Design Patterns & SOLID

The JVM guarantees enum constants are instantiated once and cannot be created reflectively or by deserialisation.

valueOf returns the existing constant — there is only one.

An abstract factory has one method per product and guarantees products come from the same family.

A builder gives readable, validated construction of an immutable result.

Each wrapper adds behaviour around the same abstraction. InputStreamReader also adapts bytes to characters.

It converts one interface (an array) into another the client expects (List).

A proxy intercepts calls and wraps them in begin/commit/rollback — which is also why self-invocation bypasses it.

Leaves and composites share one interface; composites delegate to their children recursively.

Values -128..127 are cached flyweights. For 128 you get two objects, so always use equals.

The java.sql abstraction and the driver implementations vary independently.

Each filter handles the request, passes it on, or stops the chain (e.g. returning 401).

States trigger transitions; with Strategy the client chooses the algorithm.

Commands can be stored, re-executed and reversed — often combined with Memento snapshots.

The fixed algorithm (connection, statement, iteration, cleanup, exception translation) calls your supplied step.

They give type-safe dispatch over a closed hierarchy without accept/visit boilerplate.

Code written for Rectangle breaks when given a Square, so it isn’t substitutable.

Split fat interfaces into role interfaces such as Workable and Feedable.

New payment methods are new classes; the service never changes.