Anonymous Classes

Main.java

interface Age {
    public void checkAge();
    public Student[] getStudents();
}

class Student {
    private int id;
    private String name;
    private int age;

    public int getId() { return this.id; }
    public String getName() { return this.name; }
    public int getAge() { return this.age; }

    public void setId(int id) { this.id = id; }
    public void setName(String name) { this.name = name; }
    public void setAge(int age) { this.age = age; }

    @Override public String toString() { return name + " " + age + " " + id; }
}

abstract class Students implements Age {
    private Student[] students;

    public Students(Student[] students) { this.students = students; }

    @Override public Student[] getStudents() { return this.students; }
}

public class Main {

    public static void main(String[] args) {

        Student stu = new Student();
        stu.setId(1);
        stu.setName("John");
        stu.setAge(17);

        Student[] stus = new Student[30];
        for(int i = 0; i < stus.length; i++) {
            stus[i] = stu;
        }

        Age students = new Students(stus) {
            @Override
            public void checkAge() {
                for(Student student : this.getStudents()) {
                    if (student.getAge() < 18) {
                        System.out.println(student.toString());
                    }
                }
            }
        };

        students.checkAge();
    }
}