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.

Answer 1

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);
                    }});
                }};

Answer 2

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.