Question

In short I’m trying to write an AbstractMap implementation and I encountered a syntax problem that I do not understand.

Consider the following two sketches (the identifiers with My before them are custom types).

---

(1) Not Working:

public class CustomMap extends AbstractMap<MyKey,MyValue> {
    private Set<Map.Entry<MyKey,MyValue>> data = null ;
    public static class MyMapEntry<K,V> implements Map.Entry<K,V> { 
        //...
    }
    public static class MyEntrySet<E> extends AbstractSet<E> {
        //...
    }
    public static class MyEntrySetIterator<E> implements Iterator<E> {
        //...
    }
    { // initializer block
        this.data = new MyEntrySet<MyMapEntry<MyKey,MyValue>>() ;
    }
    // (constructors and methods of the Map interface)

}

(2) Working:

public class CustomMap extends AbstractMap<MyKey,MyValue> {
    private Set<Map.Entry<MyKey,MyValue>> data = null ;
    public static class MyMapEntry<K,V> implements Map.Entry<K,V> { 
        //...
    }
    public static class MyEntrySet<E> extends AbstractSet<E> {
        //...
    }
    public static class MyEntrySetIterator<E> implements Iterator<E> {
        //...
    }
    { // initializer block
        this.data = new MyEntrySet<Map.Entry<MyKey,MyValue>>() ;
    }
    // (constructors and methods of the Map interface)

}

---

The only difference is in the way I instantiated this.data (the Set backing the Map): apparently the compiler does not accept an instance of Map.Entry (such as MyMapEntry) as the parameterized type of MyEntrySet.

The compiler complains about “incompatible types”.

I wonder why.

Please post comments in case I need to elaborate,
TIA,

FK82

Answer 1

I’ll expand on the other answers with a simpler example.

Consider

List<String> strings = new ArrayList<String>();
List<Object> objects = strings; // this is what you are doing in your constructor
objects.add("some string");

objects has declared that it will only contain Objects, and strings has declared that it will only contain Strings. So surely assigning strings to objects should be fine as all Strings are Objects? We’re even allowed to add Strings to objects without a problem.

However, what about this situation.

objects.add(new Object());

Since objects is just strings, we are adding an Object to strings, and thus breaking its declaration that it will only contain Strings.