Strategy
When one task has several valid ways to run, Strategy keeps the workflow stable while the decision-making rule can change independently.
What the book actually teaches
Head First Design Patterns is not a pattern encyclopedia — it is a course in the design principles that make patterns necessary. Every chapter starts from a concrete, slightly absurd product scenario (a duck simulator, a weather station, a coffee shop menu), lets the naive design fail, and then derives the pattern as the fix. Along the way it hammers a small set of principles: encapsulate what varies, favor composition over inheritance, program to interfaces, and strive for loose coupling between objects that interact.
The Head First format — sketches, redundant explanations, quizzes, crossword puzzles — looks unserious and is precisely why the material sticks. The authors apply cognitive science deliberately: the same idea arrives through pictures, code, conversation, and exercises until it survives contact with your actual codebase.
Who it is for
Developers who can already write working code in an object-oriented language and want to make design decisions on purpose instead of by habit. If you have ever inherited a wall of if/else branches, watched a subclass tree explode, or wired a change through nine files that should have taken one — this book names the problem you had and shows the standard repair.
It is not for absolute beginners (learn a language first) and it is not a reference manual — for that, keep the Gang of Four book nearby. Head First is the book that makes the reference manual readable.