TreeMap ClassCastException

Question

Cannot figure out where the String casting is coming from that is causing this ClassCastException. I've cleared out the map so that it only holds a single entry (115,1563) and I ensured both parameters were integers.

First I read from a file and populate the scoreMap.

private void populateScoreMap(String toConvert)
{
    Gson gson = new GsonBuilder().create();

    ScoreRecord.scoreMap = (TreeMap<Integer,Integer>) gson.fromJson(toConvert, ScoreRecord.scoreMap.getClass());

}

ScoreRecord class

public class ScoreRecord
{
    public static SortedMap<Integer,Integer> scoreMap = new TreeMap<Integer, Integer>();
}

Then I try to add an entry in the ScoreGraph class

private void addTodaysScore() {
    Integer todaysScore = getIntent().getIntExtra("score",0);
    Calendar calendar = Calendar.getInstance();
    Integer dayOfYear = calendar.get(Calendar.DAY_OF_YEAR);
    ScoreRecord.scoreMap.put(dayOfYear,todaysScore);
    }

Exception

Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
at java.lang.Integer.compareTo(Integer.java:1044)
at java.util.TreeMap.put(TreeMap.java:593)
at com.xxx.xxx.xxx.xxx.ScoreGraph.addTodaysScore(ScoreGraph.java:63)

Answer 1

The problem is that the result of ScoreRecord.scoreMap.getClass() is a Java class which does not contain the information relating to generics. In your specific case, it is just SortedMap which is equivalent to SortedMap<Object, Object> rather than SortedMap<Integer, Integer>.

What you need to do is to create what Gson calls a »type token«. This will give Gson the required hints to be able to parse your collection successfully:

private void populateScoreMap(String toConvert)
{
    Gson gson = new GsonBuilder().create();
    Type collectionType = new TypeToken<SortedMap<Integer, Integer>>(){}.getType();

    ScoreRecord.scoreMap = gson.fromJson(toConvert, collectionType);
}

This is also explained in Gson's documentation