Low Level Design

Code that works today still has to change tomorrow, and badly structured classes turn every small change into a risky edit. SOLID is five rules of thumb for keeping classes small, focused and easy to extend. Interviewers use them as the shared vocabulary for judging any design you draw.

In an LLD round you have to show your design before you code it, and a page of prose is hard to follow. UML gives a small set of standard boxes and arrows that every interviewer reads the same way. You only need class diagrams, their relationships and sequence diagrams to explain most designs.

Calling new directly everywhere ties your code to concrete classes and to messy constructor calls. Creational patterns control how and when objects are made, so that creation can change without touching the code that uses them. Singleton, Factory and Builder are the three that come up again and again in interviews and frameworks.

Most real design questions are about fitting objects together or letting behaviour vary without rewriting old code. These five patterns are named, tested answers to exactly those problems. Knowing them lets you say "this is a Strategy" in an interview and have the other person understand the whole design at once.

The machine coding or LLD round gives you an open problem like "design a parking lot" and watches how you go from vague requirements to working classes. These five problems cover the patterns, data structures and concurrency issues most interviewers look for. Practising them builds a repeatable approach you can apply to problems you have never seen.