Forced Compatibility

Description

What does it look like?

• New code directly calls legacy (outdated) interfaces
• The caller manually adapts to differences in API specifications, cluttering the logic
• Despite mismatched formats or behavior, the integration is forced for the sake of functionality

Why is it a problem?

• The calling side takes on the responsibility of adapting/transforming the data, blurring design boundaries
• Any change in the API or specification ripples across many parts of the codebase
• Makes testing or replacing the dependency difficult

Bad Example of the Anti-pattern

TypeScript

// レガシーAPI（旧仕様）
class LegacyPrinter {
  printText(text: string) {
    console.log(`*** ${text} ***`);
  }
}

// 新しいコード
class ReportGenerator {
  printSummary(text: string) {
    const printer = new LegacyPrinter(); // ← 直接依存（インターフェース不一致）
    // 無理やり旧APIに合わせる
    const legacyFormat = text.toUpperCase();
    printer.printText(legacyFormat); // ← ハードコーディング
  }
}

PHP

<?php

// レガシーAPI（旧仕様）
class LegacyPrinter {
  public function printText(string $text): void {
    echo "*** {$text} ***\n";
  }
}

// 新しいコード（無理やり接続）
class ReportGenerator {
  public function printSummary(string $text): void {
    $printer = new LegacyPrinter(); // ← 古いクラスを直接使っている
    $legacyFormat = strtoupper($text); // ← 無理やり仕様に合わせる
    $printer->printText($legacyFormat); // ← ハードコーディング
  }
}

// 利用例
$report = new ReportGenerator();
$report->printSummary("月次レポート");

Python

# レガシーAPI（旧仕様）
class LegacyPrinter:
    def print_text(self, text: str):
        print(f"*** {text} ***")

# 新しいコード（無理やり接続）
class ReportGenerator:
    def print_summary(self, text: str):
        printer = LegacyPrinter()  # ← 直接依存
        legacy_format = text.upper()  # ← 無理やり仕様に合わせる
        printer.print_text(legacy_format)  # ← 古いインターフェースにベタ依存

# 利用例
report = ReportGenerator()
report.print_summary("月次レポート")

Issues:

• ReportGenerator is tightly coupled to LegacyPrinter’s method names and formatting
• Interface mismatch is not abstracted; legacy and modern design philosophies are mixed
• Logic like converting text to uppercase is embedded in ReportGenerator, violating the SRP
• Cannot easily switch to a new printer class or external printing API
• Cannot replace with a mock printer during testing
• As usage of LegacyPrinter increases, similar transformation logic is duplicated throughout the codebase
• Any change to the legacy interface impacts all usage sites

Refactoring by Pattern

Design patterns that can address this

Pattern	Overview	Main Refactoring Approach
Adapter	Acts as a “bridge” to match interface differences	Adds a translation layer between old and new APIs
Facade	Hides subsystem complexity and provides a clean entry point	Use when you want to simplify usage
Proxy	Controls access to objects	Useful for access control, caching, or lazy calls