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Project BE2 — Android App: Calculator or Converter

← Part 3 Overview

S7 Inf A3 — Java for Graphical and Mobile Programming
Stéphane Derrode — Centrale Lyon
Part 3 — Android

Prerequisite: TD6 (Android Studio setup + first Activity) and TD7 (RecyclerView + navigation).
Duration: 2h autonomous session + personal work.
Submission: Moodle — zip of the Android project + PDF report (see Section 5).

1. Choose Your Project

Pick the subject that interests you most. Both use the same Android skills from CM6–8.

Option A — Calculator

Build a four-operation calculator for the Android platform.

Minimum requirements:

  • A display showing the current input and result
  • Digit buttons 0–9 and a decimal point
  • Operator buttons + − × ÷
  • An = button that computes and displays the result
  • A C button that clears the current input
  • Division by zero is handled gracefully (show "Error")
  • The last result is saved with SharedPreferences and restored on next launch

Suggested layout:

┌─────────────────────────────┐
│  0                          │  ← result display (large TextView)
│  0 + 0                      │  ← expression display (small TextView)
├──────┬──────┬──────┬────────┤
│  C   │  ←   │  %   │   ÷    │
├──────┼──────┼──────┼────────┤
│  7   │  8   │  9   │   ×    │
├──────┼──────┼──────┼────────┤
│  4   │  5   │  6   │   −    │
├──────┼──────┼──────┼────────┤
│  1   │  2   │  3   │   +    │
├──────┼──────┴──────┼────────┤
│  +/− │      0      │   =    │
└──────┴─────────────┴────────┘

Suggested implementation approach:

Use a state machine with three variables: 

private double operand1 = 0;
private String operator = "";  // "+", "-", "×", "÷"
private boolean newNumber = true;

Each digit button appends to the display. Each operator button stores operand1 and operator. The = button computes the result.

Option B — Currency Converter

Build a currency converter that converts between at least 4 currencies.

Minimum requirements:

  • An EditText for the amount to convert
  • A Spinner (drop-down) to select the source currency
  • A Spinner to select the target currency
  • A Convert button that displays the result
  • At least 4 currencies: EUR, USD, GBP, JPY (or your choice)
  • Exchange rates are hardcoded (no internet required)
  • The last conversion (amount + currencies) is saved with SharedPreferences and restored on next launch
  • Invalid input (empty, non-numeric) is handled with a Toast or Snackbar

Suggested layout:

┌─────────────────────────────┐
│  Currency Converter         │
├─────────────────────────────┤
│  Amount: [         ]        │
│  From:   [ EUR ▼  ]        │
│  To:     [ USD ▼  ]        │
│                             │
│       [ Convert ]           │
│                             │
│  Result: 1 EUR = 1.08 USD   │
│          100 EUR = 108 USD  │
└─────────────────────────────┘

2. Architecture Requirements

Whichever option you choose, your app must:

  • Be built with Java and Android SDK API 26+, targeting API 33
  • Use at least one custom layout designed in XML
  • Handle button clicks with setOnClickListener (lambda)
  • Persist at least one value across sessions with SharedPreferences
  • Handle at least one error case (empty input, division by zero, etc.) with Toast or Snackbar
  • Run correctly on the Pixel 6a API 33 emulator

3. Advanced Ideas (optional — appreciated)

These ideas are not required but will be noted positively:

For both projects: - Dark mode support (use ?attr/ color references in your theme) - Landscape layout variant in res/layout-land/ - Multi-language support (values-fr/strings.xml + values-en/strings.xml) - Custom app icon designed in Android Studio's Image Asset wizard - History screen: a second Activity showing previous calculations/conversions using RecyclerView

Calculator extras: - Backspace button to delete the last digit - Percentage button (x % of y) - Memory buttons (M+, M−, MR, MC) - Scientific functions (√, x², 1/x) in a second layout or scrollable panel

Converter extras: - Swap button to invert source and target currencies instantly - More conversion categories: temperature (°C/°F/K), distance (km/mi/m), weight (kg/lb/oz) - Live exchange rates from a public API (advanced — requires network permission and Retrofit)

4. Minimum Requirements Checklist

To obtain a passing grade, all of the following must work on the emulator:

Calculator: - [ ] Display shows digits as they are typed - [ ] Addition, subtraction, multiplication and division all produce correct results - [ ] Division by zero shows an error (no crash) - [ ] C button clears the display - [ ] Last result is restored on app restart

Converter: - [ ] Amount can be entered in the EditText - [ ] Both Spinners show at least 4 currencies - [ ] Convert button shows a correct result - [ ] Invalid input (empty/non-numeric) is handled gracefully - [ ] Last conversion settings are restored on app restart

5. Deliverable

Submit on Moodle before the deadline a single zip containing:

  • The complete Android Studio project (all source files)
  • A PDF report (1–2 pages) including:
  • A screenshot of your running app on the emulator
  • Your layout XML structure (a diagram or the XML itself)
  • A brief description of your implementation choices and any difficulties encountered
  • If you implemented advanced features, describe them

Zip naming: BE2_NomPrenom1_NomPrenom2.zip

Tips

  • Start with the layout — get the UI right before writing any logic.
  • For the calculator: implement one operator at a time and test thoroughly before adding the next.
  • For the converter: define your exchange rates as constants at the top of the Activity — easy to update later.
  • Use Log.d(TAG, ...) liberally during development — Logcat is your best friend.
  • Test edge cases: what happens if the user taps = twice? What if the first digit is the operator?