Item 27: Eliminate unchecked warnings

When you program with generics, you will see many compiler warnings: unchecked cast warnings, unchecked method invocation warnings, unchecked parameterized vararg type warnings, and unchecked conversion warnings. The more experience you acquire with generics, the fewer warnings you’ll get, but don’t expect newly written code to compile cleanly.

Many unchecked warnings are easy to eliminate. For example, suppose you accidentally write this declaration:

Set<Lark> exaltation = new HashSet();

Venery.java:4: warning: [unchecked] unchecked conversion
        Set<Lark> exaltation = new HashSet();
                               ^
  required: Set<Lark>
  found:    HashSet

Set<Lark> exaltation = new HashSet<>();

 Eliminate every unchecked warning that you can. If you eliminate all warnings, you are assured that your code is typesafe, which is a very good thing. It means that you won’t get a ClassCastExceptionat runtime, and it increases your confidence that your program will behave as you intended.

If you can’t eliminate a warning, but you can prove that the code that provoked the warning is typesafe, then (and only then) suppress the warning with an@SuppressWarnings("unchecked") annotation.

Always use the SuppressWarnings annotation on the smallest scope possible. Typically this will be a variable declaration or a very short method or constructor. Never use SuppressWarnings on an entire class. Doing so could mask critical warnings.

return (T[]) Arrays.copyOf(elements, size, a.getClass());

// Adding local variable to reduce scope of @SuppressWarnings
public <T> T[] toArray(T[] a) {
    if (a.length < size) {
        // This cast is correct because the array we're creating
        // is of the same type as the one passed in, which is T[].
        @SuppressWarnings("unchecked") T[] result =
            (T[]) Arrays.copyOf(elements, size, a.getClass());
        return result;
    }
    System.arraycopy(elements, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

Every time you use a @SuppressWarnings("unchecked") annotation, add a comment saying why it is safe to do so. This will help others understand the code, and more importantly, it will decrease the odds that someone will modify the code so as to make the computation unsafe. If you find it hard to write such a comment, keep thinking. You may end up figuring out that the unchecked operation isn’t safe after all.

In summary, unchecked warnings are important. Don’t ignore them. Every unchecked warning represents the potential for a ClassCastException at runtime. Do your best to eliminate these warnings. If you can’t eliminate an unchecked warning and you can prove that the code that provoked it is typesafe, suppress the warning with a @SuppressWarnings("unchecked") annotation in the narrowest possible scope. Record the rationale for your decision to suppress the warning in a comment.