Classes and objects theory + first problem: create a Server class with hostname, IP, and status

easyClasses and objects

A class is a blueprint. An object is one real thing built from that blueprint.

The class Server says every server has a hostname, an IP and a status.
Each object holds its own values, such as web-01 or db-01.
The values stored inside an object are called fields.
A special function called the constructor runs when you create an object and fills in its fields.

Instead of three loose variables per server, you keep them together in one object.

flowchart LR A["class Server: hostname, ip, status"] --> B["object: web-01, 10.0.0.1, up"] A --> C["object: db-01, 10.0.0.2, down"] classDef blueprint fill:#DBEAFE,stroke:#2563EB,color:#1E3A8A classDef obj fill:#D1FAE5,stroke:#059669,color:#065F46 class A blueprint class B obj class C obj

Common mistake: in Python, forgetting self. when saving a field.

TEXT

Wrong:
def __init__(self, hostname):
    hostname = hostname

That only sets a local variable that disappears when the constructor ends. The object never gets a hostname. Write self.hostname = hostname.

Your task: create a class Server with a hostname, an IP and a status. Build two servers: web-01 (10.0.0.1, up) and db-01 (10.0.0.2, down). Print each as <hostname> (<ip>) is <status>.

Expected output

TEXT

web-01 (10.0.0.1) is up
db-01 (10.0.0.2) is down

Approach

Define the class with a constructor, create the objects, then read their fields:

Python: the constructor is __init__. Its first parameter, self, means "this object". Fields are set with self.name = value.
Java: the constructor has the same name as the class. Fields are declared with a type. this.hostname = hostname saves a value.
C++: the constructor also has the class name. Fields are marked public: so main can read them. Java and C++ need the type of every field written out.

In the Java file, Main comes first because java Main.java starts from the first class in the file.

class Server:
    def __init__(self, hostname, ip, status):
        self.hostname = hostname
        self.ip = ip
        self.status = status


servers = [Server("web-01", "10.0.0.1", "up"), Server("db-01", "10.0.0.2", "down")]
for server in servers:
    print(f"{server.hostname} ({server.ip}) is {server.status}")

Problem statement

Approach