Interfaz.java

import java.util.Scanner;

interface CalculoCirculo {
    public final double PI = 3.1415;

    public float area();
    public float perimetro();
    public float diametro();
}

class Circulo implements CalculoCirculo {
    private float radio;

    public Circulo(float radio) {
        this.radio = radio;
    }

    public float area() {
        return (float)PI * (this.radio * this.radio);
    }

    public float perimetro() {
        return 2 * (float)PI * this.radio;
    }

    public float diametro() {
        return 2 * this.radio;
    }
}

public class Interfaz {
    public static void main(String[] args) {
        Scanner entrada = new Scanner(System.in);

        System.out.printf("Ingrese el radio: ");
        int radio = entrada.nextInt();

        Circulo circulo1 = new Circulo(radio);
        System.out.println("Mi area es: " + circulo1.area());
        System.out.println("Mi perimetro es: " + circulo1.perimetro());
        System.out.println("Mi diametro es: " + circulo1.diametro());
    }
}