Java – what does this’ static ‘mean and why

public class tt {

public class tt {
static{
    System.out.println("class tt");
    }
}

This is the first time I have met it. I want to know what it is and what it is used for

Solution

It is the static initializer of this class When the class is loaded, the static initializer runs It's like a constructor, but for classes, not individual objects

Multiple static initializers can appear in classes or in direct initializers of static variables These will be combined into one initializer in the declared order For example, whenever a class is loaded, the following will print "foo" to stdout (usually once per application)

public class Foo {

  static String a;

  static {
    a = "foo";
  }

  static String b = a;

  static {
    System.println(b);
  }

}