Double brace initialization with nested collections

Question

I know I can declare and initialize a List using double braces:

// (1)
List<Object> myList = new ArrayList<object>(){{
    add("Object1");
    add("Object2");
}};

But I want a List of <Map<Object,Object>>:

// (2)
List<Map<Object,Object>> myList = new ArrayList<Map<Object,Object>>();

How can I use double brace initialization (see (1)) with nested collections? My goal is to declare and initialize the data structure in a single line.

Also I would like to know if there are certain drawbacks when using double brace initialization I have to be aware of.

Just use "normal" initialization code; double brace initialization is difficult to read. Code is WORM (Write Once, Read Many). Note: if this is a static list, use a static initializer. — fge, Feb 09 '15 at 12:58
Why would you want to write code like that? Do you work alone? — kolossus, Feb 09 '15 at 13:00
Don't use the "double braces", because each time you do that, you are creating an useless subclass of ArrayList. — Florent Bayle, Feb 09 '15 at 13:00
possible duplicate of [What is Double Brace initialization in Java?](http://stackoverflow.com/questions/1958636/what-is-double-brace-initialization-in-java) — Rafael Winterhalter, Feb 09 '15 at 13:14

score 10 · Answer 1 · answered Feb 09 '15 at 13:09

Avoid double brace initialization as it a) surprises your colleagues and is hard to read, b) harms performance and c) may cause problems with object equality (each object created has a unique class object).

If you're working on a code style guide this kind of trickery belongs in the don't section.

If you really want to be able to create lists and maps inline, just go ahead and create a few factory methods. Could look like this:

List l = Lists.of(
    Maps.of(new Entry("foo", "bar"), new Entry("bar", "baz")),
    Maps.of(new Entry("baz", "foobar"))
);

However, Vangs example shows exactly how the syntax for your example is for double brace initialization.

Double brace code can also throw up issues in testing. How would you mock xyz if it is statically initialized, for example? — vikingsteve, Feb 09 '15 at 13:25

score 5 · Answer 2 · answered Feb 09 '15 at 13:01

5

It's not good idea, because it's hard to read, but you can do following:

List<Map<Object,Object>> myList = new ArrayList<Map<Object,Object>>() {{
                    add(new HashMap<Object, Object>() {{
                        put(key, obj);
                    }});
                }};

answered Feb 09 '15 at 13:01

Bracadabra

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@vang: thanks for your time and answer but in this you didn't initlize the list :( – Arvind Vishwakarma Feb 09 '15 at 13:06
2

What do you mean? Than what new ArrayList>() is? – Bracadabra Feb 09 '15 at 13:07
@Vang: i meant that you gave me an idea to put objects in map..it's fine but want to init. my list as well. hope you got it :) – Arvind Vishwakarma Feb 09 '15 at 13:10
2

Vangs example is correct and creates a `List>` with exactly one map having exactly one entry of `key: object`. – atamanroman Feb 09 '15 at 13:13
I'm afraid we don't understand each other) By the way it better to accept @atamanroman answer – Bracadabra Feb 09 '15 at 13:13

Double brace initialization with nested collections

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