Question

Write the first line of a class named Customer, which implements an interface named Relatable.

Short answer

Answer: The first line of the Customer class is the class declaration, which is: `public class Customer implements Relatable`.

Step-by-step solution

Define the interface Relatable

To create the interface, we write: ```java public interface Relatable { // Interface methods to be implemented by the Customer class } ```

Create the Customer class

To create the Customer class that implements the Relatable interface, we write: ```java public class Customer implements Relatable { // Customer class implementation } ``` The first line of the Customer class is the class declaration, which is: `public class Customer implements Relatable`.

Key concepts

Defining Java Interfaces

When programming in Java, an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces. Definition of interfaces is crucial when it comes to establishing a formal contract between different parts of your code.

To define an interface in Java, you use the interface keyword, followed by the name of the interface and a body enclosed in braces. Here's an example:
public interface Relatable {
// Abstract methods that classes implementing this interface must provide
}

An interface serves as a blueprint for classes. It spells out methods that the implementing classes must provide, however, it does not provide any method implementations. This approach is essential in enforcing a certain structure while providing the flexibility to have different implementations based on the specific needs of the classes.

Implementing Interfaces in Java

After defining an interface, the next step is to implement it in a Java class. Implementing an interface is a straightforward process that involves a class declaring that it will provide concrete implementations of the interface's abstract methods.

Here's how you can implement an interface in a class:
public class Customer implements Relatable {
// Class members and method implementations
}

The implements keyword is used to declare that a class will abide by the contract defined by the interface. This definition requires the class to provide bodies for all abstract methods described in the interface. If a class fails to implement any of the interface's methods, the class must be declared abstract. An interface can be implemented by any number of classes, leading to a form of polymorphism in Java known as 'interface inheritance'.

Java Class Structure

A class in Java is a blueprint for creating objects and can include variables, methods, constructors, and more. The structure of a Java class is organized and specific, as it helps to encapsulate the data and methods for the desired objects.

The class structure typically begins with access modifiers such as public, followed by the class keyword and the class name. The implements keyword is used if the class is implementing any interfaces. Following that, curly braces enclose the class body, containing the constructor, variables, and methods.

Here’s an example of a typical class structure implementing an interface:
public class Customer implements Relatable {
// Instance variables
// Constructors
// Methods defined by the Relatable interface
// Additional methods and functionality
}

A well-defined class structure is pivotal in making the code easier to read, maintain, and debug. A class that implements an interface must provide concrete implementations for all its declared methods, promoting consistent and predictable behavior across different classes that implement the same interface.