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up vote 3 down vote favorite

I am using ObjectMapper to convert a JSON String to a Map but I get ClassCastException when trying to retrieve a value:

Map<String,String[]> args = objectMapper.readValue(jsonString, HashMap.class);
String[] array = args.get(paramName);

then doing

String x = array[0];

gives

java.lang.ClassCastException: java.util.ArrayList cannot be cast to [Ljava.lang.String

How can this be when I clearly specify String[] and not ArrayList, and when the second line doesn't cause any errors...

share|improve this question
    
ArrayList is Java's implementation of a dynamic array (so therefore has an underlying String[] to store values) but I doubt that's what you're running into here. –  Roddy of the Frozen Peas Aug 21 '12 at 19:09
2  
The actual type is erased at runtime. I think yoru objectMapper returns a Map<String,ArrayList<String>>, not Map<String,String[]>. –  dasblinkenlight Aug 21 '12 at 19:10
    
@dasblinkenlight shouldn't that launch the ClassCastException at the previous line (String[] array = args.get(paramName);) –  SJuan76 Aug 21 '12 at 19:15
    
@SJuan76 Not necessarily, because the info of the type parameters is not available at runtime. –  dasblinkenlight Aug 21 '12 at 19:19

3 Answers 3

up vote 2 down vote

HashMap.class is the only information available to objectMapper at runtime.

It doesn't know that you declared a Map<String, String[]> on the left-hand side, so it does what most JSON mappers would do: It assumes you want a 1:1 map-based representation of the JSON which results in a Map<String, ArrayList<String>>.

Just use

List<String> list = args.get(paramName);
String x = list.get(0);

instead of String[] array and array[0].

Better yet, map your JSON into actual model classes, not a HashMap.

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i cannot change the type of args on the first line: Map<String, String[]>.. then when doing List<String> list = args.get(param) i expectedly get 'cannot convert from String[] to List'... how can i reconcile this –  mathematician Aug 21 '12 at 19:35
    
If you System.out.println(args) right after objectMapper.readValue(...), what does it print? –  Philipp Reichart Aug 21 '12 at 21:28
up vote 0 down vote

The object mapper does not obey (in fact cannot) the parameterized type information you used. It only knows about the class you are passing in as an argument. You'd be much better off using the generic type.

What you are actually getting back is:

Map<Object, Object> args = objectMapper.readValue(jsonString, HashMap.class);

I recommend changing your code to use something that for your arguments and then doing class casts and checking accordingly.

For example:

Map<Object, Object> args = new LinkedHashMap<Object, Object>();
Object value = args.get("foo");
String x;
if (value instanceof String[]) {
    String[] array = (String[])value;
    x = array[0];
} else if(value instanceof List) {
    List<Object> list = (List<Object>) value;
    x = String.valueOf(list.get(0));
} else {
    //error handling
}
...
share|improve this answer
up vote 0 down vote

Your diagnosis is incorrect. The line that throws the ClassCastException is

String[] array = args.get(paramName);

and not the later array access. The reason for the error to occur at this line is, I believe, obvious: the map value simply isn't an array, but is in fact an ArrayList.

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