Groovy Truth

last modified October 15, 2026

In Groovy, any value can be used where a boolean is expected, such as in an if condition or the operands of && and ||. Groovy converts the value with a set of rules called Groovy truth. This tutorial shows those rules, how to use them safely, and how to customize them for your own classes.

The Basic Rules

Null, false, zero, empty strings, empty collections, and empty maps are false. Non-empty strings, non-zero numbers, and non-empty collections are true. Any object that is not one of these special cases is true, unless its class defines a different rule.

BasicTruth.groovy

def values = [null, false, 0, "", [], [:], "text", 42, [1], [a: 1]]

values.each { v ->
    println "${String.valueOf(v).padRight(8)} -> ${v ? 'true' : 'false'}"
}

The ternary operator ? : uses Groovy truth. The output shows that only null, false, zero, and the empty values are false. This makes checks such as if (name) concise for optional inputs.

Using Groovy Truth in Conditions

Instead of comparing with null and the empty string separately, test the value directly. Be careful with numbers: 0 is false, so if (count) does not distinguish a missing count from zero.

TruthInConditions.groovy

def describe(String name) {
    if (name) {
        return "Hello, $name"
    }
    return "Hello, guest"
}

println describe("Ann")
println describe("")
println describe(null)

def count = 0
if (!count) {
    println "No items, or count was never set"
}

An empty string and null both fall through to the default greeting. The last check treats 0 and null the same way, which is exactly the ambiguity to watch for. When the difference matters, compare explicitly with count == null.

Logical Operators and Values

The &&, ||, and ! operators use Groovy truth, but they always return a boolean. To get one of the values back, use the Elvis operator ?:, which returns its left operand when that operand is true and its right operand otherwise.

LogicalValues.groovy

def user = [name: "", nick: "Ace"]

println user.name || user.nick
println user.name ?: user.nick
println user.missing ?: "no value"

user.name || user.nick is true, because the nick is not empty. The Elvis operator returns the value itself: the empty name is skipped and "Ace" is printed. Use ?: when you need a value, and || when you need a yes-or-no answer.

Groovy Truth for Custom Classes

Define an asBoolean() method to control how your objects are evaluated. Groovy calls it whenever the object is used as a condition.

CustomTruth.groovy

class Inventory {
    List<String> items = []

    boolean asBoolean() {
        !items.isEmpty()
    }
}

def stock = new Inventory()
println stock ? "In stock" : "Empty"

stock.items << "keyboard"
println stock ? "In stock" : "Empty"

Without the method, every Inventory object would be true. With asBoolean(), the object is true only when it holds items, so it reads like a collection.

Safe Checks with the Not Operator

The ! operator negates Groovy truth, so !list is true for null and for an empty list. Use it for guard clauses at the start of a method.

GuardClause.groovy

def average(List<Integer> numbers) {
    if (!numbers) {
        return null
    }
    numbers.sum() / numbers.size()
}

println average([2, 4, 6])
println average([])
println average(null)

Both the empty list and null return null from average, so the division never runs with a zero divisor. The guard reads more clearly than separate null and isEmpty() checks.

Best Practices

Use truth for presence checks: if (name) is clear for strings and collections.
Be explicit with numbers: Compare with == 0 or != null when zero is a valid value.
Remember the return value: && and || return an operand, so do not assume a boolean.
Define asBoolean deliberately: Only add it when an object has a natural empty state.

Source

Groovy Semantics Documentation | Groovy Operators Documentation

Groovy truth keeps conditions short, but it hides the difference between null, empty values, and zero. Use it for presence checks, and compare explicitly when a value such as 0 carries meaning.

Author

My name is Jan Bodnar, and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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