'Allow Math.PI as a reference to the PI constant' means that your code will have to look like this in order to work:

static double getCircumference(double radius ) {
      return Math.PI * 2 * radius;
 }
 public static double getArea(double radius) {
      return Math.PI * radius * radius;
 }

What import java.lang.Math; does is importing the class java.lang.Math so you can reference it with Math instead of the qualified version java.lang.Math. import java.lang.Math.*; does the same for Math and all nested classes, but not it's members.

Here is usage of Math.PI to find circumference of circle and Area First we take Radius as a string in Message Box and convert it into integer

public class circle {

    public static void main(String[] args) {
        // TODO code application logic here

        String rad;

        float radius,area,circum;

       rad = JOptionPane.showInputDialog("Enter the Radius of circle:");

        radius = Integer.parseInt(rad);
        area = (float) (Math.PI*radius*radius);
        circum = (float) (2*Math.PI*radius);

        JOptionPane.showMessageDialog(null, "Area: " + area,"AREA",JOptionPane.INFORMATION_MESSAGE);
        JOptionPane.showMessageDialog(null, "circumference: " + circum, "Circumfernce",JOptionPane.INFORMATION_MESSAGE);
    }

}