code: public String getString( String key ) {
        verifyIsNull();
        Object o = get( key );
        if( o != null ){
           return o.toString();
        }
        throw new JSONException( "JSONObject[" + JSONUtils.quote( key ) + "] not found." );
     }
docs: Get the string associated with a key.
  
  @param key A key string.
  @return A string which is the value.
  @throws JSONException if the key is not found.