Replace the nullable annotations with guava's Optional class.

com.sk89q.intake.argument.Namespace から始め、リンクされている Guava Optional のドキュメントと併せて、その get(Object) および get(Class) メソッドを確認します。プロジェクト内で nullable アノテーションを検索し、どの API を置き換える必要があるかを判断します。nullable アノテーションが一貫して Optional の使用に置き換えられ、影響を受ける動作が既存のテストで引き続きカバーされていれば完了です。

What's the point of Guava's Optional class?

Probably the single biggest disadvantage of null is that it's not obvious what it should mean in any given context: it doesn't have an illustrative name. It's not always obvious that null means "no value for this parameter" -- heck, as a return value, sometimes it means "error", or even "success" (!!), or simply "the correct answer is nothing". Optional is frequently the concept you actually mean when you make a variable nullable, but not always.

more: http://stackoverflow.com/questions/9561295/whats-the-point-of-guavas-optional-class

Optional class: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Optional.html

Example in com.sk89q.intake.argument.Namespace

    /**
     * Returns the value specified by the given key.
     *
     * @param key The key
     * @return The value, which may be null, including when the key doesn't exist
     */
    public Optional<Object> get(Object key) {
        if (!locals.containsKey(key)) Optional.absent();
        return Optional.of(locals.get(key));
    }

    /**
     * Get an object whose key will be the object's class.
     *
     * @param key The key
     * @param <T> The type of object
     * @return The value
     */
    @SuppressWarnings("unchecked")
    public <T> Optional<T> get(Class<T> key) {
        if (!locals.containsKey(key)) Optional.absent();
        return Optional.of((T)locals.get(key));
    }