C#
Summary - Interfaces
Combine multiple interfaces to write a payment system.
Summary - Interfaces
In this group you learned interfaces: an interface is a contract, a class implements it with :, multiple interfaces are comma-separated, default methods have a body and you saw the interface vs abstract distinction.
Interfaces unite different classes under a common contract and are the foundation of polymorphism. The next group dives into abstract classes: abstract methods, the virtual vs abstract distinction and abstract class hierarchies.
Exercise Printing interface (summary)
Define the IPrintable interface with the void Print(); method signature. Document and Main are already in place; you only add the interface.
Show example solution
Add `interface IPrintable { void Print(); }`.
Example solution:
using System;
interface IPrintable
{
void Print();
}
class Document : IPrintable
{
public void Print()
{
Console.WriteLine("Printed");
}
}
class Program
{
static void Main()
{
IPrintable d = new Document();
d.Print();
}
}
Exercise Logger (summary)
Define the FileLogger class: it implements ILogger. The public void Log(string message) method does Console.WriteLine("File: " + message);. In Main, create ILogger logger = new FileLogger(); and call logger.Log("Record").
Show example solution
ILogger { public void Log(string message) { Console.WriteLine("File: " + message); } }`. In Main write `ILogger logger = new FileLogger(); logger.Log("Record");`.
Example solution:
using System;
interface ILogger
{
void Log(string message);
}
class FileLogger : ILogger
{
public void Log(string message)
{
Console.WriteLine("File: " + message);
}
}
class Program
{
static void Main()
{
ILogger logger = new FileLogger();
logger.Log("Record");
}
}
Next step
Interfaces are a powerful tool for contracts and polymorphism. With default methods you preserve backward compatibility and with multiple interfaces you give classes different roles. The next group covers abstract classes: sharing common behavior, abstract methods and sealed constraints.
Remember: an interface says "can-do"; an abstract class says "is-a". Picking the right tool keeps the design clean.
Test yourself
-
Fill in the blank: interface method call
ILogger logger = new FileLogger();
logger.____("Record");
Show answer
ILogger logger = new FileLogger();
logger.Log("Record");
-
Which statement about interfaces is correct?
- An interface can be instantiated with new.
- An interface can hold fields.
- An implementing class must implement all the interface's methods.
- An interface can be implemented by at most one class.
Show answer
An implementing class must implement all the interface's methods. — Correct. The implementing class must provide all the interface's methods.
Next lesson: abstract class
