public static double acos(double a)

// Java program to demonstrate working
// of java.lang.Math.acos() method
import java.lang.Math;
  
class Gfg {
  
    // driver code
    public static void main(String args[])
    {
        double a = Math.PI;
  
        // Output is NaN, because Math.PI gives 3.141 value
        // greater than 1
        System.out.println(Math.acos(a));
  
        // convert Math.PI to radians
        double b = Math.toRadians(a);
  
        System.out.println(Math.acos(b));
  
        double c = 1.0;
        double d = 0.0;
        double e = -1.0;
        double f = 1.5;
  
        System.out.println(Math.acos(c));
        System.out.println(Math.acos(d));
        System.out.println(Math.acos(e));
   
       // value of f does not lie in between -1 and 1 
       // so output is NaN
        System.out.println(Math.acos(f));
    }
}

NaN
1.5159376794536454
0.0
1.5707963267948966
3.141592653589793
NaN