Coming Up for Air

Funky Object Initialization

Tuesday, October 25, 2011 |

I’ve been using a technique a lot, recently, for initializing an object a bit more succinctly. It looks pretty odd, I’ll admit, enough so that it really caught a coworker of mine off guard. If you’ve been reading my recent REST posts, you’ve seen this a few times. I like it a lot, so I thought I’d a take a quick look at it.

Here’s the technique in question:

Map<String, Object> map =  new HashMap<String, Object>() {{
    put("foo", "abc");
    put("bar", "123");

I’m not a language geek by any reasonable stretch of the imagination, so I can’t see in definitive (or maybe even accurate) terms what’s going on here, but it seems to me to be very much like the instantiation of an anonymous inner class. We’ve all seen those:

Thread t = new Thread(name) {
    public void run() {
        // foo

Nothing unusual there. What makes this so interesting is (if my educated guess is correct) is the static initializer block we’ve declared ({ put…​; put…​;}). Technically, I think we’re subclassing HashMap and specifying a static initializer. Technicalities aside, what this does is allow us to execute initialization code in the context of the newly created object before it’s returned to the enclosing code. What I like about this is that it makes the code much more concise, and can be collapsed in IDEs and editors that support such things. It also allows me to create a Map (or List or Set or Foo or…​) and initialize it inside a method call:

someObject.someMethodThatTakesAList(new List<String>() {{

That’s some fancy kung fu. :)

I didn’t invent or discover this technique, but I’ve found it to be extremely useful. Now that you have it in your toolbox, maybe you will too.



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    My name is Jason Lee. I am a software developer living in the middle of Oklahoma. I’ve been a professional developer since 1997, using a variety of languages, including Java, Javascript, PHP, Python, Delphi, and even a bit of C#. I currently work for Red Hat on the WildFly/EAP team, where, among other things, I maintain integrations for some MicroProfile specs, OpenTelemetry, Micrometer, Jakarta Faces, and Bean Validation. (Full resume here. LinkedIn profile)

    I am the president of the Oklahoma City JUG, and an occasional speaker at the JUG and a variety of technical conferences.

    On the personal side, I’m active in my church, and enjoy bass guitar, running, fishing, and a variety of martial arts. I’m also married to a beautiful woman, and have two boys, who, thankfully, look like their mother.

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