Replace the nullable annotations with guava's Optional class.

从 com.sk89q.intake.argument.Namespace 开始，结合链接的 Guava Optional 文档检查其 get(Object) 和 get(Class) 方法。在项目中搜索 nullable 注解，并确定哪些 API 需要替换；当 nullable 注解已一致地替换为使用 Optional，且受影响的行为仍由现有测试覆盖时，即视为完成。

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));
    }