CODE EXAMPLE FOR JAVA

interface java

//Since Java9, Interface now has methods: 
//public abstract, public default, public static, private, private static
//default methods have body (implementation), is inherited by subclasses. 
//static methods is the same as regular static methods.
//private methods and private static methods are helper methods.

public interface CustomInterface {
    public abstract void method1();
    public default void method2() {
        method4();  //private method inside default method
        method5();  //static method inside other non-static method
        System.out.println("default method");
    }
    public static void method3() {
        method5(); //static method inside other static method
        System.out.println("static method");
    }
    private void method4(){
        System.out.println("private method");
    } 
    private static void method5(){
        System.out.println("private static method");
    } 
}
 
public class CustomClass implements CustomInterface {
    @Override
    public void method1() {
        System.out.println("abstract method");
    }
    public static void main(String[] args){
        CustomInterface instance = new CustomClass();
        instance.method1();
        instance.method2();
        CustomInterface.method3();
    }
}

/* Output:
abstract method
private method
private static method
default method
private static method
static method
*/

// Another example of default and private methods
import java.util.function.IntPredicate;
import java.util.stream.IntStream;
 
public interface CustomCalculator 
{
    default int addEvenNumbers(int... nums) {
        return add(n -> n % 2 == 0, nums);
    }
    default int addOddNumbers(int... nums) {
        return add(n -> n % 2 != 0, nums);
    }
    private int add(IntPredicate predicate, int... nums) { 
        return IntStream.of(nums)
                .filter(predicate)
                .sum();
    }
}

public class Main implements CustomCalculator {
     public static void main(String[] args) {
        CustomCalculator demo = new Main();
         
        int sumOfEvens = demo.addEvenNumbers(1,2,3,4,5,6,7,8,9);
        System.out.println(sumOfEvens);
         
        int sumOfOdds = demo.addOddNumbers(1,2,3,4,5,6,7,8,9);
        System.out.println(sumOfOdds);
    } 
}
/* output:
20
25
*/