How to implement an interface

• To declare a class that implements an interface:
• you use the implements keyword
• you provide an implementation for each method defined by the interface
• If you forget to implement an interface method, the compiler will issue an error message.
• A class that implements an interface can use any constant defined by that interface.

The syntax for implementing an interface

public class ClassName
    implements Interface1[, Interface2]...{}

A class that implements two interfaces

import java.text.NumberFormat;

public class Employee implements Printable,
                                 DepartmentConstants
{
    private int department;
    private String firstName;
    private String lastName;
    private double salary;

    public Employee(int department, String lastName,
        String firstName, double salary)
    {
        this.department = department;
        this.lastName = lastName;
        this.firstName = firstName;
        this.salary = salary;
    }

    public void print()
    {
        NumberFormat currency =
            NumberFormat.getCurrencyInstance();
        System.out.println("Name:\t" + firstName + " " + lastName);
        System.out.println("Salary:\t" + currency.format(salary));

        String dept = "";
        if (department == ADMIN)
            dept = "Administration";
        else if (department == EDITORIAL)
            dept = "Editorial";
        else if (department == MARKETING)
            dept = "Marketing";

        System.out.println("Dept:\t" + dept);
    }
}