Command and Undo

Encapsulate requests as objects for queues, logging, and undo stacks.

Intermediate10 min read

Read these first

Decorator Pattern

Why this matters

Command interface — detailed pointwise breakdown in the section below.
Invoker — detailed pointwise breakdown in the section below.
Undo stack — detailed pointwise breakdown in the section below.
CampusOS editor — detailed pointwise breakdown in the section below.
Patterns are forces and solutions — name the force before the pattern.

Invoker

Commandexecute() / undo()

Receiver

Encapsulate requests as objects — enables undo, queue, and logging.

Overview

Encapsulate requests as objects for queues, logging, and undo stacks. Study each section below in order — every list is interview-ready reference material.

Command interface

Points to cover

Definition — what Command interface means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Invoker

Points to cover

Definition — what Invoker means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Undo stack

Points to cover

Definition — what Undo stack means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

CampusOS editor

Points to cover

Definition — what CampusOS editor means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Java

// CampusOS — apply "Command interface" on paper first, then in code
public final class CommandAndUndoExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Command interface — review this slice before mock interviews.
Invoker — review this slice before mock interviews.
Undo stack — review this slice before mock interviews.
CampusOS editor — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.