's static.

mt1, and mt2 are both declared with actual type parameters, so they're not raw types.

What's so special about raw types?

Essentially, raw types behaves just like they were before generics were introduced. That is, the following is entirely legal at compile-time.

List names = new ArrayList(); // warning: raw type!
names.add("John");
names.add("Mary");
names.add(Boolean.FALSE); // not a compilation error!

The above code runs just fine, but suppose you also have the following:

for (Object o : names) {
    String name = (String) o;
    System.out.println(name);
} // throws ClassCastException!
  //    java.lang.Boolean cannot be cast to java.lang.String

Now we run into trouble at run-time, because names contains something that isn't an instanceof String.

Presumably, if you want names to contain only String, you could perhaps still use a raw type and manually check every add yourself, and then manually cast to String every item from names. Even better, though is NOT to use a raw type and let the compiler do all the work for you, harnessing the power of Java generics.

List<String> names = new ArrayList<String>();
names.add("John");
names.add("Mary");
names.add(Boolean.FALSE); // compilation error!

Of course, if you DO want names to allow a Boolean, then you can declare it as List<Object> names, and the above code would compile.

See also

Java Tutorials/Generics

How's a raw type different from using