double acos(double d)

public class Test { 

   public static void main(String args[]) {
      double degrees = 45.0;
      double radians = Math.toRadians(degrees);

      System.out.format("The value of pi is %.4f%n", Math.PI);
      System.out.format("The arccosine of %.4f is %.4f degrees %n", Math.cos(radians),
         Math.toDegrees(Math.acos(Math.cos(radians))));
   }
}

The value of pi is 3.1416
The arccosine of 0.7071 is 45.0000 degrees