Comments and Style

Comments

#!/usr/bin/env groovy
// A single-line comment
def taxRate = 0.2 // after code

/*
 * A block comment
 * spanning several lines
 */

/**
 * Returns the price including tax.
 * @param price the net price
 * @return the gross price
 */
def withTax(price, rate) {
    price * (1 + rate)
}

println withTax(100, taxRate)

120.0

• // and /* ... */ work as in Java.
• /** ... */ comments are Groovydoc, turned into HTML documentation by the groovydoc tool and shown by your IDE.
• A shebang line (#!/usr/bin/env groovy) is allowed as the very first line, so a script can be run directly on Linux and macOS after chmod +x.

Idiomatic Groovy style

The official style guide (on groovy-lang.org) boils down to: write less ceremony than Java.

• Leave out semicolons, and public (it's the default).
• Use def for local variables when the type is obvious; use types for method signatures and public APIs.
• Use single quotes for plain strings, double quotes only when interpolating.
• Use == for equality, ?. and ?: for nulls, and Groovy Truth (if (list)) instead of explicit checks.
• Omit return on the last line of short methods and closures.
• Use list/map literals and collection methods (collect, findAll) instead of manual loops.
• Indent with four spaces; camelCase names; PascalCase classes.
• Keep parentheses when leaving them out would hurt readability — println foo(bar) is fine, println foo bar is not.

Tools

• CodeNarc — a static analysis tool with hundreds of rules for Groovy style, bugs, and complexity; it has Gradle and Maven plugins.
• IntelliJ IDEA — reformats Groovy code and flags common problems.
• npm-groovy-lint — CodeNarc packaged as a command-line tool, handy for linting and formatting Jenkinsfiles.

What's next

You now know the core of Groovy. Try the Beginner Projects, then continue to Level 2 — Intermediate, which starts with closures in depth.