IntStream is = IntStream.of(1, 3, 5);
int x = is.filter(i->i%2 == 0).average(); //1
System.out.println(x);
int y = is.filter( i->i%2 != 0 ).sum();//2
System.out.println(y);
is = IntStream.of(1, 3, 5, 9);
int z = is.filter( i->i%3 != 0 ).count();//3
System.out.println(z);

class Student {
    public static enum Grade {A, B, C, D, F}
    private String name;
    private Grade grade;
    public Student(String name, Grade grade) {
        this.name = name;
        this.grade = grade;
    }
    public String toString() {
        return name + ":" + grade;
    }
}
class TestClass {
    public static void main(String[] args) {
        var ls = Arrays.asList(new Student("S1", Student.Grade.A),
                new Student("S2", Student.Grade.A),
                new Student("S3", Student.Grade.B),
                new Student("S4", Student.Grade.C),
                new Student("S5", Student.Grade.F));
        var group = ls.stream()
                .filter(student -> student.getGrade() != Student.Grade.F)
                .collect(Collectors.groupingBy(Student::getGrade,
                        Collectors.mapping(Student::getName, Collectors.toList())));
        System.out.println(group);
    }
}

var collection = new HashSet<>();
collection.add(1);
var list1 = List.of(collection);
collection.add(2);
var list2 = List.copyOf(collection);
System.out.println(list1);
System.out.println(list2);

var collection = Arrays.asList(1,2,3);

class X {
        public void print(List<Integer> lst) {
            System.out.println("X list " + lst);
        }
    }
class Y extends X {
        public void print(List<Integer> lst) {
            System.out.println("Y list " + lst);
        }
    }
public class TestClass {
    public static void main(String[] args) {
        X x = new Y();
        Y y = new Y();
        var list = new ArrayList<>();
        list.add(1);
        x.print(list);
        y.print(list);
    }
}

public List<Integer> getCollection() {
    return new ArrayList<Integer>();
           }

public List<? super Integer> getCollection() {
        return new ArrayList<Integer>();
    }
public List<? extends Integer> getCollection() {
        return new ArrayList<Integer>();
    }
public ArrayList<Integer> getCollection() {
        return new ArrayList<Integer>();
    }

interface Bird {
     default String fly() {
        return "fly";
    }
}
interface Parrot extends Bird {
     public String fly();
}
public class RainbowParrot implements Parrot {
    public static void main(String[] args) {
        Bird b = new RainbowParrot();
        System.out.println(b.fly());
    }
}