I'll Be Backend
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Principles

DRY: Don't Repeat Yourself

DRY means every piece of knowledge, such as a business rule, lives in exactly one place in the code. When a rule is copied into several places, changing it means finding every copy, and the copy you miss becomes a bug. These lessons take a free shipping rule that three classes check on their own and move it into one home, one small refactoring at a time.

Pick your language

DRY: one rule, one place in Java, step 2: Members ship free from 25Java · ~4 minDRY in JavaDRY: one rule, one place in Kotlin, step 2: Members ship free from 25Kotlin · ~4 minDRY in KotlinDRY: one rule, one place in C#, step 2: Members ship free from 25C# · ~4 minDRY in C#DRY: one rule, one place in C++, step 2: Members ship free from 25C++ · ~4 minDRY in C++DRY: one rule, one place in Python, step 2: Members ship free from 25Python · ~3 minDRY in PythonDRY: one rule, one place in Ruby, step 2: Members ship free from 25Ruby · ~4 minDRY in RubyDRY: one rule, one place in PHP, step 2: Members ship free from 25PHP · ~4 minDRY in PHPDRY: one rule, one place in TypeScript, step 2: Members ship free from 25TypeScript · ~3 minDRY in TypeScriptDRY: one rule, one place in Go, step 2: Members ship free from 25Go · ~4 minDRY in GoDRY: one rule, one place in Rust, step 2: Members ship free from 25Rust · ~4 minDRY in RustDRY: one rule, one place in Swift, step 2: Members ship free from 25Swift · ~4 minDRY in Swift

When to use it

Use it when

One business rule, number or decision shows up in more than one feature.

Skip it when

Two lines look alike but follow different rules, like a shipping fee and a gift wrap fee that both cost 4.99.

Spot it by

The same literal or condition in several classes, and changes that start with a search.