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Lone braces inside a class?

I couldn't find anything about this online, because I didn't know what to search for, but Java doesn't mark this code as having an error:

public class Test {
    // ...

    {
        int test;
    }

    // ...
}

Does this serve any purpose in Java? If so, what?

Solution

  • This is called an instance initializer (JLS section 8.6)

    When creating an object the instance initializer is run after super constructors but before the called constructor of the class they are defined in. See JLS 12.5 Creation of New Class Instances. Specifically, instance initializers are evaluated in step 4 of the object creation process. The key point is that instance initiailzers always get called no matter what constructor is used to instantiate the object.

    There are also static initializers which are similar but marked with the static keyword. 

    public class Test {

    static {
       // Do something interesting on class load.
    }

}

    In my experience static intiailzers are more common as you can use them to setup complex class state (like linking JNI libraries) when the class is loaded.