📦Command Pattern

Encapsulate a request as an object — parameterize clients with queues, requests, and operations.

📌 Key IdeaTurn an action into an object with execute() / undo(). Invoker triggers; Receiver does real work. Enables macro recording, job queues, undo stacks, transactional behaviour.

🖼

Visual

📦 Invoker ─▶ Command ─▶ Receiver

  button.onClick(new PasteCommand(editor))
  queue.submit(new SendEmailCommand(...))  // jobs

Decouples who triggers from what runs — command is first-class data.

✅

When to Use

You need undo/redo — stack commands or paired reversals.
Schedule, queue, or log operations (audit trail).
GUI actions as objects (Action, Runnable, Swing Action).
Transactional semantics — compose commands.

🚫

When NOT to Use

Trivial one-line calls with no undo/queue — pattern weight not worth it.

🚨

Anti-Pattern (What NOT to Do)

Huge switch on string actionBadjava

void onAction(String a) {
    switch(a) { case "paste": ... } // no undo, no queue

Actions aren't objects — hard to extend.

✅

The Right Way

Command objectsGoodjava

interface Command { void execute(); void undo(); }

class PasteCommand implements Command {
    private final Editor e;
    public void execute() { e.paste(); }
    public void undo() { e.deletePaste(); }
}

Invoker stores history for undo.

⚖️

Pros & Cons

👍 Pros

Macro transactions and composite commands.
Clean separation invoker / receiver.
Natural fit for message queues and event sourcing snapshots.

👎 Cons

Many small classes — boilerplate.

🔀

Common Confusions Cleared Up

Command vs Strategy

Both encapsulate algorithms. Strategy varies algorithm family for one context (pluggable policy). Command encapsulates requests as objects — emphasis on invocation timing, queuing, undo — different narrative.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

CQS / event sourcing — relationship to Command?

🧾

Quick Cheat Sheet

IntentRequest as object; undo; queue.

ActorsInvoker, Command, Receiver.

vs StrategyRequest lifecycle vs algorithm swap.

🔗