The limits of double-brace initialization

Submitted by rherrick
28 Dec 2011

One thing I miss in Java is C#'s in-line object initialization syntax, where you can create the new object and set properties all in the same statement. So take something like this:




	
	private class Cat





	{



	    // Auto-implemented properties.



	    public int Age { get; set; }



	    public string Name { get; set; }



	}



	 



	Cat cat = new Cat { Age = 10, Name = "Fluffy" };



	That's nice and Java didn't seem to have it. Well, it does but it's just not very well advertised. It's called double-brace initialization. It actually works a bit differently, but is similar enough. Once you've created a new object (including passing parameters to the constructor), you just follow up with double braces and insert a bunch of statements without qualifying the owning object:





	
	GridBagConstraints SPANNING = new GridBagConstraints() {{





	    gridx = 0;



	    gridwidth = 2;



	    insets = new Insets(8, 0, 0, 0);



	}};



	Great! But... I'm not really sure why this happens, but delegates and listeners don't seem to work properly. So I did something like this:



	 


private final JDateChooser _dateChooser = new JDateChooser() {{ addPropertyChangeListener("date", this); }};



	The containing class implements the PropertyChangeListener interface. With this configuration, my propertyChange() method isn't invoked when a property is changed. If I remove the double-brace initialization, though, and move listener configuration into the constructor, all is well.



	 



	So an interesting limitation on this technique. I don't know if this is such a great idea anyways: lots of people are unfamiliar with the syntax, there are some questions about performance, and the Java site itself doesn't seem to be a great deal of help in this regard. But, FWIW...