Question

What is Scala equivalent of Java’s static block ?

Answer 1

Code in the constructor (that is, the body) of the companion object is not precisely the same as code in a static initialiser block of a Java class. In the example below, I create an instance of A, but the initialization does not occur.

scala> object Test { class A; object A { println("A.init") }}        
defined module Test

scala> new Test.A
res3: Test.A = Test$A@3b48a8e6

scala> Test.A
A.init
res4: Test.A.type = Test$A$@6e453dd5

To trigger construction of the companion object when the first instance of the class is created, you could access it from the class constructor.

scala> object Test { class A { A }; object A { println("A.init") }}
defined module Test

scala> new Test.A                                                  
A.init
res5: Test.A = Test$A@4e94a28e

scala> new Test.A
res6: Test.A = Test$A@30227d4e

In many circumstances, the difference would not matter. But if you are launching missiles (or other side effects), you might care!