Design Patterns in Java

A few patterns that show up constantly in real Java codebases:

Singleton — exactly one instance of a class, accessed globally:

class AppConfig {
    private static final AppConfig INSTANCE = new AppConfig();
    private AppConfig() {}
    static AppConfig getInstance() { return INSTANCE; }
}

Factory — a method that decides which concrete class to instantiate, hiding the decision from the caller:

static Shape createShape(String kind) {
    return switch (kind) {
        case "circle" -> new Circle(1.0);
        case "square" -> new Square(1.0);
        default -> throw new IllegalArgumentException("Unknown shape: " + kind);
    };
}

Strategy — swap an algorithm's behavior via an interface, without an if/else chain at the call site (this is exactly what a Comparator passed to .sort() already does).

Common mistakes

• Reaching for a pattern because it's a "known pattern," not because it solves an actual problem in front of you — an unneeded Singleton or Factory just adds indirection.