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Throw Exception in Java

beginner· Exceptions · Error Handling

Problem

The throw statement is what actually raises an exception at the exact point a failure is detected — it's the action, distinct from merely declaring that a method might fail.

Validate a quantity, and use throw to raise an exception the moment it turns out to be invalid.

Input

quantity = -3

Output

Rejected: Quantity cannot be negative: -3

Java Program

Java

public class ThrowExceptionDemo {
    static void validateQuantity(int quantity) {
        if (quantity < 0) {
            throw new IllegalArgumentException("Quantity cannot be negative: " + quantity);
        }
        System.out.println("Quantity accepted: " + quantity);
    }

    public static void main(String[] args) {
        try {
            validateQuantity(-3);
        } catch (IllegalArgumentException e) {
            System.out.println("Rejected: " + e.getMessage());
        }
    }
}

▶ Run Code

Output

Rejected: Quantity cannot be negative: -3

Core Logic

Checking the precondition first, and using throw the moment it fails, raises the exception at the exact line where the problem was actually detected.

How It Works

1validateQuantity(-3) checks quantity < 0 before doing anything else with the value.
2throw new IllegalArgumentException(...) is the statement that actually creates the exception object and raises it right there — nothing happens after it in that method call.
3The caller's try/catch is what stops the exception from propagating further and crashing the program.
4If the quantity had been valid, throw would never execute and the method would print its success message instead.

For quantity = -3, the check fails immediately, so throw raises the exception, caught by the caller and printed as "Rejected: Quantity cannot be negative: -3".

💡

Key Point: throw is the single statement that actually raises an exception — a method's throws clause only declares that this might happen, it doesn't make anything happen on its own.

Key Concepts

throw statementprecondition checkIllegalArgumentException

Approach 2: Java 8

Java

import java.util.Optional;

public class ThrowExceptionOptional {
    static int validateQuantity(int quantity) {
        return Optional.of(quantity)
            .filter(q -> q >= 0)
            .orElseThrow(() -> new IllegalArgumentException("Quantity cannot be negative: " + quantity)); // built only if the filter rejected the value
    }

    public static void main(String[] args) {
        try {
            int accepted = validateQuantity(-3);
            System.out.println("Quantity accepted: " + accepted);
        } catch (IllegalArgumentException e) {
            System.out.println("Rejected: " + e.getMessage());
        }
    }
}

▶ Run Code

Output

Rejected: Quantity cannot be negative: -3

Core Logic

Optional.filter().orElseThrow() expresses 'validate, then throw' as one chained expression instead of an explicit if-check followed by a separate throw statement.

How It Works

1Optional.of(quantity) wraps the value to be validated.
2.filter(q -> q >= 0) keeps it only if it's non-negative, becoming empty otherwise — the same condition the manual if check tested.
3.orElseThrow(() -> new IllegalArgumentException(...)) returns the quantity if present, or invokes the lambda to build and throw the exception if the check failed.
4The throw keyword is still there, just moved inside the lambda that orElseThrow() calls when needed.

For quantity = -3, the filter produces an empty Optional, so orElseThrow() invokes the lambda and raises the same exception, caught and printed identically.

💡

Key Point: The exception is only constructed if the Optional is actually empty — orElseThrow() defers building it until the moment it's needed, rather than always allocating it up front.

Key Concepts

Optionalfilter()orElseThrow()

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Throws Keyword

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