[Class Report] System Development (Year 3) – Week 42
~Entering the World of APIs: Foundational Understanding for Connecting to External Services Safely~
From Week 42, the Year 3 curriculum officially began in earnest.
As an introduction to this stage, we focused on what APIs are / why we use them / how to use them safely,
taking the first step toward extending the design skills acquired in Years 1–2 to external service integration.
■ Teacher’s Introduction: “An API Is a Promise with the Outside World”
Mr. Tanaka:
“An API is not a magic box.
It’s a collection of agreements about how you call it, what comes back, and what happens when it fails.”
The teacher first lined up:
- The applications we have built ourselves
- External services such as weather, maps, translation, and generative AI
and explained that APIs are designs for crossing boundaries.
■ Today’s Goals
- Understand the basic concepts of APIs (request / response)
- Be able to read the structure of REST APIs (URL, methods, parameters)
- Learn the risks involved in using APIs (failures, limits, security)
- Understand where APIs fit into the designs learned in Year 2 (DAO / Service)
■ Exercise ①: What Is an API? Thinking with Familiar Examples
The teacher posed questions using everyday apps.
- Where does a weather app get its weather data?
- How does a map app display your current location?
- Where does a translation app perform language conversion?
Students reaffirmed that “everything is not built inside the app.”
Student A: “So our apps can borrow functions from the outside world too.”
■ Exercise ②: Reading the Basic Structure of REST APIs
Next, we learned how to read API documentation.
Basic Elements of a REST API
- Endpoint: where to access (URL)
- HTTP Methods: what you want to do
- GET (retrieve)
- POST (create)
- PUT / PATCH (update)
- DELETE (remove)
- Parameters: conditions and input values
- Response: returned data (often JSON)
Example (for conceptual understanding)
GET /weather?city=Tokyo
→ { "temp": 18, "condition": "Cloudy" }
Student B: “This is similar to the DAO concept we learned in Year 2!”
→ Teacher: “Exactly. Think of an API as an external DAO.”
■ Exercise ③: Practicing Reading API Responses (JSON)
Students were given simple JSON data and worked on identifying:
- Which information should be displayed on screen
- Which information should be stored
- Which information might be useful in the future
{
"location": "Tokyo",
"current": {
"temp": 18,
"humidity": 60,
"wind": 3.2
},
"updated_at": "2026-01-08T10:00:00Z"
}
Student C: “The idea that we don’t have to use everything is refreshing.”
■ Exercise ④: Key Precautions When Using APIs (Failure Is the Default)
Here, the teacher emphasized the weak points of APIs.
Things You Must Always Consider When Using APIs
- Network errors can occur
- Responses can be slow
- Specifications can change
- There are usage limits (rate limits)
- API keys are dangerous if leaked
Mr. Tanaka:
“The exception handling, logging, and fallback strategies you learned in Year 2
are exactly what you’ll need in the API era.”
■ Exercise ⑤: Placing APIs into Our Own Architecture
On the whiteboard, we drew the system architecture we built in Year 2
and added an API class (ExternalService).
UI
↓
Service
↓
External API Client
↓
External Service
Key points:
- API calls are made from the Service layer
- The UI never calls APIs directly
- The Service layer decides how to handle failures
Student D: “It’s great that our Year 2 design works as-is.”
■ Discussion: What APIs Would You Like to Use?
Finally, each group discussed APIs they wanted to try.
- Weather APIs
- Translation APIs
- Map / location APIs
- News APIs
- Generative AI APIs
Student E: “I want to use generative AI in a way that’s useful but not dangerous.”
■ Teacher’s Closing Message
“APIs are technologies for borrowing power from the outside world.
That’s why design, separation of responsibilities, and preparation for failure are so important.
The design skills you acquired in Year 2
are your strongest weapons for connecting with the outside world in Year 3.”
■ Homework (for Next Week)
- Choose one API you’re interested in and research briefly:
- What it can do
- What input it requires
- What data it returns
- Draw a diagram showing where that API fits into your system
- List three problems that might occur if the API fails
■ Preview of Next Week: Actually Calling an API (Hands-On)
Next week, we’ll take on our first real API implementation.
You’ll send requests, receive responses,
and experience what happens when errors occur.
Week 42 was the moment
when our systems stood at the doorway to connecting with the outside world.
Students gained an understanding of both the possibilities and responsibilities of APIs,
taking a major step forward into the full-scale learning of Year 3.