The best way to static initialization of Maps in google collections

Question

What is the best way to static initialization of modifiable Maps? I found only

ImmutableMap.of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5)

But this way created immutable map and contains fixed list of parameters.

Related - http://stackoverflow.com/questions/5947869/like-immutablemap-of-for-hashmap-in-maps — Premraj, Sep 01 '12 at 16:11

卢声远 Shengyuan Lu · Accepted Answer · 2012-09-02T12:55:13.737

22

If you do want an of code fashion, you could use:

myMap = Maps.newHashMap(ImmutableMap.of(k1, v1, k2, v2...));

In addition, ImmutableMap.Builder is other choice to create a Map from complex source:

myMap = Maps.newHashMap(new ImmutableMap.Builder<K, V>()
                   .put(k1, v1) //One k-v pair 
                   .putAll(otherMap) //From other Map
                   .put(Maps.immutableEntry(k2, v3)) //From a Map Entry
                   ...
                   .build());

Plus: My code is not original intention of ImmutableMap. If Nawa insists on using Guava library ;)

edited Sep 02 '12 at 12:55

answered Sep 01 '12 at 14:06

卢声远 Shengyuan Lu

31,208
22
85
130

That's a bit wasteful on the garbage creation front, though. In the first case, you create a temporary `ImmutableMap` whenever you want to create a `HashMap`, and in the second case, you create an `ImmutableMap.Builder` (which contains a mutable `Map`), which then creates an `ImmutableMap` before finally creating the `HashMap`... – Frank Pavageau Sep 02 '12 at 12:18
@FrankPavageau Yes, you are right. My code is not original intention of ImmutableMap. I mean if Nawa insists on using Guava library ;) – 卢声远 Shengyuan Lu Sep 02 '12 at 12:48

Frank Pavageau · Answer 2 · 2012-08-31T15:43:22.453

4

You don't actually need static initialization. What's wrong with the following ?

Map<K, V> map = Maps.newHashMap();
map.put(k1, v1);
map.put(k2, v2);
// More put() calls

// map is now ready to use.

You can create a helper method around it if needed, but you can only create so many different versions (for 1 entry, 2 entries, etc.). At some point it's not helping anymore.

edited Aug 31 '12 at 15:43

answered Aug 31 '12 at 15:37

Frank Pavageau

11,477
1
43
53

Well, no. Either you write a static method for each number of entries you want to initialize the `Map` with, or you write a static method taking `Object[]` or `Object...` as a parameter and make sure its length is a multiple of 2 (to get an integer number of entries), but then you lose any type checking. I say stick with the sequence of `put()`s. – Frank Pavageau Aug 31 '12 at 15:55

score 3 · Answer 3 · answered Aug 31 '12 at 15:56

There is no point in having this for mutable collections. The only reason I can think of is that you want to have a shorthand for this when creating initially small mutable maps. Write your own utility methods if you need this quiet often:

public static <K,V> HashMap<K,V> newHashMap(K k1, V v1) {
    HashMap<K, V> map = new HashMap<>();
    map.put(k1, v1);
    return map;
}

public static <K,V> HashMap<K,V> newHashMap(K k1, V v1, K k2, V v2) {
    HashMap<K, V> map = new HashMap<>();
    map.put(k1, v1);
    map.put(k2, v2);
    return map;
}

...

Override it as long as you think it is still readable. Because of mixture from keys and values this gets unreadable fast in my opinion - even with proper formatting. Guava guys stopped this at 5 key-value pairs what is too much imho.

The best way to static initialization of Maps in google collections

3 Answers3