# A map with a default value.
#
# ~~~~
# var dm = new DefaultMap[String, Int](10)
# assert dm["a"] == 10
# ~~~~
#
# The default value is used when the key is not present.
# And getting a default value does not register the key.
#
# ~~~~
# assert dm["a"] == 10
# assert dm.length == 0
# assert dm.has_key("a") == false
# ~~~~
#
# It also means that removed key retrieve the default value.
#
# ~~~~
# dm["a"] = 2
# assert dm["a"] == 2
# dm.keys.remove("a")
# assert dm["a"] == 10
# ~~~~
#
# Warning: the default value is used as is, so using mutable object might
# cause side-effects.
#
# ~~~~
# var dma = new DefaultMap[String, Array[Int]](new Array[Int])
#
# dma["a"].add(65)
# assert dma["a"] == [65]
# assert dma.default == [65]
# assert dma["c"] == [65]
#
# dma["b"] += [66]
# assert dma["b"] == [65, 66]
# assert dma.default == [65]
# ~~~~
class DefaultMap[K, V]
	super HashMap[K, V]

	# The default value.
	var default: V

	redef fun provide_default_value(key) do return default
end