What is an exception 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.

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

Series 1 (Recommended learning: java course )

In order to better understand what is abnormal, 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.

package ch11;
import Java.util.Scanner;
public class TestO1
{
    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 of exceptions and usage principles

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

Java internal error exception occurred, an exception generated by the Java virtual machine.

Exceptions caused by errors in the program code written, 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.

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.

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