Prompt Engineering Basics

[Role]         You are a senior Java developer.
[Context]      I'm building a Spring Boot REST API.
[Task]         Write a service method that...
[Constraints]  Use Java 21 records. Follow SOLID principles.
[Output Format] Return only the Java code in a code block.

❌ Vague: "Write a Java class for users"

✅ Specific:
"Write a Java 21 record class named UserDto with fields:
  - id (String)
  - email (String, validated)
  - name (String)
  - createdAt (Instant)
Include a static factory method fromUser(User user)."

❌ No context: "How do I fix this error?"

✅ With context:
"I'm using Spring Boot 3.2 with MongoDB.
I get this error when calling userRepository.save():
  [paste exact error]
My User entity is: [paste code]
What's causing this and how do I fix it?"

"Explain the difference between HashMap and TreeMap.
Format:
- First, a one-sentence summary
- Then a comparison table with: time complexity, ordering, null keys
- Finally, one code example showing when to use each"

"Convert these SQL queries to JPQL.

SQL: SELECT * FROM users WHERE age > 18
JPQL: SELECT u FROM User u WHERE u.age > 18

SQL: SELECT name, email FROM users WHERE active = true
JPQL: [convert this one]"

"You are a code reviewer with 10 years of Java experience.
Review this code for:
1. Performance issues
2. Security vulnerabilities
3. Code style (per Google Java Style Guide)

Be critical but constructive. List issues by severity."

System: You are an expert Spring Boot developer.
        Generate production-ready code with proper error handling.
        Use Java 21 features where appropriate.
        Do not add explanatory comments to the code.

User: Create a Spring Boot REST endpoint that:
- Accepts POST /api/v1/users
- Validates: email (not blank, valid format), name (not blank, max 100 chars)
- Creates user in MongoDB
- Returns 201 with the created user
- Returns 409 if email already exists
- Returns 400 for validation errors

❌ "Give me everything you know about microservices"
   → Too broad; answer will be shallow

❌ "Write good code" → No constraints

❌ "Is this correct?" [no code pasted] → Missing context

❌ "Fix all bugs" → Unclear scope

Code Review:
"Review this [language] code for: bugs, performance, security, readability.
[code]
List issues as: [severity] | [line] | [description] | [fix]"

Explain Code:
"Explain what this code does in 3 parts:
1. What it does (1 sentence)
2. How it works (step by step)
3. Potential issues (if any)
[code]"

Debug:
"I expected: [expected]
Actual result: [actual]
Here's the code and error:
[code + error]
What's wrong and how do I fix it?"