Static field with program code

Question

I have learned that in a class you can declare variables and methods. They can both be declared as static if that is prefered.

Now I have encountered a program example I do not really understand. After some variable declaration in the class there is a field declared as static and inside there is program code.

When is this code executed? My guess is that the code is executed like following when a new object is created:

Memory allocation for the instance variables.
Execution of the contructors connected to the instance variables.
Exection of the static field
Execution of the constructor (if "= new Constructor() is used when the object is created)

If I execute

MyCars myCars = new MyCars();

the following will happen in this order?

public class MyCars
{
    private Car volvo = new Car()
 //              (1)       (2)
    static
    {
        volvo.setNumberOfWheels = 4;
 //                  (3)
    }

    public MyCars()
    {
         volvo.setBrand = "Volvo";
         volvo.setModel = "XC70";
                 (4)
    }

Here is the original code:

public class SettingsSetter extends ListActivity
{
    private static Map<Integer,String> menuActivities=new HashMap<Integer,String>();
    private static List<BooleanSetting> settings=new ArrayList<BooleanSetting>();

    static 
    {
        menuActivities.put(R.id.app,
            Settings.ACTION_APPLICATION_SETTINGS);
        menuActivities.put(R.id.security, Settings.ACTION_SECURITY_SETTINGS);
        menuActivities.put(R.id.wireless,
            Settings.ACTION_WIRELESS_SETTINGS);
        menuActivities.put(R.id.all,
            Settings.ACTION_SETTINGS);

        settings.add(new BooleanSetting(Settings.System.INSTALL_NON_MARKET_APPS,
            "Allow non-Market app installs", true));
        settings.add(new BooleanSetting(Settings.System.HAPTIC_FEEDBACK_ENABLED,
            "Use haptic feedback", false));
        settings.add(new BooleanSetting(Settings.System.ACCELEROMETER_ROTATION,
            "Rotate based on accelerometer", false));
    }

Is your example supposed to make volvo static like your real code? — corsiKa
– corsiKa, Commented Jan 3, 2012 at 9:01

Amadan · Accepted Answer · 2012-01-03 09:05:58Z

5

Static initialisers are executed when the class is loaded, so before any instances are created. Your first code is in error, since volvo is an instance (non-static) variable, and the static block cannot access it. Revisiting your example:

public class MyCars
{
    private static Car audi = new Car()
 //                     (2)
    private Car volvo = new Car()
 //              (5)       (6)
    static
    {
        audi.setNumberOfWheels = 4;
 //                  (3)
    }
    public MyCars()
    {
         volvo.setBrand = "Volvo";
         volvo.setModel = "XC70";
 //              (7)
    }
}

MyCars myCars = new MyCars();
// (1)                (4)

edited Jan 3, 2012 at 9:05

answered Jan 3, 2012 at 8:56

Amadan

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6 Comments

Eric1101000101 Over a year ago

Thank you @Amadan (and all other). Now it's more clear to me. Is this correct: Your number 1 is load class and number 4 is instance creation? If I write MyCars myCars; MyCars myNeighboursCars; Will the class be loaded twice?

Amadan Over a year ago

No, a class is only loaded once - and thus, static blocks and initialisers are only executed once.

Eric1101000101 Over a year ago

Interesting this. Are the class loaded at first use (1 above) or are all the classes in the project loaded directly before main is executed?

Amadan Over a year ago

AFAIK, first use. Test for yourself: class A { static { System.out.println("A"); } } and then class B { public static void main(String[] args) { A a; System.out.println("B"); a = new A(); } } - the output is "B" then "A", so the class is loaded just before the first object of A is constructed (not declared, not at start of the program). You can also load it manually (for example, using Class.forName("B")) - by which you can force class load to happen earlier than the default class loader would make it.

Eric1101000101 Over a year ago

I tried that out and it works as you tell me. With that in mind, shouldn't the answer above be revised. At 1 we only reserve memory for a pointer (that later will be pointing to the object "new MyCars()"). 4 -> 2, 2 -> 3, 3 -> 4.

|

Michael Borgwardt · Accepted Answer · 2012-01-03 08:56:54Z

0

That's not a field, it's a static initializer. It will be executed once when the class is first loaded. It will not be executed when an instance of the class is initialized.

BTW, your MyCars code will not compile because a static initializer cannot refer to instance variables, as it will be executed before there are any instances of the class.

answered Jan 3, 2012 at 8:56

Michael Borgwardt

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Comments

Milo Casagrande · Accepted Answer · 2012-01-03 09:02:05Z

0

It is a static block, ora a static initialization block, that is executed when the class is loaded into the JVM. Whatever you have inside, it will be automatically executed at load time.

It is also possible to have multiple static blocks inside the same class, the JVM garauntees you that they will be executed in the order they appear.

answered Jan 3, 2012 at 9:02

Milo Casagrande

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