C# to Java - Dictionaries?

Question

Is it possible in Java to make a Dictionary with the items already declared inside it? Just like the below C# code:

   Dictionary<string, int> d = new Dictionary<string, int>()
    {
        {"cat", 2},
        {"dog", 1},
        {"llama", 0},
        {"iguana", -1}
    };

How do I do this and what type do I use? I've read that Dictionary is obsolete.

Sean Reilly · Accepted Answer · 2017-11-28T13:42:43.347

75

This will do what you want:

Map<String,Integer> map = new HashMap<String, Integer>(){{
    put("cat", 2);
    put("dog", 1);
    put("llama", 0);
    put("iguana", -1);
}};

This statement creates an anonymous subclass of HashMap, where the only difference from the parent class is that the 4 entries are added during instance creation. It's a fairly common idiom in the Java world (although some find it controversial because it creates a new class definition).

Because of this controversy, as of Java 9 there is a new idiom for conveniently constructing maps: the family of static Map.of methods.

With Java 9 or higher you can create the map you need as follows:

Map<String, Integer> map = Map.of(
    "cat", 2,
    "dog", 1,
    "llama", 0,
    "iguana", -1
);

With larger maps, this alternative syntax may be less error-prone:

Map<String, Integer> map = Map.ofEntries(
    Map.entry("cat", 2),
    Map.entry("dog", 1),
    Map.entry("llama", 0),
    Map.entry("iguana", -1)
);

(This is especially nice if Map.entry is statically imported instead of being referenced explicitly).

Besides only working with Java 9+, these new approaches are not quite equivalent to the previous one:

They don't allow you to specify what Map implementation is used
They only create immutable maps
They don't create an anonymous subclass of Map

However, these differences shouldn't matter for many use cases, making this a good default approach for newer versions of Java.

edited Nov 28 '17 at 13:42

answered Jul 31 '11 at 00:12

Sean Reilly

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2

And here is an explanation on what this is doing. http://techpolesen.blogspot.com/2007/08/java-initializers-add-elements-to.html – Josh Smeaton Jul 31 '11 at 00:18
Hmmm... double braces syntax is not generally considered good style – Luigi Plinge Jul 31 '11 at 00:21
@Luigi You're right, I'll change my answer (won't do anything though :P) – fireshadow52 Jul 31 '11 at 00:22
3

The extra braces are not a style-thing. They are both functional and necessary to denote the code as an initializer block. – Affe Jul 31 '11 at 00:42
@Affe - you are aware we are creating a new anonymous class for each such initialization? And what will happen if you try to serialize anything in the map? And the performance impact? (http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) Even the writer of the article linked to above says he wouldn't use it in production code. – Luigi Plinge Jul 31 '11 at 00:53
1

I was talking to fireshadow, who thought you meant "use one brace, not two" not "don't anonymously subclass JavaSE library classes just to save yourself a bit of typing." – Affe Jul 31 '11 at 01:07
1

@Josh Smeaton: Nice article, although there is a tiny mistake in it: the inner braces are an instance initializer, not a static initializer. – Sean Reilly Jul 31 '11 at 08:11

fireshadow52 · Answer 2 · 2011-07-31T00:47:06.543

12

Map<String,Integer> map = new HashMap<String, Integer>(){{
put("cat", 2);
put("dog", 1);
put("llama", 0);
put("iguana", -1);
}};

edited Jul 31 '11 at 00:47

answered Jul 31 '11 at 00:13

fireshadow52

6,298
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This doesn't compile, your answer was correct before you edited it. – Affe Jul 31 '11 at 00:42

Luigi Plinge · Answer 3 · 2011-07-31T00:39:27.807

Bite the bullet and type out the map name!

    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("cat", 2);
    map.put("dog", 1);
    map.put("llama", 0);
    map.put("iguana", -1);

You could also do something like this, which might save some typing with a long list:

    Object[][] values = {
        {"cat", 2},
        {"dog", 1},
        {"llama", 0},
        {"iguana", -1}
    };

    for (Object[] o : values) {
        map.put((String) o[0], (Integer) o[1]);
    }

score 3 · Answer 4 · edited Dec 06 '14 at 15:39

If you use the Guava library, you can use its ImmutableMap class, either by itself (examples 1 and 2), or as an initializer for a HashMap (examples 3 and 4):

Map<String, Integer> map1 = ImmutableMap.<String, Integer> builder()
    .put("cat", 2)
    .put("dog", 1)
    .put("llama", 0)
    .put("iguana", -1)
    .build();
Map<String, Integer> map2 = ImmutableMap.of(
    "cat", 2,
    "dog", 1,
    "llama", 0,
    "iguana", -1
);
Map<String, Integer> map3 = Maps.newHashMap(
    ImmutableMap.<String, Integer> builder()
    .put("cat", 2)
    .put("dog", 1)
    .put("llama", 0)
    .put("iguana", -1)
    .build()
);
Map<String, Integer> map4 = Maps.newHashMap( ImmutableMap.of(
    "cat", 2,
    "dog", 1,
    "llama", 0,
    "iguana", -1)
);

score 0 · Answer 5 · answered Jul 31 '11 at 00:42

Java7 almost introduced "collection literals" that would allow syntax like that. They'll probably try to shove it in Java8. I have no idea what is wrong with these people.

This can be easily achieved by some kind of wrapper API

Map<String,Integer> map = Maps.<String,Integer>empty()
    .put("cat", 2).put("dog",1)....;

Not too bad. I would prefer something like

map("cat", "dog", ... )
.to(  1,     2,   ... );

This kind of thing must have been implemented by various people, unfortunately the standard API doesn't inculde such things.

C# to Java - Dictionaries?

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