All 23 GoF design patterns in Java, each as one short Java 21 file that runs.

• All 23 patterns, one file each: 22–56 lines (median 32), with records, sealed interfaces and switch pattern matching instead of getter boilerplate.
• 34 JUnit 5 tests in 23 test classes pass with mvn test on Java 21 and Java 25 (same output on both). No Maven? run-all.sh / run-all.cmd compile with javac and run all 23.
• For every pattern: the problem, the idea, when NOT to use it, the code smell it replaces, the real output of the file and where the JDK itself uses it.
• 69 interview questions with short answers, a one-page when-to-use chart, drills.csv for flashcards.
• Rules for Claude Code and Cursor that spot where a pattern fits in your own code.

Free on the page: every pattern, its code and output, the chart and the questions. One-time $19: the Maven project ZIP, the 30-page PDF, the flashcards, the AI rules and printing the chart. No subscription.