Do you ever store a null value? If not, you can just do:
Foo value = map.get(key);
if (value != null) {
...
} else {
// No such key
}
Otherwise, you could just check for existence if you get a null value returned:
Foo value = map.get(key);
if (value != null) {
...
} else {
// Key might be present...
if (map.containsKey(key)) {
// Okay, there's a key but the value is null
} else {
// Definitely no such key
}
}