what are exceptions in java

Exception in Java, also known as exception, is an event that occurs during program execution, which interrupts the normal instruction flow of the executing program. In order to handle running errors in the program promptly and effectively, exception classes must be used.

Exception introduction:

In a program, errors may occur from various situations that the programmer did not expect, or that are beyond the programmer's control. Environment, such as user's bad data, trying to open a file that does not exist, etc. In order to handle running errors in the program promptly and effectively, Java specifically introduced the exception class.

Example:

In order to better understand what an exception is, let’s look at a very simple Java program. The following sample code allows the user to enter an integer within 1~3, and prompts an input error in other cases.

import java.util.Scanner;

public class Test01 {
    public static void main(String[] args) {
        System.out.println("请输入您的选择：（1~3 之间的整数）");
        Scanner input = new Scanner(System.in);
        int num = input.nextInt();
        switch (num) {
        case 1:
            System.out.println("one");
            break;
        case 2:
            System.out.println("two");
            break;
        case 3:
            System.out.println("three");
            break;
        default:
            System.out.println("error");
            break;
        }
    }
}

Under normal circumstances, the user will enter a number between 1 and 3 according to the system prompts. However, if the user does not input as required, for example, enters a letter "a", an exception will occur when the program is run, and the running results are as follows.

请输入您的选择：（1~3 之间的整数）
a
Exception in thread "main" java.util.InputMismatchException
        at java.util.Scanner.throwFor(Unknown Source)
        at java.util.Scanner.next(Unknown Source)
        at java.util.Scanner.nextInt(Unknown Source)
        at java.util.Scanner.nextInt(Unknown Source)
        at text.text.main(text.java:11)

Causes and usage principles of exceptions

There are three main reasons why an exception occurs in Java:

1. Java internal error occurs, Java Exceptions generated by the virtual machine.

2. Exceptions caused by errors in the written program code, such as null pointer exceptions, array out-of-bounds exceptions, etc. This kind of exception is called an unchecked exception, and it is generally necessary to handle these exceptions centrally in certain classes.

3. Exceptions generated manually through the throw statement are called checked exceptions and are generally used to inform the caller of the method some necessary information.

Java handles exceptions through object-oriented methods. During the running of a method, if an exception occurs, the method will generate an object representing the exception and hand it to the runtime system. The runtime system will look for the corresponding code to handle the exception.

We call the process of generating an exception object and submitting it to the runtime system as throwing an exception. The runtime system searches the method's call stack until it finds an object that can handle that type of exception. This process is called catching the exception.

Java exceptions force users to consider the robustness and security of their programs. Exception handling should not be used to control the normal flow of the program. Its main function is to capture exceptions that occur when the program is running and handle them accordingly. When writing code to handle exceptions that may occur in a certain method, you can follow the following three principles:

1. Use the try catch statement in the current method declaration to catch exceptions.

2. When a method is overridden, the method that overrides it must throw the same exception or a subclass of the exception.

3. If the parent class throws multiple exceptions, the overriding method must throw a subset of those exceptions and cannot throw new exceptions.

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