DAO Pattern — Sample

A minimal demonstration of the DAO (Data Access Object) pattern in Java + Spring Boot.

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What It Demonstrates

DAO is a pattern that hides data access details behind an interface. Business logic works against the UserRepository interface and has no knowledge of where or how data is stored — whether in a HashMap, ArrayList, PostgreSQL, or anything else.

The project illustrates this with a concrete example: the same UserRepository interface is implemented two ways — via HashMap and via ArrayList. Behavior is identical, implementation is different.

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Structure

The entire project intentionally fits in a single file — so the pattern is visible in full, without unnecessary noise:

DaoArchitecture.java
│
├── User                          # Model — record with id and name
├── UserRepository                # DAO contract — interface with findById / findAll / save / delete
├── InMemoryMapUserRepository     # Implementation via HashMap — O(1) lookup by id
├── InMemoryListUserRepository    # Implementation via ArrayList — sequential scan
└── DaoArchitecture               # Spring Boot entry point + demo()

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Key Points

Interface as contract. UserRepository defines what to do — not how. The calling code (demo()) is written against the interface and has no idea which implementation is in use.

Two implementations — one behavior. InMemoryMapUserRepository stores data in a HashMap — O(1) lookup by id. InMemoryListUserRepository stores data in an ArrayList — sequential scan at O(n). From the outside, there is no difference.

Substitutability. Swapping the implementation requires changing a single line in demo(). Nothing else changes — that is the point of DAO.

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Running

./mvnw spring-boot:run

Console output:

MAP:  Optional[User[id=1, name=jack]]
LIST: Optional[User[id=1, name=jack]]

Both implementations return the same result.