Initializing Java Maps

Last week as I was performing a code review when I stumbled upon a little gem. I use Java almost everyday but I had never seen a concise way to initialize a Map in Java. For arrays I do: 

String[] allowed = new String[] {"text/html", "text/json"};

But what about Maps?

import java.util.*;
 
public class Router {
  public static final Map<String, Handler> routes;
 
  static {
    Map<String, Handler> map = new HashMap<String, Handler>();
    map.put("/account", new AccountHandler());
    map.put("/feed", new FeedHandler());
    map.put("/opml", new OpmlHandler());
    routes = Collections.unmodifiableMap(map);
  }
}

So did you know you could do this?

public static final Map<String , Handler> routes = 
           new HashMap<String , Handler>() {{
    put("/account", new AccountHandler());
    put("/feed", new FeedHandler());
    put("/opml", new OpmlHandler());
}};

I did not. I looked around the web looking for the precise answer and found very few examples of this syntax. I found a post by code_poet referring to the feature as “initializing block” and describing it as creating a subclass and initializing the map upon construction. I even skimmed the Java Specification Language looking for more details, but no luck. Any better explanations?

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Published:
03.08.08 / 2pm
Category:
Technology