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Table of Contents
2. When should an exception be thrown? Don't throw it around!
3. Custom exception class: Make errors more meaningful
4. Be polite to throw exceptions: don't just polish it.
Home Java javaTutorial How to throw an exception?

How to throw an exception?

Jun 27, 2025 am 01:15 AM

The core of throwing exceptions is to discover problems, create exception objects and throw them. The syntax is similar in different languages ??but the keywords are slightly different. 1. The basic syntax requires the use of throw or raise keywords followed by exception objects, and the exact exception type is selected; 2. Exceptions should be thrown when the parameters are illegal, the status is inconsistent, the resources are unavailable, or there should be no state, and not all errors are applicable; 3. Custom exception classes can improve the meaning of errors, which is convenient for debugging and targeted processing; 4. Exception throwing requires documentation instructions, reasonable capture and necessary logs to avoid hidden dangers caused by empty catch.

How to throw an exception?

Exceeding exceptions is actually an active interruption of the process and prompting an error message when the program runs. Different programming languages ??have different syntax, but the core idea is the same: discover problems → create exception objects → throw exceptions .

Let’s talk about how to throw exceptions and what to pay attention to from several common perspectives.


1. Basic syntax: How to correctly throw an exception?

In most mainstream languages, the keyword of the statement that throws an exception is throw , followed by an exception object. for example:

  • Java :

     throw new IllegalArgumentException("Argument is invalid");
  • Python :

     raise ValueError("Value Error")
  • C# :

     throw new ArgumentException("parameter error");
  • JavaScript (Node.js or browser) :

     throw new Error("Error");

The key points are two:

  • To throw an "exception object", not a string or number;
  • The exception type should be as accurate as possible, such as using ValueError to indicate the wrong value, and using TypeError to indicate the type error.

2. When should an exception be thrown? Don't throw it around!

Not all errors require exception thrown, and some cases are more suitable for returning error codes or default values. The following situations are more suitable for throwing exceptions:

  • The parameters are illegal or missing, causing the function to be unable to execute normally;
  • The status does not meet the logical requirements, such as login must be logged in before operation;
  • External resources are unavailable, such as the file does not exist or the network request fails;
  • A state that should not appear, such as switch does not overwrite all enum values.

For example, in a payment interface, if the amount passed in is less than or equal to 0, it is necessary to directly throw an exception, because this is not a situation that should occur in the normal process.


3. Custom exception class: Make errors more meaningful

Although the system comes with the exception type is easy to use, it is sometimes not specific enough. For example, if you write a user registration module, you can create a UserRegistrationError class, inherited from a general exception class, so that it is easier for the caller to judge the error type.

For example, in Python, you can write it like this:

 class UserRegistrationError(Exception):
    pass

def register_user(email):
    If not email:
        raise UserRegistrationError("The mailbox cannot be empty")

There are several benefits to doing this:

  • The error type is clearer, making it easier to debug and log analysis;
  • You can carry more information, such as error codes, original input, etc.;
  • The upper layer code can use try-except to catch specific types of exceptions and perform targeted processing.

4. Be polite to throw exceptions: don't just polish it.

Throwing an exception is just the first step, and more importantly, someone can catch it. So there are two things to pay attention to when writing code:

  • Clarify the documentation instructions : If the function you write will throw exceptions, it is best to explain them clearly in the comments or documents;
  • Use try-catch reasonably : catch exceptions in the right place, do some cleaning work or downgrade processing;
  • Don't swallow exceptions : It is very dangerous to not do anything after the catch reaches the exception (commonly known as "empty catch").

For example, this Java code is very bad:

 try {
    doSomething();
} catch (Exception e) {
    // Do nothing}

At least a log should be printed or notified to the developer.


Basically that's it. Exceeding exceptions is a very common operation. It looks simple, but if you use it well, you need to understand the program flow, error boundaries and exception design principles.

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