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This chapter is from the book

This chapter is from the book

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();

The compiler will gently remind you what you did wrong:

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

You can then make the indicated correction, causing the warning to disappear. Note that you don’t actually have to specify the type parameter, merely to indicate that it’s present with the diamond operator (<>), introduced in Java 7. The compiler will then infer the correct actual type parameter (in this case, Lark):

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

Some warnings will be much more difficult to eliminate. This chapter is filled with examples of such warnings. When you get warnings that require some thought, persevere! 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 ClassCastException at 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. If you suppress warnings without first proving that the code is typesafe, you are giving yourself a false sense of security. The code may compile without emitting any warnings, but it can still throw a ClassCastException at runtime. If, however, you ignore unchecked warnings that you know to be safe (instead of suppressing them), you won’t notice when a new warning crops up that represents a real problem. The new warning will get lost amidst all the false alarms that you didn’t silence.

The SuppressWarnings annotation can be used on any declaration, from an individual local variable declaration to an entire class. 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.

If you find yourself using the SuppressWarnings annotation on a method or constructor that’s more than one line long, you may be able to move it onto a local variable declaration. You may have to declare a new local variable, but it’s worth it. For example, consider this toArray method, which comes from ArrayList:

public <T> T[] toArray(T[] a) {
    if (a.length < size)
       return (T[]) Arrays.copyOf(elements, size, a.getClass());
    System.arraycopy(elements, 0, a, 0, size);
    if (a.length > size)
       a[size] = null;
    return a;

If you compile ArrayList, the method generates this warning:

ArrayList.java:305: warning: [unchecked] unchecked cast
       return (T[]) Arrays.copyOf(elements, size, a.getClass());
  required: T[]
  found:    Object[]

It is illegal to put a SuppressWarnings annotation on the return statement, because it isn’t a declaration [JLS, 9.7]. You might be tempted to put the annotation on the entire method, but don’t. Instead, declare a local variable to hold the return value and annotate its declaration, like so:

// 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;

The resulting method compiles cleanly and minimizes the scope in which unchecked warnings are suppressed.

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.

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