How to throw exception in Java?

Throw an exception through the throw keyword, syntax: throw new ExceptionName(message). Steps: 1. Check the condition; 2. If the condition is not met, throw an exception: throw new ExceptionName(message); 3. Define a custom exception: inherit Exception and provide the constructor and getMessage() method.

How to throw exceptions in Java

In Java, you can throw exceptions through the throw keyword abnormal. An exception is an event that indicates that a program has encountered a problem that prevents it from continuing to execute normally.

Syntax

throw new ExceptionName(message);

Where:

• ExceptionName is the class name to throw an exception.
• message is an optional exception message that provides detailed information about the exception.

Practical case

Suppose we have a method to check whether the username is valid. If the username is invalid, we need to throw an exception to indicate the error.

public static void checkUsername(String username) throws InvalidUsernameException {
    // 检查用户名是否有效
    if (username == null || username.isEmpty()) {
        throw new InvalidUsernameException("用户名不能为空");
    }
}

When calling the checkUsername method, if the incoming username is invalid, an InvalidUsernameException exception will be thrown.

Custom exceptions

Java provides a Exception base class for all exceptions. However, we can also create our own custom exception classes to provide more detailed information for specific types of errors.

Here are the steps to create a custom exception InvalidUsernameException:

1. Create a new class that inherits from Exception.
2. Provide a constructor to receive exception messages.
3. Redefine the getMessage() method as needed.

public class InvalidUsernameException extends Exception {

    public InvalidUsernameException(String message) {
        super(message);
    }

}

Using Custom Exceptions

Custom exceptions can be thrown and handled in the same way as built-in exceptions.

try {
    checkUsername("admin");
} catch (InvalidUsernameException e) {
    System.out.println(e.getMessage());
}

Best Practices

• Throw only exceptions that may actually affect program execution.
• Develop a clear exception handling strategy to ensure that exceptions are handled correctly.
• Use custom exceptions to provide more details about specific errors.

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