Is Map.containsKey() useful? [duplicate]

Question

I'm wondering, does it make any sense to check for a particular key before trying to access it. Example:

Map myMap ....
if myMap.containsKey(key) {
   Object value = myMap.get(key);
   .....
}

Example of not using containsKey:

Object value = myMap.get(key);
if (value != null) {
 ......
}

EDIT: to clarify on null keys and values. Let's say, that map doesn't allow null keys and null values, so this two examples be identical.

Answer 1

Yes - keys can have null values:

Map myMap = ...;
myMap.put("foo", null);
if (myMap.containsKey("foo")) {
   Object value = myMap.get(key); // value is null
}

Object value = myMap.get("foo");
if (value != null) {
    // you won't get here
}

You could argue (and I'd agree) that not distinguishing a null value from a non-existent entry was a pretty bad design decision when they first made Java's collection API.

(Some maps - Hashtable and ConcurrentHashMap, at least - don't allow null values, which makes containsKey less important there, but it's still a nice readability improvement over m.get(k) == null.)

Answer 2

In case your map allows for null values, with just get(key) you can't differentiate between key/value pair that has value = null and "no matching key/value pair exists".