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Consider the following code snippet implementing the Command Pattern with undo/redo stacks. After executing the commands: Add 5, Add 3, Undo, Redo, what is the final state of the receiver?

easy🧾 Code Trace Q12 of Q15

OOP & Design Patterns - Command Pattern - Undo/Redo, Request Queuing & Logging

Consider the following code snippet implementing the Command Pattern with undo/redo stacks. After executing the commands: Add 5, Add 3, Undo, Redo, what is the final state of the receiver?

A5

B8

C3

D0

Step-by-Step Solution

Solution:

Step 1: Trace command executions
Execute Add 5 -> state = 5; Execute Add 3 -> state = 8.
Step 2: Trace undo and redo
Undo removes Add 3 -> state reverts to 5; Redo reapplies Add 3 -> state returns to 8.
Final Answer:
Option B -> Option B
Quick Check:
State after redo is 8 [OK]

Quick Trick: Undo then redo restores the last undone command [OK]

Common Mistakes:

MISTAKES

Forgetting redo reapplies the command
Off-by-one in stack operations

Trap Explanation:

PITFALL

Some think undo permanently removes commands or redo does nothing after new commands.

Interviewer Note:

CONTEXT

Checks candidate's ability to mentally execute command pattern with undo/redo stacks.

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