Java generics assignability

Question

I am struggling a bit on assign-ability of generic type parameters.

Here are how my type definitions look:

public static interface CellValue<T> {
    T getValue();
    Class<?> getType();
}

public static class DoubleCell implements CellValue<Double> {
    Double value;

    public DoubleCell(Double value) {
        super();
        this.value = value;
    }

    @Override
    public Double getValue() {
        return value;
    }

    @Override
    public Class<?> getType() {
        return Double.class;
    }
}

public static class FormulaCell implements CellValue<String> {
    String value;

    public FormulaCell(String value) {
        this.value = value;
    }

    @Override
    public String getValue() {
        return value;
    }

    @Override
    public Class<?> getType() {
        return String.class;
    }
}

Now, why do the following declarations work (to me, this is expected behaviour, but I may have misunderstood why it works)

Map<String, ? extends CellValue<?>> m = new HashMap<String, FormulaCell>();
Map<String, ? extends CellValue<?>> m2 = new HashMap<String, DoubleCell>();

..while the following do not?

Map<Integer, Map<String, ? extends CellValue<?>>> n = new HashMap<Integer, Map<String, FormulaCell>>();
Map<Integer, Map<String, ? extends CellValue<?>>> n2 = new HashMap<Integer, Map<String, DoubleCell>>();

Would [this](https://stackoverflow.com/questions/19219635/java-hashmap-nested-generics-with-wildcards) be helpful? — Michał Krzywański, Dec 03 '19 at 13:12
Thanks! This was helpful. Nested wildcards are something I'll try to stay away from. — DebD, Dec 04 '19 at 06:14

score -2 · Answer 1 · answered Dec 03 '19 at 12:50

-2

Try Map<Integer,? extendsMap<String, ? extends CellValue<?>>> instead.

answered Dec 03 '19 at 12:50

rzwitserloot

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Java generics assignability

1 Answers1