Data Types

Java developers already think in terms of types, but Clojure changes where “type work” happens. You still run on the JVM and you still have Java classes under the hood, but most application code lets values and operations drive the design. You reach for explicit type control when interop, correctness, or performance demands it.

This section focuses on the scalar values you will see everywhere: numbers, strings, characters, booleans, nil, and JVM-backed variants such as ratios, BigInt, and BigDecimal.

Java mental model: Clojure is dynamically typed, but it’s not “typeless”—values have concrete JVM classes, and you can always inspect them at the REPL.

What to pay attention to

• Truthiness: only false and nil are falsy. Everything else is truthy.
• nil vs null: nil is the idiomatic “no value” in Clojure, but Java APIs may still return/accept null—interop code should be defensive.
• Numbers: the numeric tower is real. You will see 1 as a Long, 1N as a BigInt, 1M as a BigDecimal, and ratios like 1/3.

1(class 1)    ;; => java.lang.Long
2(class 1N)   ;; => clojure.lang.BigInt
3(class 1/3)  ;; => clojure.lang.Ratio

As a Java engineer, the main skill is deciding which details matter in normal Clojure code and which details matter only at boundaries. Truthiness, nil, numeric literals, and Java string interop matter immediately. Primitive specialization, unchecked math, and type hints can usually wait until profiling or interop makes them necessary.

In this section

• Numbers in Clojure for Java Developers
  Learn Clojure's numeric literals, ratios, BigInt and BigDecimal values, overflow behavior, equality rules, and JVM interop concerns without importing Java's primitive-first habits.
• Strings in Clojure for Java Developers
  Work with Clojure strings as JVM strings: immutable text values, clojure.string functions, regex operations, formatting, nil handling, and Java interop boundaries.
• Characters, Booleans, and Truthiness
  Understand Clojure character literals, boolean values, truthiness, nil, predicates, and conditional forms through the differences Java developers notice first.