Meet Chai Corner
Imagine a tiny tea shop called Chai Corner (a made-up shop, used only to teach). One cashier takes orders. One barista makes the drinks. A little bell rings when a drink is ready. Customers can add milk or sugar, use a coupon, and get a message when their order is done.
That whole shop is a low-level design (LLD). High-level design decides which shops exist and how they talk to each other. LLD decides what happens inside one shop: who does which job, and how the shop can change (a new coupon, a new payment machine) without being rebuilt from scratch.
We will use Chai Corner for every idea in this post. First we explain it like you are five (ELI5: "explain like I'm five"), then give the precise technical meaning, then show a short piece of Java. The code uses Java 17 features such as record, and every snippet was compiled and run.
The next posts apply these ideas: concurrency, then two sets of machine-coding case studies.
1. The four OOP ideas, in the tea shop
| Idea |
Chai Corner (ELI5) |
Technical meaning |
Interview trap |
| Encapsulation |
The cash drawer is behind the counter. You hand money to the cashier; you do not reach in. |
Keep state private; expose behaviour |
Public mutable fields everywhere |
| Abstraction |
You say "one tea, please". You do not need to know how the stove works. |
Depend on a simple interface, hide detail |
Leaky interfaces that expose storage choices |
| Inheritance |
A "Masala Tea" is a kind of tea, so it starts with the normal tea recipe. |
A subtype reuses and specialises a parent |
Deep hierarchies used just for code reuse |
| Polymorphism |
Say "serve it" to any drink. Tea is poured; a cold coffee is shaken. Same words, different action. |
Same call, different behaviour by type |
Long instanceof or switch chains on type |
Prefer composition over inheritance: build behaviour by holding collaborators ("has a") instead of extending classes ("is a") (overview). In the shop: the cashier has a card machine. The cashier is not a kind of card machine. If the shop changes machines, the cashier does not change. Inheritance couples child to parent; composition lets you swap parts at runtime.
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