How to declare that method is returning Collections.unmodifiableMap

Question

I'm returning an unmodifiable map from a method. How do I make sure that any code that tries to modify the returned map will get a compile time error, as opposed to a run time error?

My class:

public class Foo
{
    private final Map<String, Bar> aMap;

    public Foo()
    {
    ...
    }

    public Map<String, Bar> getMap(){
        return Collections.unmodifiableMap(aMap);
    }
}

I want this to create a compile time error:

Foo foo = new Foo();
Map<String, Bar> m = foo.getMap();
m.put('key',null); // user should be notified that the map is unmodifiable here, not at run time

Can I change the return type? Can I add an appropriate annotation?

Answer 1

You could create a class like

class ReadOnlyMap<K, V> {

    private final Map<K, V> map;

    public ReadOnlyMap(Map<K, V> map) {
       this.map = map;
    }

    public V get(Object key) {
        return map.get(key);
    }

    // also implement size() method
    // and everything else you want to expose
}

and use it as the return type of your method. Then your method would be

public ReadOnlyMap<String, Bar> getMap() {
    return new ReadOnlyMap(aMap);
}

Beware that this won't hinder the caller to mutate the value objects, if they're not instances of immutable classes.