Instance initializer and static initializer blocks

Question

Hi guys,
I read about the above mentioned in the JLS and also in a book before, but I still don't quite understand, what is the use of these. I sort of have a rough idea, but not exactly. I mean, what is the purpose of the instance initializer and static initializer blocks, how can it be useful? I understand I can execute pieces of code that will initialize instance and static variables accordingly, but how is it different then to using a constructor to initialize these fields? Are these pieces of code executed before any constructor is executed, or when otherwise?
Sorry for my noob, I'm learning.
PR.

Answer 1

Static initializers are useful for initializing a class when the initialization is more complex than simply setting a single variable, or when that initialization can throw a checked exception.
public class Foo {
  private static final Bar bar;
  static {
    try {
      bar = new Bar();
      bar.doSomeInitializationStuff();
    catch (SomeCheckedExceptionThatBarThrows e) {
      throw new ExceptionInInitializerError(e);
}Here we could not do the two-step new Bar() + doSomeInit() stuff in the line where we declare the bar variable. Additionally, assuming that one or both of those can throw a checked exception, we could not do that on the declaration line; we need the static initializer to wrap that in the appropriate unchecked exception.
This allows us to do more complex class initialization when the class is loaded than we could do with a simple variable initialization.
Instance initializers are useful if you want to perform the same steps in every constructor and don't want to have to repeat the code in each constructor. Instance initializers are executed as the first step of each constructor (or maybe it's after any super() calls, I forget).