public class Main {

  public static void main(String[] args) {

    System.out.println(Math.acos(0.64));

    System.out.println(Math.acos(-0.4));

    System.out.println(Math.acos(0));

    System.out.println(Math.acos(1));

    System.out.println(Math.acos(-1));

    System.out.println(Math.acos(2));

  }

}

​