检查是否存在HashMap密钥

Question

In Java, having a HashMap fully filled in with data of such form:

HashMap<Integer, int[]> map = new HashMap<Integer, int[]>(1000000, 1);

what is faster when checking the existence of a random key, say 100:

if (map.get(100) == null))

or

if (!map.containsKey(100))

?

Question is interesting from the micro-optimization point of view.

Answer 1

The containsKey should be very slightly slower because it results in an extra function call (it just calls getEntry ) (it could get optimised away, I'm not sure whether Java will do so). containsKey looks like:

public boolean containsKey(Object key) {
  return getEntry(key) != null;
}

But note that containsKey could on the other hand be very slightly faster on other Map implementations (but probably not those in the standard Java API).

Generally my implementations look like: (avoiding the need for containsKey )

int[] arr = map.get(100);
if (arr == null) // doesn't exist
  // do stuff
else // exists
  // do stuff with arr

The below would definitely be slower than the above: (if the items you look for exist a reasonable amount of the time)

if (!map.containsKey(100)) // doesn't exist
  // do stuff
else // exists
{
  int[] arr = map.get(100);
  // do stuff with arr
}

Edit: Thanks to zvzdhk for providing the source of containsKey . I should actually have checked.

Answer 2

Actually both approaches are the same. If you look in java.util.HashMap source code you can find next containsKey realization:

public boolean containsKey(Object key) {
    return getEntry(key) != null;
}

Answer 3

这两个只在return类型上有所不同，除了map.get(key)可以返回null ，如果它是一个键，但是map.containsKey(key)将返回boolean ，它可以用来distingush两个可能的map.get(key)返回null 。

Answer 4

There is no difference between this two approaches. The main difference only what you are going to do next. If you need the value then, of course by get you will have the value.