Program & UI Design (College Board AP® Computer Science Principles): Revision Note
Program design
What is program design?
Program design is the process of planning the structure and behavior of a program before writing code
The design of a program incorporates investigation to determine its requirements, what the program must do and who it is for
Design activities help developers think through the solution before committing to an implementation, reducing errors later
Investigation: understanding user constraints & interests
Method | Description | Example |
|---|---|---|
Collecting data through surveys | Collecting opinions and preferences from a large group of users | Asking students to rate desired features of a new revision app using an online form |
Interviews | In-depth conversation with individual users to understand their needs in detail | Talking with a teacher about what a grading tool must do |
Direct observations | Watching users interact with an existing system to identify problems | Recording how users navigate a website and where they get stuck |
User testing | Trying early versions of the program with users to gather feedback | Giving users a prototype and noting where they struggle to complete a task |
Requirements & specifications
What are program requirements?
Program requirements describe how a program functions and may include a description of the user interactions the program must provide (for example, how a user logs in, submits an answer, or navigates between screens)
Requirements are identified through investigation, by studying user constraints and interests
What is a specification?
A program's specification defines the requirements for the program
In a development process, the design phase outlines how to accomplish a given specification
A clear specification gives the development team a shared target for what the program must do
Design activities
What happens during program design?
Common design activities include:
Activity | Purpose | Example |
|---|---|---|
Brainstorming | Generating a range of ideas before committing to one approach | A team lists ten possible ways to structure a quiz app before agreeing on an approach |
Planning and storyboarding | Mapping out the program's features and the user's journey through the screens | Listing features in priority order and sketching each screen of the app |
Organizing the program into modules and functional components | Breaking the program into smaller, independent parts that can be developed and tested separately | Separating a program into a login module, a data processing module, and a display module |
Creation of diagrams that represent the layouts of the user interface | Visually showing how the user will interact with the program | Sketching wireframes of each screen showing buttons, menus, and input fields |
Development of a testing strategy for the program | Deciding in advance how the program will be tested and what outputs are expected for given inputs | Writing down that entering −1 should display "Invalid input" before any code is written |
Examiner Tips and Tricks
When a question describes a team watching users, asking questions, or distributing questionnaires before building a program, it is describing investigation — the way developers identify user constraints and interests before writing any code
Remember that a specification defines what the program must do, while the design phase (including its activities) outlines how to build it — the AP exam may ask you to distinguish between these
Design activities happen before coding begins — if a question describes a team drawing diagrams, listing features, or dividing a program into modules and functional components, that is design, not development
For the AP Create Performance Task, you will need to describe the purpose and intended user of your program, your design decisions, and how you structured your solution — investigation and design activities are the foundation of all of these written responses
Worked Example
Before writing any code, a development team holds a meeting to divide their program into separate modules, each handling a different feature. They also write down what inputs and outputs each module will need.
Which design activity does this best describe?
(A) Investigation, because they are collecting information about user needs
(B) Testing, because they are deciding what outputs to expect
(C) Modularity and planning, because they are structuring the program into independent parts before development
(D) Iterative development, because they are refining the program through repeated cycles
[1]
Answer:
(C) Modularity and planning, because they are structuring the program into independent parts before development [1 mark]
The scenario describes two design activities working together: organizing the program into modules and functional components (dividing into modules), and planning (defining inputs and outputs for each module before development).
That combination is what makes (C) correct — investigation happens earlier, testing would come after code is written, and iterative development refers to revising a program through cycles, not structuring it.
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