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");

Solution 1

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;

}

Solution 2

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.

Solution 3

You could use ImmutableMap.Builder from Google collections library.

Solution 4

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.

Solution 5

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.

View more solutions

Comments

[700 Software]

Most likely caveat is that will not be Serializable.

[home]

+1. interesting, I did not see that before. Question is if it makes life really easier :-)

[Eugene Kuleshov]

This is generally not a good idea for any kind of production code. It also creates anonymous class every time.

[Nathan Hughes]

@George: it's not that it isn't Serializable, it's that a compiler warning complains it doesn't have a serialUID.

[Eugene Kuleshov]

This breaks Java generics type safety.

[Amir Raminfar]

I don't see why you would want to reinvent ImmutableMap.of(...) in google's collection

[700 Software]

@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."

[Maurício Linhares]

I would reinvent to avoid adding yet another unnecessary library full of unnecessary classes when all i want is a single method.

[Maurício Linhares]

Great idea @Robert, added it now.

[Eugene Kuleshov]

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.

[Maurício Linhares]

Eugene, that's a matter of opinion, I don't find it useful to add a library instead of typing 15 lines of code.

[Paŭlo Ebermann]

The deserialized map will have the original values for these keys, even if changed later (before serializing).

[Jochen Bedersdorfer]

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.

[Mladen Adamovic]

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.