Google Java Style Guide

Official Google Java coding standards for consistent, maintainable code.

Golden Rules

1. 2-space indentation — no tabs
2. Column limit: 100 characters
3. Use @Override whenever applicable
4. No wildcard imports — import specific types
5. Braces required even for single-statement blocks
6. One top-level class per file
7. Prefer interfaces for type definitions

Quick Reference

Naming Conventions

Element	Convention	Example
Packages	lowercase.dotted	com.example.project
Classes/Interfaces	UpperCamelCase	UserService
Methods	lowerCamelCase	getUserById
Variables	lowerCamelCase	userCount
Constants	UPPER_SNAKE_CASE	MAX_RETRIES
Type parameters	Single letter	T, RequestT

Classes

// ✓ CORRECT
public class Animal {
    private final String name;

    public Animal(String name) {
        this.name = name;
    }

    public String speak() {
        return name + " makes a sound.";
    }
}

Control Structures

// ✓ CORRECT - braces always required
if (condition) {
    doSomething();
}

// ✗ INCORRECT - no braces
if (condition)
    doSomething();

// ✓ CORRECT - enhanced for loop
for (String item : items) {
    process(item);
}

Exception Handling

// ✓ CORRECT
try {
    processData(input);
} catch (IOException e) {
    logger.error("IO error: {}", e.getMessage());
    throw new ServiceException("Failed", e);
}

// ✓ CORRECT - try-with-resources
try (InputStream in = new FileInputStream(file)) {
    return IOUtils.toByteArray(in);
}

// ✗ INCORRECT
try {
    ...
} catch (Exception e) {    // too broad
    e.printStackTrace();   // don't use printStackTrace
}

Lambdas and Streams

// ✓ CORRECT
List<String> names = users.stream()
    .filter(u -> u.isActive())
    .map(User::getName)
    .sorted()
    .collect(Collectors.toList());

// ✓ CORRECT - method references
users.forEach(System.out::println);

Javadoc

/**
 * Finds a user by their unique identifier.
 *
 * @param userId the user's unique ID
 * @return an Optional containing the user, or empty if not found
 */
public Optional<User> findUserById(long userId) { ... }

Imports

// ✓ CORRECT
import java.util.List;
import java.util.Optional;

// ✗ INCORRECT
import java.util.*;  // no wildcard imports

Common Mistakes

Mistake	Correct Approach
Wildcard imports	Specific imports only
Omitting braces	Always use braces
Missing @Override	Always annotate overridden methods
e.printStackTrace()	Use a proper logger
Catching Exception broadly	Catch specific exceptions
4-space indentation	Use 2-space indentation

When to Use This Guide

• Writing new Java code
• Refactoring existing Java
• Code reviews
• Setting up Checkstyle rules
• Onboarding new team members

Install

npx skills add testdino-hq/google-styleguides-skills/java

Full Guide

See java.md for complete details, examples, and edge cases.