Use of Initializers vs Constructors in Java

Question

So I've been brushing up on my Java skills as of late and have found a few bits of functionality that I didn't know about previously. Static and Instance Initializers are two such techniques.

My question is when would one use an initializer instead of including the code in a constructor? I've thought of a couple obvious possibilities:

• static/instance initializers can be used to set the value of "final" static/instance variables whereas a constructor cannot

• static initializers can be used to set the value of any static variables in a class, which should be more efficient than having an "if (someStaticVar == null) // do stuff" block of code at the start of each constructor

Both of these cases assume that the code required to set these variables is more complex than simply "var = value", as otherwise there wouldn't seem to be any reason to use an initializer instead of simply setting the value when declaring the variable.

However, while these aren't trivial gains (especially the ability to set a final variable), it does seem that there are a rather limited number of situations in which an initializer should be used.

One can certainly use an initializer for a lot of what is done in a constructor, but I don't really see the reason to do so. Even if all constructors for a class share a large amount of code, the use of a private initialize() function seems to make more sense to me than using an initializer because it doesn't lock you into having that code run when writing a new constructor.

Am I missing something? Are there a number of other situations in which an initializer should be used? Or is it really just a rather limited tool to be used in very specific situations?

Answer 1

As you mentioned, it's not useful in a lot of cases and as with any less-used syntax, you probably want to avoid it just to stop the next person looking at your code from spending the 30 seconds to pull it out of the vaults.

On the other hand, it is the only way to do a few things (I think you pretty much covered those).

Static variables themselves should be somewhat avoided anyway--not always, but if you use a lot of them, or you use a lot in one class, you might find different approaches, your future self will thank you.

Answer 2

I most often use static initializer blocks for setting up final static data, especially collections. For example:

public class Deck {
  private final static List<String> SUITS;

  static {
    List<String> list = new ArrayList<String>();
    list.add("Clubs");
    list.add("Spades");
    list.add("Hearts");
    list.add("Diamonds");
    SUITS = Collections.unmodifiableList(list);
  }

  ...
}

Now this example can be done with a single line of code:

private final static List<String> SUITS =
  Collections.unmodifiableList(
    Arrays.asList("Clubs", "Spades", "Hearts", "Diamonds")
  );

but the static version can be far neater, particularly when the items are non-trivial to initialize.

A naive implementation may also not create an unmodifiable list, which is a potential mistake. The above creates an immutable data structure that you can happily return from public methods and so on.

Answer 3

Anonymous inner classes can't have a constructor (as they're anonymous), so they're a pretty natural fit for instance initializers.

Answer 4

Static initializers are useful as cletus mentioned and I use them in the same manner. If you have a static variable that is to be initialized when the class is loaded, then a static initializer is the way to go, especially as it allows you to do a complex initialization and still have the static variable be final. This is a big win.

I find "if (someStaticVar == null) // do stuff" to be messy and error prone. If it is initialized statically and declared final, then you avoid the possibility of it being null.

However, I'm confused when you say:

static/instance initializers can be used to set the value of "final" static/instance variables whereas a constructor cannot

I assume you are saying both:

• static initializers can be used to set the value of "final" static variables whereas a constructor cannot
• instance initializers can be used to set the value of "final" instance variables whereas a constructor cannot

and you are correct on the first point, wrong on the second. You can, for example, do this:

class MyClass {
    private final int counter;
    public MyClass(final int counter) {
        this.counter = counter;
    }
}

Also, when a lot of code is shared between constructors, one of the best ways to handle this is to chain constructors, providing the default values. This makes is pretty clear what is being done:

class MyClass {
    private final int counter;
    public MyClass() {
        this(0);
    }
    public MyClass(final int counter) {
        this.counter = counter;
    }
}

Answer 5

A static initializer is the equivalent of a constructor in the static context. You will certainly see that more often than an instance initializer. Sometimes you need to run code to set up the static environment.

In general, an instance initalizer is best for anonymous inner classes. Take a look at JMock's cookbook to see an innovative way to use it to make code more readable.

Sometimes, if you have some logic which is complicated to chain across constructors (say you are subclassing and you can't call this() because you need to call super()), you could avoid duplication by doing the common stuff in the instance initalizer. Instance initalizers are so rare, though, that they are a surprising syntax to many, so I avoid them and would rather make my class concrete and not anonymous if I need the constructor behavior.

JMock is an exception, because that is how the framework is intended to be used.

Answer 6

Just to add to some already excellent points here. The static initializer is thread safe. It is executed when the class is loaded, and thus makes for simpler static data initialization than using a constructor, in which you would need a synchronized block to check if the static data is initialized and then actually initialize it.

public class MyClass {

    static private Properties propTable;

    static
    {
        try 
        {
            propTable.load(new FileInputStream("/data/user.prop"));
        } 
        catch (Exception e) 
        {
            propTable.put("user", System.getProperty("user"));
            propTable.put("password", System.getProperty("password"));
        }
    }

versus

public class MyClass 
{
    public MyClass()
    {
        synchronized (MyClass.class) 
        {
            if (propTable == null)
            {
                try 
                {
                    propTable.load(new FileInputStream("/data/user.prop"));
                } 
                catch (Exception e) 
                {
                    propTable.put("user", System.getProperty("user"));
                    propTable.put("password", System.getProperty("password"));
                }
            }
        }
    }

Don't forget, you now have to synchronize at the class, not instance level. This incurs a cost for every instance constructed instead of a one time cost when the class is loaded. Plus, it's ugly ;-)