IllegalArgumentException

IllegalArgumentException is a runtime exception in the Java programming language indicating that a method has been passed an illegal or inappropriate argument. It is a subclass of RuntimeException, and therefore is unchecked. It signals that a precondition of a method has been violated by the caller, such as an argument value being out of range, of an invalid type, or otherwise inconsistent with the method’s contract. As a result, callers are not required to catch it, although they may choose to handle it or let it propagate to reveal programming errors.

Common usage patterns include validating parameters at the start of a method and throwing new IllegalArgumentException with a descriptive message when constraints are violated. For null values the conventional choice is often NullPointerException or to use Objects.requireNonNull, depending on the contract. NumberFormatException is a subclass of IllegalArgumentException, used when a string cannot be parsed as a numeric value.

The class provides several constructors: IllegalArgumentException(), IllegalArgumentException(String message), IllegalArgumentException(String message, Throwable cause), and IllegalArgumentException(Throwable cause). In API design, documenting argument constraints in the method’s Javadoc and preferring early validation helps maintain clear error reporting.

There is no direct alternative language-wide equivalent; other platforms have similar concepts, such as ArgumentException in .NET or ValueError in Python, but IllegalArgumentException is specific to Java’s standard exception hierarchy.