An interface can extend another interface. In the same way a class can extend another class. If an entity signs a contract for the extended interface, all of the requirements of that interface and the interface it extends, must be met. A hasWheels interface extends the vehicle interface, meaning that along with wheels being a required numeric property, any entity that signs that hasWheels contract must have a numeric speed property and may have a Boolean flux capacitor and may have an accelerate method. Classes can implement multiple interfaces giving us fine control over our reusable components. The bus class shown on line 22 implements the hasWheels interface, along with the hasPayingPassengers interface. Values are given to all properties that are required to meet all of the contracts in the constructor and implementations are given for any methods that this class needs to provide. We then create an instance of the class on the right of the screen and use some of its methods logging the results out as we go. The interfaces can be reused across multiple classes. That covers what we need to start using interfaces. Looking at how we can define particular properties and or functions, that objects or classes must implement when we sign that contract. It's included looking at function types, optional properties, access property checks, indexable types, apply an interface to classes and extending interfaces. It'd be worth going getting some practice on that now.

Lectures

• An Introduction to Interfaces
• Function Types
• Optionals
• Excess Property Checks
• Indexable Types
• Interfaces and Classes