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OIBSIP — Android Development — Task 5

Stopwatch

A precise Android stopwatch with start, pause, reset, lap-time recording, and rotation safety.

🎯 Objective

Build an Android stopwatch that can start, pause, resume, and reset, and that can record lap times showing both the split (since the previous lap) and the running total. The timer must continue correctly even if the device is rotated.

🛠 Tools Used

Tool / Library Purpose
Android Studio IDE
Java Application logic
XML UI layouts
AndroidX RecyclerView Lap times list
Handler + Runnable Periodic UI tick (~31 ms)
Material Components 1.12.0 MaterialButton, MaterialCardView, MaterialToolbar
AGP 8.5.0 / Gradle 8.7 / JDK 17 Build toolchain
Min SDK 24 • Target SDK 34 API compatibility

🔧 Steps Performed

  1. Project setup — Empty Activity (Java), Material 3 DayNight theme.
  2. UI layout — large monospace timer display in a Material card, a 3-button row (Start / Hold / Resume, Lap, Reset), a "Lap times" header, and a RecyclerView for laps with an empty-state text.
  3. Time-keeping model — two long fields:
    • elapsedBeforeStart — accumulated milliseconds before the current run segment
    • startedAt — System.currentTimeMillis() when the current segment started
    • Current elapsed = running ? elapsedBeforeStart + (now - startedAt) : elapsedBeforeStart
  4. Tick loop — a Handler re-posts a Runnable every 31 ms (centisecond resolution) that re-formats tvTime while running.
  5. Primary button states — the same button cycles: Start → Hold → Resume, label switching based on current state. Lap is enabled only while running. Reset is enabled while running or after stopping.
  6. Lap recording — each Lap tap inserts the current elapsed time at the top of an ArrayList<Long>; the RecyclerView shows newest first with two columns per row: + split (this total − previous total) and total in HH:MM:SS.cs.
  7. Rotation safety — onSaveInstanceState persists elapsedBeforeStart, startedAt, running flag, and the lap array as a long[]. After rotation the activity rebuilds and resumes ticking from exactly the right moment.
  8. Format helper — format(long ms) returns a fixed HH:MM:SS.cs string using String.format(Locale.US, …).
  9. Cleanup — onDestroy removes the tick callback so no leaks occur.

✅ Outcome

A working stopwatch app featuring:

  • Start / Hold (pause) / Resume / Reset controls
  • Lap times recorded with both split and total
  • HH:MM:SS.cs monospace display (centisecond precision)
  • Rotation-safe via onSaveInstanceState
  • Material 3 card UI + Day/Night auto theme
  • Enabled/disabled button states matching the stopwatch state

📂 Project Structure

java/com/intern/stopwatch/
└── MainActivity.java        # tick handler + RecyclerView LapAdapter

res/layout/
├── activity_main.xml        # timer card + 3-button row + RV
└── item_lap.xml             # Lap n  |  +split  |  total

⏱ Quick demo flow

  1. Tap Start → timer runs.
  2. Tap Lap a few times → split & total appear in the list.
  3. Tap Hold → freezes at the current time; button becomes Resume.
  4. Tap Resume → continues exactly where it paused.
  5. Rotate the screen → still ticking from the right instant.
  6. Tap Reset → everything zeroed out.