Java: When is a static initialization block useful?

java static-initialization
Adam Matan picture Adam Matan · Feb 21, 2012 · Viewed 105.9k times · Source

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

public class staticTest {

    static String s;
    static int n;
    static double d;

    static {
        s = "I'm static";
        n = 500;
        d = 4000.0001;
    }
    ...

And individual static initialization:

public class staticTest {

    static String s = "I'm static";
    static int n    = 500;
    static double d = 4000.0001;

    ....

Answer

Rich O'Kelly picture Rich O'Kelly · Feb 21, 2012

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
}

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