On static and non-static initializing blocks in Java

Question

I originally thought that static blocks were for static variables but the compiler allows both A and B to compile and run, what gives?
A

   private static final Map<String,String> m = new HashMap<String,String>();

        {
            m.put("why", "does");
            m.put("this","work");
        }

B

 private static final Map<String,String> m = new HashMap<String,String>();

        static{
               m.put("why", "does");
               m.put("this","work");
             }

Running System.out.println(Main.m.toString()); for A prints

{}

but running the same for B prints out in Yoda-speak

{this=work, why=does}

Is there an alternative to static and non-static blocks?

Dimitri Dewaele
– Dimitri Dewaele

2013-08-07 06:43:05 +00:00
Commented Aug 7, 2013 at 6:43 — Dimitri Dewaele
– Dimitri Dewaele, Commented Aug 7, 2013 at 6:43

Community · Accepted Answer · 2017-05-23 12:25:39Z

13

The non static block is executed when an "instance" of the class is created.

Thus

System.out.println(Main.m.toString());

prints nothing because you haven't created an instance.

Try creating an instance first

 Main main = new Main();

and you'll see the same message as B

As you know class variables (declared using static) are in scope when using instance blocks.

1 Comment

non sequitor Over a year ago

Which makes perfect logical sense

erickson · Accepted Answer · 2009-10-15 20:43:45Z

6

In A, you have an instance initializer. It will be executed each time you construct a new instance of A.

If multiple threads are constructing A instances, this code would break. And even in a single thread, you normally don't want a single instance to modify state that is shared by every instance. But if you did, this is one way to achieve it.

answered Oct 15, 2009 at 20:43

erickson

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1 Comment

aperkins Over a year ago

Erickson is correct - create a new instance, and they will be equivalent. And if you change the values for those keys around, and then create another new instance, they will be replaced with the original values again.

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