OIBSIP — Android Development — Task 3

Calculator

A modern Android calculator that evaluates full mathematical expressions with operator precedence, parentheses, and scientific functions.

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🎯 Objective

Build an Android calculator that goes beyond chained single-operation logic — it should accept a full expression (e.g. 3 + 4 * (2 - 1)), respect operator precedence, support parentheses, and offer scientific operations (sin, cos, tan, √).

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🛠 Tools Used

Tool / Library	Purpose
Android Studio	IDE
Java	Application logic
XML	UI layouts
exp4j 0.4.8	Mathematical expression parser & evaluator
AndroidX GridLayout	4×6 button keypad
Material Components 1.12.0	Material 3 buttons & toolbar
Vibrator service	Haptic feedback on keypress
AGP 8.5.0 / Gradle 8.7 / JDK 17	Build toolchain
Min SDK 24 • Target SDK 34	API compatibility

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🔧 Steps Performed

1. Project setup — Empty Activity (Java), Material 3 DayNight theme, added the dependency net.objecthunter:exp4j:0.4.8 and androidx.gridlayout:gridlayout:1.0.0.
2. UI layout — large result display + smaller expression preview on top, then a 4-column GridLayout keypad (24 buttons across 6 rows: scientific row, clear/parentheses/÷, 7-8-9-×, 4-5-6-−, 1-2-3-+, ⌫-0-.-=).
3. Button styles — CalcKey (numbers, outlined), CalcKeyTonal (scientific & clear), CalcKeyOp (operators, filled), CalcKeyEq (equals, bold) defined in values/styles.xml.
4. Expression buffer — append each token to a StringBuilder; backspace erases function names like sin( in one tap.
5. Pretty render — display * as ×, / as ÷, - as − while typing, while keeping the raw operator chars in the buffer for the parser.
6. Trigonometric helpers — wrap sin(, cos(, tan( so the argument is interpreted in degrees (multiplied by π/180 before evaluation).
7. Evaluation — pass the prepared expression to exp4j's ExpressionBuilder and call .evaluate(); show Error on NaN, infinity, or Exception.
8. Haptic feedback — every button calls Vibrator.vibrate(15 ms, default amplitude).
9. Continuation — after pressing =, the next operator continues from the previous result; the next digit starts a fresh expression.
10. Theming — Day/Night auto themes, monospace result text for that calculator-feel.

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✅ Outcome

A working calculator app with:

• Full expression parsing with correct precedence (no left-to-right hacks)
• Parentheses, unary minus, decimals
• Scientific: sin, cos, tan (degrees), √
• Backspace that handles whole function tokens
• Haptic key feedback
• Error handling for divide-by-zero, NaN, infinity, malformed input
• Material 3 + Day/Night theming

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🧪 Test inputs

Input	Expected
2 + 3 * 4	14
(5 + 5) / 2	5
sin(30)	0.5
√(144)	12
9.5 / 0	Error (no crash)

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📂 Project Structure

app/src/main/
├── AndroidManifest.xml          # VIBRATE permission
├── java/com/intern/calculator/
│   └── MainActivity.java        # button wiring + exp4j eval
└── res/
    ├── layout/activity_main.xml # 4×6 GridLayout keypad
    ├── values/styles.xml        # button styles
    └── values/, values-night/   # day & night themes