The Low-Level Design Round

Amazon runs a design round that has nothing to do with load balancers or sharded databases. The interviewer hands you a single product, "design Amazon Locker," and asks you to model it in code. What are the classes, what does each one hold, and which methods does other code call on them? No service diagrams, no traffic estimates. Just objects and how they connect. This is low-level design, also called object-oriented design, and at the SDE II level it often takes the slot a second coding round would have filled.

The prompts are consistent year to year. Amazon Locker with getPackage() and putPackage() is the one that comes up by name. A few others round out the set: parking lot, an elevator bank (where you model a tick() that steps the simulation forward), a vending machine, and a shopping cart with a discount engine. A Premium question reservation system and Stack Overflow show up too. They share a shape. Each is a small real-world thing with a few kinds of objects and some rules about what can happen when. Each also has an obvious place where the design either stays clean or turns into a tangle as you add features.

The Amazon angle

Amazon does not run your code in either design round, so the editor never tells you whether your classes even compile. What gets scored is how cleanly you reason through the design and how readable the code on the screen is. Readability matters everywhere in the loop, but here it is everything. Two more things are Amazon-specific. The 16 Leadership Principles are scored inside this round too, so expect a "tell me about a time you had to extend a system someone else designed" before the modeling starts.