title: Convert Json subobject to a HashMap with Jackson
body: <p>I would like to convert the <code>bpi</code> field of the following json string:</p>
  
  <pre><code>{
      "bpi": {
          "2017-10-15": 5697.3917,
          "2017-10-16": 5754.2213,
          "2017-10-17": 5595.235,
          "2017-10-18": 5572.1988,
          "2017-10-19": 5699.5838,
          "2017-10-20": 5984.0863,
          "2017-10-21": 6013.2288,
          "2017-10-22": 5984.9563,
          "2017-10-23": 5895.2988,
          "2017-10-24": 5518.85,
          "2017-10-25": 5733.9038,
          "2017-10-26": 5888.145,
          "2017-10-27": 5767.68,
          "2017-10-28": 5732.825,
          "2017-10-29": 6140.5313,
          "2017-10-30": 6121.8,
          "2017-10-31": 6447.6675
      },
          "disclaimer": "This data was produced from the CoinDesk Bitcoin Price Index. BPI value data returned as USD.",
          "time": {
          "updated": "Nov 1, 2017 13:49:57 UTC",
          "updatedISO": "2017-11-01T13:49:57+00:00"
      }
  }
  </code></pre>
  
  <p>into a hashmap - preferably <code>Map&lt;Date, Double&gt;</code> but <code>Map&lt;String, String&gt;</code> will do as well (then I would have to process it later on). I read somewhere how to convert json arrays to java arrays but since the desired array is wrapped in the  <code>bpi</code> sub field I dont know how to access it. Could you please help me with that?</p>
  
  <p>Thanks a lot</p>