Better Map Constructor

Question

Is there a more streamlined way to do the following?

Map<String, String> map = new HashMap<String, String>();
map.put("a", "apple");
map.put("b", "bear");
map.put("c", "cat");

I'm looking for something closer to this.

 Map<String, String> map = MapBuilder.build("a", "apple", "b", "bear", "c", "cat");

are the keys/values hardcoded like that? if so, why not something like loading a json string http://stackoverflow.com/questions/443499/json-to-map — David, Jul 26 '11 at 17:30
possible duplicate of [Is there a best practice for writing maps literal style in Java?](http://stackoverflow.com/questions/3798083/is-there-a-best-practice-for-writing-maps-literal-style-in-java), and [initialize java HashSet values by construction](http://stackoverflow.com/questions/2041778/initialize-java-hashset-values-by-construction) — Paŭlo Ebermann, Jul 26 '11 at 19:18
@David: That is a great for constants. Especially for me because I use JSON frequently and often have the methods available. — 700 Software, Jul 26 '11 at 19:54

Nathan Hughes · Answer 1 · 2012-04-19T17:21:58.733

16

There's always double-brace initialization:

Map<String, String> map = new HashMap<String, String>(){{
    put("a", "apple"); put("b", "bear"); put("c", "cat");}};

There are problems with this approach. It returns an anonymous inner class extending HashMap, not a HashMap. If you need to serialize the map then know that serialization of inner classes is discouraged.

edited Apr 19 '12 at 17:21

answered Jul 26 '11 at 17:18

Nathan Hughes

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Most likely caveat is that will not be Serializable. – 700 Software Jul 26 '11 at 17:19
+1. interesting, I did not see that before. Question is if it makes life really easier :-) – home Jul 26 '11 at 17:19
This is generally not a good idea for any kind of production code. It also creates anonymous class every time. – Eugene Kuleshov Jul 26 '11 at 17:21
@George: it's not that it isn't Serializable, it's that a compiler warning complains it doesn't have a serialUID. – Nathan Hughes Jul 26 '11 at 17:22
@Nathan, I did not know you could serialize anonymous subclasses, but even then, I wouldn't recommend it. *["Serialization of inner classes (i.e., nested classes that are not static member classes), including local and **anonymous classes**, is strongly discouraged for several reasons."](http://download.oracle.com/javase/6/docs/platform/serialization/spec/serial-arch.html#4539)* – 700 Software Jul 26 '11 at 17:28
The deserialized map will have the original values for these keys, even if changed later (before serializing). – Paŭlo Ebermann Jul 26 '11 at 19:22

Maurício Linhares · Accepted Answer · 2011-07-26T17:31:09.933

13

No, there isn't, but I wrote a method to do exactly this, inspired by Objective-C NSDictionary class:

public static Map<String, Object> mapWithKeysAndObjects(Object... objects) {

    if (objects.length % 2 != 0) {
        throw new IllegalArgumentException(
                "The array has to be of an even size - size is "
                        + objects.length);
    }

    Map<String, Object> values = new HashMap<String, Object>();

    for (int x = 0; x < objects.length; x+=2) {
      values.put((String) objects[x], objects[x + 1]);
    }

    return values;

}

edited Jul 26 '11 at 17:31

answered Jul 26 '11 at 17:18

Maurício Linhares

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2

This breaks Java generics type safety. – Eugene Kuleshov Jul 26 '11 at 17:24
2

I don't see why you would want to reinvent ImmutableMap.of(...) in google's collection – Amir Raminfar Jul 26 '11 at 17:27
7

I would reinvent to avoid adding **yet another unnecessary library full of unnecessary classes** when all i want is a single method. – Maurício Linhares Jul 26 '11 at 17:29
Great idea @Robert, added it now. – Maurício Linhares Jul 26 '11 at 17:31
Maurício, looking at number of iterations in your code, you had saved some time and bugs by using a proven library. Besides, you may find it actually useful and it would help toy write a cleaner and safer code. – Eugene Kuleshov Jul 26 '11 at 17:43
Eugene, that's a matter of opinion, I don't find it useful to add a library instead of typing 15 lines of code. – Maurício Linhares Jul 26 '11 at 17:52
1

Guava is bloated v18 adds 2.5MB jar file (without sources), if you are going to use 1% of it, I'd rather use custom sollutions like this one for faster compile/deploy times. – Mladen Adamovic Nov 20 '14 at 08:41

score 9 · Answer 3 · answered Jul 26 '11 at 17:18

9

You could use ImmutableMap.Builder from Google collections library.

answered Jul 26 '11 at 17:18

Eugene Kuleshov

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score 3 · Answer 4 · answered Dec 09 '19 at 08:36

Java 9 adds Map.of, such as:

Map<String, String> map = Map.of("a", "apple", "b", "bear", "c", "cat");

Up to 10 entries are supported. For more entries you can use the overload taking Entry:

Map<String, String> map 
    = Map.ofEntries
        (Map.entry("a", "apple")
        , Map.entry("b", "bear")
        , Map.entry("c", "cat"));

Note that these methods do not return a HashMap. It returns an optimized immutable map.

score 2 · Answer 5 · edited May 23 '17 at 11:55

2

You could always use double brace initialization:

Map<String, String> map = new HashMap<String, String>() {{
    put("foo", "bar");
    put("baz", "qux");
}}

But bear in mind this might not be efficient according to these answers.

edited May 23 '17 at 11:55

Community

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answered Jul 26 '11 at 17:21

whirlwin

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whoever came up with the name 'double brace initialization' should be shot. This is a valid declaration of an anonymous class combined with a non-static initializer block. – Jochen Bedersdorfer Jul 26 '11 at 19:38

Better Map Constructor

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