How to get all key and values from nested JSON in java

Hi I need to read all key, values from nested JSON, where ever there is inner JSON. I need that values ignoring the key.From below JSON i need Key values for nested JSON, like: responseStatus-passed, "statusCode":"200", "retrieveQuoteResponse":null,"quoteGuid":null, etc.ignoring the start key value like: responsePreamble, quoteProductList which has a nested json inside it.

{
    "responsePreamble": {
        "responseStatus": "Passed",
        "statusCode": "200",
        "responseMessage": "Records Found"
    },
    "retrieveQuoteResponse": null,
    "totalQuoteProductCount": 2,
    "quoteProductList": {
        "quoteGuid": null,
        "quantity": 180
}

Code:

ObjectReader reader = new ObjectMapper().readerFor(Map.class); 
Map<String, Map<String, String>> employeeMap = reader.readValue(jsonObject); 
for (Entry<String, Map<String, String>> empMap : employeeMap.entrySet()) { 
    Map<String, String> addMap = empMap.getValue(); 
    if(addMap!=null) { 
        for (Entry<String, String> addressSet : addMap.entrySet()) {
            System.out.println(addressSet.getKey() + " :: " + addressSet.getValue()); 
        } 
    } 
}

OutPut:

responseStatus :: Passed
statusCode :: 200
responseMessage :: Records Found
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.util.Map
    at com.im.api.tests.CompareTwoJsons.main(CompareTwoJsons.java:78)