Java Instance Initializers

I've always been aware of java static initializers, and have been quite comfortable around them

public class Foo {
    private static final Map<String, Object> CACHE;
    static {
        CACHE = new HashMap<String,Object>();
        CACHE.put("foo", new Object());
        CACHE.put("bar", new Object());
    }
}

But I saw this the other day

public class Foo {
    private final List<String> emailAddresses;
    {
        emailAddresses = new ArrayList<String>();
        emailAddresses.add("foo@bar.com");
    }
}

Did some research and looks like an instance initializer, which is just like a static initializer except is run every time an instance of the class is created.

Very useful if there are multiple constructors for a class, and you don't want to keep running the same initialization code over and over again.

Some interesting examples here