Classes

Generics

Generics allow you to create a class that can hold many different kinds of data types, and specify an upper bound for what kind of interfaces are allowed to be stored and manipulated.

Nested Classes

You can also have classes defined within another classes. If the nested class is static, then it can be instantiated from the class itself just like any other static methods. Otherwise, you need an instance of the class to access the nested class.

Static

public class Employee {
    public static class EmployeeComparator <T extends Employee> implements Comparator<Employee> {
        private String sortType;

        public EmployeeComparator(String sortType) {
            this.sortType = sortType;
        }

        @Override
        public int compare(Employee o1, Employee o2) {
            if (sortType == "yearStarted") {
                // sort by year started
            }
            // able to access private variables in Employee, and vice versa
            return o1.name.compareTo(o2.name);
        }
    }

    private int employeeId;
    private String name;
    private int yearStarted;
}

public static void main() {
    List<Employee> employees = new ArrayList<>();

    employees.sort(new Employee.EmployeeComparator<>());
}

Non-static

public class StoreEmployee extends Employee {
    private String store;

    public class StoreComparator <T extends Employee> implements Comparator<StoreEmployee> {
        public int compare(StoreEmployee o1, StoreEmployee o2) {
            int result = o1.store.compareTo(o2.store);
            if (result == 0) {
                return new Employee.EmployeeComparator<>("yearStarted").compare(o1, o2);
            }
            return result;
        }
    }
}

public static void main() {
    List<StoreEmployee> storeEmployees = new ArrayList<>();

    var genericEmployee = new StoreEmployee();
    var comparator = genericEmployee.new StoreComparator<>();
    var comparator = new StoreEmployee().new StoreComparator<>();
    storeEmployees.sort(comparator);
}