CODEWITHSUNDEEP
javagenericswildcardbounded-wildcard
Lucija
Lucija

Reputation: 5

Why List<Map<? extends Number,? extends Number>> is not working for List<Map<Integer,Double>>

I am iterating through a map whose keys are charts and values are data sets which will be displayed on charts. Data sets are lists of maps because I have multiple XYSeries displayed on each of my Charts (one series - one map with x and y values). In some charts x-axis/y-axis values are Doubles, and in others Integers. Thus, my data sets are type < ? extends Number>. What am I doing wrong? 

for (Map.Entry<Chart, List<Map<? extends Number, ? extends Number>>> entry : tcInstance.getMapChartDataset().entrySet()) {
        switch (entry.getKey().getTitle()) {
            case something:
                entry.setValue(listOfMaps1);
                break;
            case something else:
                entry.setValue(listOfMaps2);
                break;
           // other case options
         }
}

These are the declarations of lists of maps:

static List<Map<Integer, Double>> listOfMaps1 = new ArrayList<>();
static List<Map<Double, Double>> listOfMaps2 = new ArrayList<>();

I expected values to be set, but instead I got these errors which tell that the method setValue is not applicable for the arguments (List>) (and the same error for the arguments (List>).

Upvotes: 0

Views: 76

Answers (1)

Andy Turner
Andy Turner

Reputation: 140318

A List<Map<Integer,Double>> is not a List<Map<? extends Number,? extends Number>>.

If it were, I could do this:

List<Map<Integer,Double>> list = new ArrayList<>();
List<Map<? extends Number,? extends Number>> listX = list;  // Doesn't work, pretend it does.

Map<Double,Integer> map = new HashMap<>();
map.put(0.0, 0);

listX.add(map);

for (Map<Integer, Double> e : list) {
  Integer i = e.keySet().iterator().next();  // ClassCastException!
}

You would get a ClassCastException because e has a Double key, not Integer as expected.

Ideone demo

If you add an extra upper bound to the wildcarded list:

List<? extends Map<? extends Number,? extends Number>>
     ^-------^ here

then you wouldn't have been able to add map to listX, so it would be safe.

Upvotes: 3

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