Java: When is a static initialization block useful?

Question

What\'s the difference between initialization within a static block:

public class staticTest {

    sta         


        

Answer 1

A static initialization blocks allows more complex initialization, for example using conditionals:

static double a;
static {
    if (SomeCondition) {
      a = 0;
    } else {
      a = 1;
    }
}

Or when more than just construction is required: when using a builder to create your instance, exception handling or work other than creating static fields is necessary.

A static initialization block also runs after the inline static initializers, so the following is valid:

static double a;
static double b = 1;

static {
    a = b * 4; // Evaluates to 4
}

Answer 2

Exception handling during initialization is another reason. For example:

static URL url;
static {
    try {
        url = new URL("https://blahblah.com");
    }
    catch (MalformedURLException mue) {
        //log exception or handle otherwise
    }
}

This is useful for constructors that annoyingly throw checked exceptions, like above, or else more complex initialization logic that might be exception-prone.

Answer 3

The static code block enables to initialize the fields with more than instuction, initialize fields in a different order of the declarations and also could be used for conditional intialization.

More specifically,

static final String ab = a+b;
static final String a = "Hello,";
static final String b = ", world";

will not work because a and b are declared after ab.

However I could use a static init. block to overcome this.

static final String ab;
static final String a;
static final String b;

static {
  b = ", world";
  a = "Hello";
  ab = a + b;
}

static final String ab;
static final String a;
static final String b;

static {
  b = (...) ? ", world" : ", universe";
  a = "Hello";
  ab = a + b;
}

Answer 4

Technically, you could get away without it. Some prefer multiline initialisation code to go into a static method. I'm quite happy using a static initialiser for relatively simple multistatement initialisation.

Of course, I'd almost always make my statics final and point to an unmodifiable object.

Answer 5

In your example, there is no difference; but often the initial value is more complex than is comfortably expressed in a single expression (e.g., it's a List<String> whose contents are best expressed by a for-loop; or it's a Method that might not exist, so exception-handlers are needed), and/or the static fields need to be set in a specific order.

Answer 6

static block can be used to initialize singleton instance, to prevent using synchronized getInstance() method.