Test — Module 6: Methods and Interfaces¶

Topic: [[go]] Module: [[go/6. Methods and Interfaces]]

Before You Begin¶

[!WARNING] Do not look at ANSWERS.md or your notes during the test. The purpose of this test is to surface what you truly know vs. what you think you know. An inflated score is worthless. An honest score shows you exactly where to focus next.

Instructions: 1. Close all notes, the module README, and any reference materials. 2. Set a timer. Note your start time below. 3. Attempt every question — partial credit exists. 4. After finishing, grade yourself using ANSWERS.md. 5. Record your score in the Grading Record at the bottom.

Start time: ___________ End time: ___________ Total time: ___________

Scoring Table¶

Passing score: 15/22 (68%)  ·  Target score: 18/22 (82%)

Section 1: Recall (5 questions × 1 pt = 5 pts)¶

Quick recall questions. Answer from memory in 1–3 sentences each.

1.1 What is the difference between a method and a function in Go? Give a one-line example of each that does the same thing.

Your answer:

1.2 Complete the method-set table from memory:

Your answer:

1.3 Does Go require a type to explicitly declare which interfaces it implements (like Java's implements)? How does Go determine whether a type satisfies an interface?

Your answer:

1.4 What is the any type in Go? When was it introduced, and what is it an alias for?

Your answer:

1.5 Name three important standard library interfaces covered in this module and state how many methods each has.

Your answer:

Section 2: Conceptual Understanding (3 questions × 2 pts = 6 pts)¶

Demonstrate that you understand the "why", not just the "what".

2.1 Explain why *T can call value-receiver methods, but T cannot call pointer-receiver methods when stored in an interface. What is the underlying reason for this asymmetry?

Your answer:

2.2 Describe the internal representation of an interface value. What are the two components? Explain what each component contains for: (a) a nil interface, (b) an interface holding os.Stdout (a *os.File), and © an interface holding a typed nil (*os.File)(nil).

Your answer:

2.3 What is the "accept interfaces, return structs" principle? Explain why it applies to both parameters (accept interfaces) and return values (return structs) with a concrete justification for each direction.

Your answer:

Section 3: Applied / Practical (2 questions × 3 pts = 6 pts)¶

Show that you can use your knowledge to solve real problems.

3.1 Write a complete Go program that: - Defines an interface Shape with methods Area() float64 and Name() string - Implements Shape with two types: Circle (with a Radius float64 field) and Square (with a Side float64 field) - Writes a function largestShape(shapes []Shape) Shape that returns the shape with the largest area - In main, creates a slice of at least 4 shapes and prints the name and area of the largest one

Your answer:

3.2 Write a function mustRead(r io.Reader, n int) []byte that reads exactly n bytes from r, panics with a descriptive message if fewer than n bytes are available (or any error occurs), and returns the bytes. Then write a brief main demonstrating it with a strings.Reader. Also write a version tryRead(r io.Reader, n int) ([]byte, error) that returns an error instead of panicking.

Your answer:

Section 4: Scenario / Debugging (1 question × 3 pts = 3 pts)¶

Analyze a realistic scenario and identify what's wrong or what should be done.

4.1 A colleague shows you this code and says "the error handling is broken — it always reports an error even for valid inputs":

type DBError struct {
    Code int
    Msg  string
}

func (e *DBError) Error() string {
    return fmt.Sprintf("DB error %d: %s", e.Code, e.Msg)
}

func queryDB(id int) error {
    var dbErr *DBError
    if id <= 0 {
        dbErr = &DBError{Code: 404, Message: "record not found"}
    }
    return dbErr
}

// Caller:
if err := queryDB(42); err != nil {
    log.Fatal("unexpected error:", err)  // always fires!
}

a) Identify the exact bug (which line, and why it is a bug).

b) Explain what the interface value queryDB(42) returns — describe its internal state using the (type, value) model.

c) Write the corrected queryDB function.

Your answer:

Section 5: Essay / Discussion (1 question × 2 pts = 2 pts)¶

Open-ended. There is more than one good answer. Show your reasoning.

5.1 "Define interfaces at the consumer, not the producer." Explain what this means with a concrete example involving two packages. Why is this better than the alternative? What problem does it solve that exists in languages with explicit implements declarations?

Consider at least two perspectives: coupling between packages, and the practical ability to add a new implementation without modifying existing code.

Your answer (aim for 4–7 sentences):

Section 6: Bonus Challenge (1 question × +5 pts)¶

[!NOTE] This question is optional. It will not count against you if you skip it or get it wrong. Only attempt it after completing Sections 1–5.

6.1 Design and implement a small plugin system using interfaces:

1. Define an interface Plugin with methods Name() string and Execute(input string) (string, error)
2. Implement two plugins: UpperPlugin (converts input to uppercase) and ReversePlugin (reverses the input string)
3. Write a Pipeline struct that holds a []Plugin and has an Execute(input string) (string, error) method that passes the input through each plugin in sequence (output of one is input of next); return on first error
4. In main, create a pipeline of [UpperPlugin, ReversePlugin], run it on "hello world", and print the result
5. Bonus within the bonus: make Pipeline itself implement Plugin (its Name() returns the names of all plugins joined by "→") and verify this by passing a Pipeline as a Plugin to a function that accepts Plugin

Your answer:

Self-Assessment¶

After grading your test, answer these questions honestly:

What did I get right and why?

Your reflection:

What did I get wrong and why did I make that mistake?

Your reflection:

What should I review before moving to the next module?

Your reflection:

What score did I get? Do I feel it accurately reflects my understanding?

Your reflection:

Grading Record¶

Append a new row each time you take this test. Do not overwrite previous attempts.

Grade scale: A ≥ 90% (≥20/22)  ·  B ≥ 80% (≥18/22)  ·  C ≥ 70% (≥15/22)  ·  D ≥ 60% (≥13/22)  ·  F < 60%

See ANSWERS.md for the answer key. Review it only after completing the test.