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java.lang.IllegalArgumentException

java.lang.IllegalArgumentException

DodaTech 3 min read

java.lang.IllegalArgumentException is thrown when a method receives an argument that is outside the acceptable range or doesn’t meet the method’s preconditions.

What It Means

The method you called has specific requirements for its parameters (non-null, positive, within a range, etc.), and the value you passed violates one of those constraints. The method throws this exception before processing the invalid input.

Why It Happens

  • You passed a negative number where a positive value is required.
  • You passed a string that exceeds a maximum length limit.
  • You passed an enum value that doesn’t match expected constants.
  • A time or date value is out of the valid range.
  • You passed a null where the method requires non-null (though this often throws NullPointerException depending on the implementation).

How to Fix It

1. Read the error message

The exception message usually tells you exactly which parameter is invalid and what the expected range is:

// Example error:
// IllegalArgumentException: days: 367 (expected: 1-366)

2. Validate inputs before calling the method 

// BUG: passing invalid age
public void setAge(int age) {
    if (age < 0 || age > 150) {
        throw new IllegalArgumentException("age: " + age + " (expected: 0-150)");
    }
    this.age = age;
}

// Caller must validate first
public void registerUser(int age) {
    if (age < 0 || age > 150) {
        System.out.println("Invalid age: " + age);
        return;
    }
    setAge(age);
}

3. Use Objects.requireNonNull for null checking 

public void processOrder(Order order, Discount discount) {
    // Clear pre-condition checks at the top
    Objects.requireNonNull(order, "order must not be null");
    Objects.requireNonNull(discount, "discount must not be null");

    // Guard against invalid values
    if (order.getTotal() < 0) {
        throw new IllegalArgumentException(
            "order total must be non-negative, got: " + order.getTotal()
        );
    }

    // Business logic below — safe to proceed
}

4. Check API documentation for parameter constraints 

// The Javadoc might specify:
// @throws IllegalArgumentException if start > end
// @throws IllegalArgumentException if step <= 0

IntStream.rangeClosed(10, 5)     // throws: start 10 > end 5
IntStream.iterate(0, i -> i - 1) // fine — no constraint on step

5. Use Preconditions from Google Guava 

import com.google.common.base.Preconditions;

public void setPercentage(double value) {
    Preconditions.checkArgument(
        value >= 0.0 && value <= 100.0,
        "percentage must be between 0 and 100, got: %s", value
    );
    this.percentage = value;
}
What's the difference between IllegalArgumentException and NullPointerException?
IllegalArgumentException means the argument value is invalid (wrong range, wrong format). NullPointerException specifically means the argument is null where an object reference is required. Many methods throw NullPointerException via Objects.requireNonNull() for null checks and IllegalArgumentException for value range checks.
Should I throw IllegalArgumentException in my own code?
Yes — it’s the standard way to signal that a caller passed invalid arguments. It’s a runtime exception, so it doesn’t require a throws declaration. Document the constraints in your Javadoc using @throws IllegalArgumentException.
Why does some invalid input not throw an exception?
Many methods use defensive programming and silently clamp or default invalid values. For example, Math.max(5, -1) returns 5 without complaint. Only methods that explicitly validate their parameters will throw IllegalArgumentException. Always check the documentation.
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