CRUD APIs using Spring Boot

@RestController
@RequestMapping("/api/users")
class UserController {
}

import org.springframework.web.bind.annotation.*;
import java.util.*;

@RestController
@RequestMapping("/api/users")
class UserController {

  private List<String> users = new ArrayList<>(Arrays.asList("John", "Alex"));

  // READ ALL
  @GetMapping
  public List<String> getAllUsers() {
    return users;
  }

  // READ BY ID
  @GetMapping("/{id}")
  public String getUser(@PathVariable int id) {
    return users.get(id);
  }

  // CREATE
  @PostMapping
  public String createUser(@RequestBody String user) {
    users.add(user);
    return "User Created";
  }

  // UPDATE
  @PutMapping("/{id}")
  public String updateUser(@PathVariable int id, @RequestBody String user) {
    users.set(id, user);
    return "User Updated";
  }

  // DELETE
  @DeleteMapping("/{id}")
  public String deleteUser(@PathVariable int id) {
    users.remove(id);
    return "User Deleted";
  }
}

// Output examples:
// GET    /api/users
// POST   /api/users
// PUT    /api/users/1
// DELETE /api/users/1

• 1 Create – add new data
• 2 Read – retrieve data
• 3 Update – modify data
• 4 Delete – remove data

• 1 Client sends HTTP request.
• 2 Controller handles request.
• 3 Service processes logic.
• 4 Repository interacts with database.

• 1 POST – Create
• 2 GET – Read
• 3 PUT – Update
• 4 DELETE – Delete

• 1 Core of backend development.
• 2 Used in all web systems.
• 3 Easy integration with frontend.
• 4 Standard REST practice.

• 1 Used in web applications backend.
• 2 Used in mobile app APIs.
• 3 Used in microservices systems.
• 4 Used in enterprise applications.
• 5 SaaS products use CRUD APIs using Spring Boot in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply CRUD APIs using Spring Boot with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use CRUD APIs using Spring Boot carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the CRUD APIs using Spring Boot rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Not validating input data.
• 2 Using wrong HTTP methods.
• 3 Skipping service layer.
• 4 Hardcoding data instead of database integration.
• 5 Skipping the small working example before adding framework code.
• 6 Ignoring null, empty, duplicate, and boundary inputs.
• 7 Mixing business logic, input handling, and output formatting in one place.
• 8 Using broad error handling that hides the real failure.
• 9 Forgetting to test the behavior after refactoring.
• 10 Adding clever code that future maintainers will struggle to read.

• 1 Use Service layer for business logic.
• 2 Use Spring Data JPA for database operations.
• 3 Validate request body.
• 4 Return proper HTTP status codes.
• 5 Start with clear requirements and one minimal working example.
• 6 Use meaningful names that explain business intent.
• 7 Keep examples small enough to debug line by line.
• 8 Validate input at every trust boundary.
• 9 Handle errors explicitly and preserve useful context.
• 10 Prefer simple control flow over deeply nested logic.
• 11 Separate domain logic from I/O and framework code.
• 12 Write tests for normal, boundary, and failure cases.
• 13 Review security assumptions before production use.
• 14 Measure performance before optimizing.
• 15 Document non-obvious decisions close to the code or in project notes.
• 16 Use official documentation when behavior is version-specific.
• 17 Keep dependencies current and remove unused code.
• 18 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 19 Log operational events without exposing sensitive data.
• 20 Design examples so learners can safely modify and rerun them.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place CRUD APIs using Spring Boot inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates CRUD APIs using Spring Boot.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice CRUD APIs using Spring Boot with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.