Question

Why does the declaration

Set<Set<String>> var = new HashSet<Set<String>>();

work but the declaration

Set<Set<String>> var = new HashSet<HashSet<String>>();

choke?

I’m aware that ‘top level’ (not sure if that’s the correct phrase here) generics in a declaration play by different rules than those inside the pointy brackets, but I’m interested to learn the reason. Not an easy question to google, so I thought I’d try you guys.

Answer 1

It’s because you could circumvent the type system if it were allowed. The property you desire is called covariance. If collections were covariant then you’d be able to do this:

Set<Set<String>> var = new HashSet<HashSet<String>>();

var.add(new TreeSet<String>());

A TreeSet is a type of Set, and so static type checking would not prevent you from inserting a TreeSet into var. But var expects HashSets and HashSets only, not any old type of Set.

Personally, I always write your first declaration:

Set<Set<String>> var = new HashSet<Set<String>>();

The outer class needs to have a conrete implementation, but there’s usually no need to nail down the inner class to HashSet specifically. If you create a HashSet of Sets you are good to go. Whether you then proceed to insert a series of HashSets into var is your choice later in the program, but no need to restrict the declaration.

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For what it’s worth, arrays in Java are covariant, unlike the collection classes. This code will compile and will throw a runtime exception instead of being caught at compile time.

// Arrays are covariant, assignment is permitted.
Object[] array = new String[] {"foo", "bar"};

// Runtime exception, can't convert Integer to String.
array[0] = new Integer(5);