Calculator CLI

Introduction

Build a command-line calculator that parses expressions like 2 + 3 * 4 and evaluates them. You'll practice parsing, the stack-based algorithm, exception handling and packaging a JAR.

Informative example

Core evaluation loop:

public class Calculator {
    public double evaluate(String expr) {
        Deque<Double> stack = new ArrayDeque<>();
        for (String token : tokenize(expr)) {
            if (isNumber(token)) stack.push(Double.parseDouble(token));
            else if (isOperator(token)) applyOp(stack, token);
        }
        return stack.pop();
    }
}

// Extend: support parentheses, history, --help flag
// Package: mvn package → java -jar calculator.jar "10 + 5"

Best practices

• Start with + and - only; add * / and parentheses incrementally.
• Write tests for each operator before implementing.
• Handle divide-by-zero with a clear error message.

Purpose of this lesson

Build a CLI calculator to practice parsing, stacks and packaging a JAR.

Step-by-step explanation

1. Implement tokenization.
2. Add + and - with tests.
3. Add * / and parentheses.
4. Package with Maven/Gradle.

Interactive workflow diagram

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Calculator CLI daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Calculator CLI — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.