code: public ContextPair getPair(String key) {
          if (!this.contains(key)) {
              return null;
          }
          return new ContextPair(key, get(key));
      }
docs: Obtains a pair based on the given key. In there is no key, this method
  returns null
  
  @param key The key
  @return the ContextPair for the given key.