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OOP Calculator (Java)

Functional calculator in Java applying encapsulation and inheritance. Clean structure for future features and UI improvements.

View code

The calculator below is a JavaScript port of the Java classes, so it runs in your browser. The Java version, with a Swing interface and tests, is in the code.

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Keyboard works too: 0–9 + − * / Enter ⌫ Esc

History

    Calculations you finish with = appear here.

    How it’s built

    «abstract»Operation
    • Addition
    • Subtraction
    • Multiplication
    • Division
    Calculator holds an Operation and calls apply(), without knowing which subclass it is.
    • Inheritance: each operation extends the abstract Operation class and supplies its own symbol and apply() method. Adding a new operation means adding a subclass, without changing the calculator.
    • Encapsulation: Calculator keeps its state in private fields. The interface can only press keys and read the display, so the state can’t become inconsistent.
    • Separation of concerns: the Swing CalculatorPanel (and this web page) only forward key presses, so the user interface can be replaced without touching the arithmetic.
    • Tested: the same behaviours are checked in Java (CalculatorTest) and JavaScript, including left-to-right chaining, rounding and division by zero.