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
– fge, Commented Feb 9, 2015 at 12:58
Why would you want to write code like that? Do you work alone? — kolossus
– kolossus, Commented Feb 9, 2015 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
– Florent Bayle, Commented Feb 9, 2015 at 13:00
possible duplicate of What is Double Brace initialization in Java? — Rafael Winterhalter
– Rafael Winterhalter, Commented Feb 9, 2015 at 13:14

atamanroman · Accepted Answer · 2015-02-09 13:09:59Z

10

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.

answered Feb 9, 2015 at 13:09

atamanroman

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1 Comment

vikingsteve Over a year ago

Double brace code can also throw up issues in testing. How would you mock xyz if it is statically initialized, for example?

Bracadabra · Accepted Answer · 2015-02-09 13:01:11Z

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 9, 2015 at 13:01

Bracadabra

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5 Comments

Arvind Vishwakarma Over a year ago

@vang: thanks for your time and answer but in this you didn't initlize the list :(

Bracadabra Over a year ago

What do you mean? Than what new ArrayList<Map<Object,Object>>() is?

Arvind Vishwakarma Over a year ago

@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 :)

atamanroman Over a year ago

Vangs example is correct and creates a List<Map<Object, Object>> with exactly one map having exactly one entry of key: object.

Bracadabra Over a year ago

I'm afraid we don't understand each other) By the way it better to accept @atamanroman answer

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