InterfaceUsage.java

package com.winterbe.java8.samples.lambda;

/**
 * @author Benjamin Winterberg
 * @Message interface default define；static define
 */
public class InterfaceUsage {

    public static void main(String[] args) {
        Formula formula1 = new Formula() {
            @Override
            public double calculate(int a) {
                return sqrt(a * 100);
            }
        };

        double result1 = formula1.calculate(100); // 100.0
        double result2 = formula1.sqrt(-23); // 0.0
        int result3 = Formula.positive(-4); // 0.0

        // Formula formula2 = (a) -> sqrt(a * 100);

        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
    }
}

/**
 * can define function in a interface class; function can be static.
 */
interface Formula {

    double calculate(int a);

    default double sqrt(int a) {
        return Math.sqrt(positive(a));
    }

    static int positive(int a) {
        return a > 0 ? a : 0;
    }
}