Alternative to double brace initialization

Question

I have a nested collection in the form:

HashMap<String, HashMap<String, List<String>>> errorList;

Now I initialize it inline using double braces like this

errorList.put(tempName, new HashMap<String, List<String>>() {{
    put("upl", new ArrayList<String>() {{ add("Y"); add("Upload Success"); }});
}});

This lies in a foreach loop with the value of tempName changing in every iteration. I did this because i couldn't use instances of List<String> or HashMap<String,List<String>> because every time i changed the value in that instance it is reflected in the collection it is nested in. So i am forced to create new instances with double brace initialization.

Thing is: I want to use a list object. Instead of

new ArrayList<String>() {{ add("Y"); add("Upload Success"); }}

I want to use a variable.

How can I do this?

I’m afraid I haven’t fully understood your problem. Could you [create a Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve), please? — Ole V.V., May 10 '17 at 12:06
This code is scary at least, you should have objects instead of so many collections. This is just unmaintainable. — Marc, May 10 '17 at 12:25
I'm just using it inside a spring controller so that i can pass a list of errors to populate an Error table in the view(jsp). The scope is within that controller method and view. I figured this data object would grant me large flexibility in the view side. — kaattaalan, May 10 '17 at 12:32
Possible duplicate of [Efficiency of Java "Double Brace Initialization"?](https://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) — kaattaalan, May 31 '17 at 06:21

score 5 · Accepted Answer · edited Dec 13 '21 at 05:52

Instead of:

new ArrayList<String>() {{ add("Y"); add("Upload Success"); }}

you may use:

Arrays.asList("Y", "Upload Success")

This gives you a fixed-size list. If you want to be able to add or remove elements later, convert it into an ArrayList:

new ArrayList<>(Arrays.asList("Y", "Upload Success"))

And of course you can put this list into its own variable before putting it into your map structure.

If you want to put either [Y, Upload Success] or [N, Upload Failed] and make sure the lists aren’t shared between map entries, here’s a suggestion: First, outside your loop:

final List<String> successList = Arrays.asList("Y", "Upload Success");
final List<String> failureList = Arrays.asList("N", "Upload Failed");

Then inside your loop:

if (wasSuccessful) {
    errorList.put(tempName,
                  Collections.singletonMap("upl", new ArrayList<>(successList)));
} else {
    errorList.put(tempName,
                  Collections.singletonMap("upl", new ArrayList<>(failureList)));
}

You may take the idea one step further and build the maps outside the loop. And again, if you want the inner map to be a HashMap, just convert into one: new HashMap<>(Collections.singletonMap("upl", new ArrayList<>(successList))).

You notice I have avoided the double brace initialization completely. While it’s brief, it has an overhead both conceptually and performancewise. You are creating a new anonymous subclass each time, which I don’t find warranted.

could you tell me what singletonMap does in this code?. The inner hashmap has other keys also (4 of them). They represent different error types present in some uploaded data — kaattaalan, May 10 '17 at 12:34
Such a question is answered most easily be consulting [the `Collections` documentation](http://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#singletonMap-K-V-). And yes, since you want to put more elements in it, convert it to a `HashMap` as I described. — Ole V.V., May 10 '17 at 12:38

Alternative to double brace initialization

1 Answers1

Linked