EC
AssertionError
JavaFATALNotableAssertion

Assertion failed — internal invariant violated

Quick Answer

Enable assertions with java -ea in development and test environments; do not use assert for production input validation (use IllegalArgumentException instead).

What this means

Thrown when an assert statement fails. Assertions are disabled by default in Java — they must be enabled with -ea (enableassertions). Use AssertionError to signal violated invariants in test code or defensive programming.

Why it happens
  1. 1An assert expression evaluates to false when assertions are enabled (-ea)
  2. 2Code flow reached a branch that should be unreachable

Fix

Use assert for internal invariants only

Use assert for internal invariants only
// Invariant check — never reachable in correct code
switch (status) {
    case OPEN, CLOSED -> handle(status);
    default -> throw new AssertionError("Unexpected status: " + status);
}

Why this works

Throwing AssertionError explicitly in default/else branches documents the assumption that no other values can arrive, and surfaces bugs immediately if they do.

Code examples
Enable assertions in JVM argsjava
java -ea com.example.Main
# Or in Maven Surefire:
<argLine>-ea</argLine>
Do not use assert for validationjava
// Bad — skipped in production (assertions disabled by default)
assert userId != null : "userId required";

// Good — always runs
Objects.requireNonNull(userId, "userId required");
Same error in other languages
AssertionErrorin Python→panic!in Rust→
Sources
Official documentation ↗

Java SE Documentation

Content generated with AI assistance and reviewed for accuracy. Found an error? hello@errcodes.dev

← All Java errors