java - Android:I do not understand: static { foo();}

Sometimes I see something like it, in a class in Android:

static
{
    foo();
}

• What does this do?




• Why?

Answer

That's a static block. It is executed the first time that class is referenced on your code, and it is calling a static method called foo(). You can find more about the static block here. As mentioned by @CommonsWare, you can initialize a static field in two different ways, inline a declaration time

static ArrayList test = new ArrayList() {{
    add("A");
    add("B");
    add("C");
}};

but as you can see it is not easy to read. If you use a static block instead

  static ArrayList test;
  static {
        test = new ArrayList<>();
        test.add("a");
        test.add("b");
        test.add("c");
    } 

or as in your question have

static ArrayList test;
static {
  foo();
}

private static void foo() {
    test = new ArrayList<>();

    test.add("a");
    test.add("b");
    test.add("c");
}