Arrays.asList also for maps?

Question

I have the below code:

Map<String, Map<Double, String>> map = new HashMap<>();
Map<Double,String> Amap = new HashMap<>();
map.put(getValuesTypes.FUT(), HERE);

Instead of creating a Map first and put it at "HERE", I'm looking for a function like I could use with a List there Arrays.asList(...) so that i can just enter at "Here" ?.asMap({1.0,"A"}, {2.0,"B"})

With Java 9, you will be able to use collection factory methods for creating immutable maps, see http://download.java.net/java/jdk9/docs/api/java/util/Map.html#of-K-V- and its variants. — Tordanik, Jul 13 '17 at 16:01

score 24 · Accepted Answer · answered Nov 06 '16 at 13:33

24

You can initialize HashMap like this.

new HashMap<Double, String>() {
    {
        put(1.0, "ValA");
        put(2.0, "ValB");
    }
};

answered Nov 06 '16 at 13:33

nakano531

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7

Be awere that it has hidden cost of creating extra map instance. It is called double brace initialization. – jakubbialkowski Nov 09 '16 at 20:36
4

with Java 9 you can simply do `new HashMap(Map.of( 1.0, "A", 2.0, "B" ));` – Mike D3ViD Tyson Aug 22 '18 at 15:34
1

What it does is it basically creates an anonymous class from HashMap class and inside your anonymous class you use a static block. I personally would stay away from this. – Ivan Kaloyanov Mar 13 '20 at 08:31

score 10 · Answer 2 · answered Nov 06 '16 at 13:38

10

With Guava

Map<Double, String> map = ImmutableMap.of(1.0, "A", 2.0, "B");

answered Nov 06 '16 at 13:38

Jiri Kremser

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Olimpiu POP · Answer 3 · 2018-04-11T07:21:52.670

Guava's ImmutableMap.of(..) can help in this direction:

ImmutableMap.of(1, "a");

in the JDK there is only Collections.singletonMap(..), but this provides you just a map with a sole pair.

There was a discussion in guava project to contain a Maps.asMap(Object... varArgs), bit it was stopped. So, ImmutableMap.of(...) is the way to go.

EDIT since JDK 9

In JDK 9 there were added new methods that do the same thing: Map.of(K,V)

Andrew Tobilko · Answer 4 · 2016-11-06T13:59:34.313

1

There is no literal to initialize a map in that way. But you could use an anonymous class generating on the spot:

map.put(getValuesTypes.FUT(), new HashMap<Double, String>() {{
    put(1.0, "A");
    put(2.0, "B");
}});

though it's not recommended. I would suggest to use Guava's ImmutableMap:

map.put(getValuesTypes.FUT(), ImmutableMap.<Double, String>of(1.0, "A", 2.0, "B"));

If a number of pairs is greater than 5, you should use their builder:

map.put(getValuesTypes.FUT(), 
        ImmutableMap.<Double, String>builder().put(1.0, "A")/* 5+ puts */.build());

edited Nov 06 '16 at 13:59

answered Nov 06 '16 at 13:35

Andrew Tobilko

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Without the need to import extra libs, you can use `java.util.Collections` like so `Collections.unmodifiableMap(myHashMapInstance)` – Jaxon Aug 15 '17 at 03:09

score 0 · Answer 5 · answered Nov 06 '16 at 13:47

0

You can only initialize a new map by using an anonymous class. i.e.

new HashMap<K, V>() {{ put(key, value); put(key, value); }};

answered Nov 06 '16 at 13:47

Vikram Jakhar

712
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Arrays.asList also for maps?

5 Answers5

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