Initializing a HashMap with default values?

Question

I'm implementing the A* search algorithm given here, https://en.wikipedia.org/wiki/A*_search_algorithm

This line indicates we need to initiliaze a map with the default values of INFINITY,

gScore := map with default value of Infinity

So I tried that here,

Map<State, Double> gScore = new HashMap<State, Double>(Double.POSITIVE_INFINITY);

This does not work however the following does;

Map<State, Double> gScore = new HashMap<State, Double>((int) Double.POSITIVE_INFINITY);

I'm wondering why, and what impact (if any) it will have on my implementation.

Louis Wasserman · Accepted Answer · 2016-02-27 02:00:20Z

4

There is no way to initialize a map with a default value in Java, and your second version will not create a map with a default value of infinity, but instead will try to create an infinitely large map. (Not really, but it'll try creating the largest map possible.)

Instead, modify the algorithm: anytime you do map.get(key), check if the value is null and, if so, replace it with infinity.

answered Feb 27, 2016 at 2:00

Louis Wasserman

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VGR Over a year ago

Map.getOrDefault is useful for this.

Stephen C · Accepted Answer · 2016-02-27 02:32:13Z

1

I concur with @Louis that the best solution is to check the result of the get call to see if it is null.

However, it is also possible to create a subclass of HashMap with an override for the get method that returns a default value if super.get(key) returns null. But beware of anomalies; e.g.

Iterating the map will give you only the entries that are "really there".
If there is a real entry with a null value (because you called put(key, null)) you won't get a null when you call get for the entries key. But but the entry will show up in the iteration ... with value null.

So, from an OO design perspective, a better approach (to extending HashMap) would be to create an application-specific class that only exposes a subset of the Map functionality.

edited Feb 27, 2016 at 2:32

answered Feb 27, 2016 at 2:13

Stephen C

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Comments

Eran · Accepted Answer · 2018-04-29 08:02:11Z

1

Instead of explicitly putting Double.POSITIVE_INFINITY for all the possible keys (which may be inefficient), you can use putIfAbsent.

In any place where you have:

value = map.get(key);

you can change it to:

value = map.putIfAbsent(key,Double.POSITIVE_INFINITY);

This will behave as if the value of any key not present in the Map (or any key whose current value is null) is Double.POSITIVE_INFINITY. It will put the value Double.POSITIVE_INFINITY in the Map for any such keys.

It will save you the need to check if map.get(key) returns null.

answered Apr 29, 2018 at 8:02

Eran

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