Replace the nullable annotations with guava's Optional class.

### 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`

``` java
/**
* 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 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 The type of object
* @return The value
*/
@SuppressWarnings("unchecked")
public Optional get(Class key) {
if (!locals.containsKey(key)) Optional.absent();
return Optional.of((T)locals.get(key));
}
```

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。

索引モデルが issue の本文から書いたものです。