Initializing a dictionary with a specific set of data cleanly in Java

Note: The original answer is over 8 years old and Java has come a long way since then. As of now I'd recommend:

var map = Map.of(
    "BC", List.of("AB"),
    "AB", List.of("BC", "SK"),
    "SK", List.of("AB", "MB"),
    "MB", List.of("SK")
);

This is the best technique I know of:

    Map<String, String> myMap = new HashMap<String, String>() {{
        put("foo", "bar");
        put("key", "value");
        //etc
    }};

Note the double braces -- this is commonly called double brace initialization.

What you're actually doing is creating an anonymous inner class that extends HashMap, and your new subclass contains an initializer block, in which you can call any arbitrary code that is guaranteed to be executed before the instance can be used.

Also note the 'diamond operator' cannot be used with anonymous classes, for whatever reason.

This is a nice technique for test classes, but I tend to avoid it for production code.

EDIT: Thought I should answer your actual question!

double-brace initialization is probably the best solution in "pure" Java, your Map would specifically look like:

    Map<String, List<String>> westernCanadaAdjList = new HashMap<String, List<String>> () {{
        put("BC", new ArrayList<String>(){{ add("AB"); }});
        put("AB", new ArrayList<String>(){{ add("BC"); add("SK"); }});
        put("SK", new ArrayList<String>(){{ add("AB"); add("MB"); }});
        put("MB", new ArrayList<String>(){{ add("SK"); }});
    }};

... Still not super awesome. Java really does need a Map literal, and it does not have one.

For production code, I'd use a Guava's MultiMap, but honestly populating it with literals isn't much better:

    Multimap<String, String> multimap = ArrayListMultimap.create();
    multimap.put("BC", "AB");
    multimap.put("AB", "BC");
    multimap.put("AB", "SK");
    multimap.put("SK", "SK");
    multimap.put("SK", "SK");
    multimap.put("SK", "SK");

I recently faced a similar issue. I represented the data as a 2d array, relatively easy to type and parse, and wrote a utility method to parse it into the data structure. e.g. for your case

static String[][] CANADA_DATA = {
  {"BC"," AB"},
  {"AB","BC","SK"},
  // rest of Canada here
}

Example code

public Map<String, List<String>> parseIt() {

   Map<String, List<String>> map = new HashMap();

   for (String[] provinceData : CANADA_DATA ) {
      String name = provinceData [0];
      ArrayList neighbors = new ArrayList(Arrays.asList(provinceData ));
      neighbors.remove(0);  // remove ourself
      map.put(name, neighbors);
   } 

   return map; 
}

Obviously you can change the data format and parsing code to fit your specific needs.

I agree with Louis and didn't intend to add anything.

The use of streams in this case allows you to compact the code into one line but I realize this is not an answer to your question (just to closest I could think of).

        Map<String, List<String>> adjList = Stream.of(
            new SimpleEntry<>("Edmonton", Arrays.asList("E N1", "E N2", "E N3")), 
            new SimpleEntry<>("Vancouver", Arrays.asList("V N1", "V N2", "V N3")))
            .collect(Collectors.toMap((e) -> e.getKey(), (e) -> e.getValue()));

Yes, you can: Parse it as json:

import com.fasterxml.jackson.databind.ObjectMapper;

String json = "{'BC': ['AB']," +
              "'AB': ['BC', 'SK']," +
              "'SK': ['AB', 'MB']," +
              "'MB': ['SK']"
              "}";

Map<String, Object> map = new ObjectMapper().readValue(json, HashMap.class);