Characters, Booleans, and Truthiness

Characters and booleans look simple, but Java developers usually hit two Clojure-specific differences quickly:

• Character literals use backslash syntax, not single quotes.
• Conditionals use truthiness, where only false and nil are falsey.

Those two details affect parsing, validation, and every if, when, and cond you write.

Characters

Clojure characters are JVM Character values:

1(class \a)
2;; => java.lang.Character

Use char? to test for a character:

1(char? \a)
2;; => true
3
4(char? "a")
5;; => false

Convert between characters and code points deliberately:

1(int \A)
2;; => 65
3
4(char 65)
5;; => \A

int returns the numeric value of the JVM character code unit. Do not describe this as “ASCII” in general; Clojure strings and characters are Unicode-aware because they sit on JVM strings and chars.

Booleans

Clojure has true and false:

1(class true)
2;; => java.lang.Boolean

Predicates conventionally end with ?:

1(require '[clojure.string :as str])
2
3(number? 42)
4(string? "hello")
5(nil? nil)
6(str/blank? "   ")

That naming convention is not enforced by the compiler, but it is strong Clojure style. Java developers should read a trailing ? as “this function returns a truthy or falsey answer.”

Truthiness

This is the important rule:

Java developers often expect 0, empty strings, or empty collections to behave like false. They do not.

1(if 0
2  :truthy
3  :falsey)
4;; => :truthy
5
6(if []
7  :truthy
8  :falsey)
9;; => :truthy

Use explicit predicates when emptiness matters:

1(if (seq items)
2  :has-items
3  :empty)

seq returns a sequence when a collection has elements and nil when it is empty, which makes it useful in conditionals.

Conditional Forms

Examples:

1(defn display-name [user]
2  (if-let [name (:name user)]
3    name
4    "Anonymous"))

if-let binds a value only when it is truthy. This is common for optional map values:

1(defn classify-score [score]
2  (cond
3    (nil? score) :missing
4    (>= score 90) :excellent
5    (>= score 70) :passing
6    :else :needs-review))

Java Interop and Null

Clojure uses nil; Java APIs use null. At interop boundaries, treat them as the same absence signal but do not let absence leak unchecked into core logic:

1(defn safe-length [s]
2  (if (nil? s)
3    0
4    (.length ^String s)))

In pure Clojure code, prefer returning nil for absence and using predicates or some-> to handle optional values:

1(some-> user
2        :email
3        str/lower-case)

Practice

1. Evaluate (if 0 :yes :no), (if "" :yes :no), and (if [] :yes :no).
2. Rewrite a Java if (list.isEmpty()) check using seq or empty?.
3. Write initial that returns the first character of a nonblank string or nil.
4. Write safe-active? that treats missing :active? as false.

Key Takeaways

• Character literals use backslash syntax such as \a, \space, and \newline.
• Only false and nil are falsey in Clojure.
• Empty strings, zero, and empty collections are truthy.
• Use predicates such as empty?, seq, nil?, and str/blank? when you need a specific condition.
• Java null appears at interop boundaries; normalize absence before it spreads.

Quiz: Characters, Booleans, and Truthiness