Raw Types in Java

When generics arrived in Java 5, millions of lines of existing code used plain List and Map. To keep all that code compiling, Java allowed generic types to be used without type arguments. A raw type is exactly that: List instead of List<String>. Raw types switch off generic type checking for that variable, so they bring back the runtime bugs generics were designed to remove.

What a Raw Type Allows

import java.util.ArrayList;
import java.util.List;
 
public class Raw {
    public static void main(String[] args) {
        List names = new ArrayList();
        names.add("Navika");
        names.add(42);
        for (Object o : names) {
            String s = (String) o;
            System.out.println(s.toUpperCase());
        }
    }
}

Compiling it gives only a gentle note:

Note: Raw.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

With -Xlint:unchecked, the compiler explains each risky line:

Raw.java:7: warning: [unchecked] unchecked call to add(E) as a member of the raw type List
        names.add("Navika");
                 ^
  where E is a type-variable:
    E extends Object declared in interface List

And running it crashes, far from the real mistake on line 8:

NAVIKA
Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')
    at Raw.main(Raw.java:10)

Raw vs <Object> vs <?>

These three look similar but mean different things:

If you want "a list of anything" as a parameter, use List<?>; it's as flexible as the raw type but keeps the compiler on your side.

Where Raw Names Are Fine

Some places can't take type arguments at all, so the raw name is correct there:

Common Mistakes

Forgetting <> when creating objects: new ArrayList(). Use new ArrayList<>().

Implementing raw interfaces: implements Comparable. Write implements Comparable<Student>.

Ignoring "uses unchecked or unsafe operations". Recompile with -Xlint:unchecked and fix each warning.

Practice

1. Easy: Fix Raw.java by adding type arguments. What happens to the names.add(42) line?
2. Harder: Write a method that takes a raw List and returns its size, then change it to take List<?>. Show that both accept a List<String> and a List<Integer>.