Explain the Strategy pattern with an example.

Question

Accepted Answer

Summary

Strategy puts interchangeable algorithms behind one interface; a context holds one strategy and delegates to it, so the algorithm can be chosen at runtime without if/else. Shipping cost rules (standard, express, pickup) are a typical example.

Longer Answer

Each strategy is testable alone, and new ones are added without editing the context (Open/Closed). Selection usually happens once, through configuration or a lookup table. In Python, TypeScript, modern Java and PHP a strategy is often just a function or lambda; comparators passed to sort functions are strategies.

Code Example

STRATEGIES = {  # strategies as plain functions in a lookup table
    "standard": lambda subtotal: 0 if subtotal >= 5000 else 499,
    "express": lambda subtotal: 1299,
    "pickup": lambda subtotal: 0,
}


def total(subtotal, method):
    return subtotal + STRATEGIES[method](subtotal)


for method in ["standard", "express", "pickup"]:
    print(method, total(4200, method))

type Shipping = (subtotal: number) => number;

const strategies: Record<string, Shipping> = { // strategies as plain functions in a lookup table
  standard: (subtotal) => (subtotal >= 5000 ? 0 : 499),
  express: () => 1299,
  pickup: () => 0,
};

const total = (subtotal: number, method: string) => subtotal + strategies[method](subtotal);

for (const method of ["standard", "express", "pickup"]) console.log(method, total(4200, method));

import java.util.List;
import java.util.Map;
import java.util.function.IntUnaryOperator;

public class Main {
    static final Map<String, IntUnaryOperator> STRATEGIES = Map.of( // strategies as lambdas in a lookup table
        "standard", subtotal -> subtotal >= 5000 ? 0 : 499,
        "express", subtotal -> 1299,
        "pickup", subtotal -> 0);

    static int total(int subtotal, String method) { return subtotal + STRATEGIES.get(method).applyAsInt(subtotal); }

    public static void main(String[] args) {
        for (String method : List.of("standard", "express", "pickup")) System.out.println(method + " " + total(4200, method));
    }
}

<?php
$strategies = [ // strategies as closures in a lookup table
    "standard" => fn(int $subtotal) => $subtotal >= 5000 ? 0 : 499,
    "express" => fn(int $subtotal) => 1299,
    "pickup" => fn(int $subtotal) => 0,
];

$total = fn(int $subtotal, string $method) => $subtotal + $strategies[$method]($subtotal);

foreach (["standard", "express", "pickup"] as $method) echo "$method ", $total(4200, $method), "\n";

Key Points

Interchangeable algorithms behind one interface.
Context delegates; no if/else.
Often just a function.
Pick via lookup table or configuration.

Deep dive: Design Patterns, Explained Simply: Strategy

Explain the Strategy pattern with an example.

Summary

Longer Answer

Code Example

Key Points

Explanation

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