What is an exception?

Some exceptions are caused by user errors, some are caused by program errors, and others are caused by physical errors.

Exception handling keywords:

try, catch, finally, throw, throws

Notes:

1. Error is not an exception, but a breakaway A matter of programmer control.

2. All exception classes are subclasses inherited from the java.lang.Exception class.

3. The exception class has two main subclasses: IOException class and RuntimeException class.

4. Java has many built-in exception classes.

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Grammar:

try{
//需要監(jiān)聽的代碼塊
}
catch（異常類型 異常名稱/e）{
//對捕獲到try監(jiān)聽到的出錯的代碼塊進行處理
throw 異常名稱/e; //thorw表示拋出異常
throw new 異常類型（“自定義”）;
}
finally{
//finally塊里的語句不管異常是否出現(xiàn)，都會被執(zhí)行
}
修飾符 返回值 方法名 () throws 異常類型{  
//throws只是用來聲明異常,是否拋出由方法調(diào)用者決定
//代碼塊
}

Sample code: (try, catch and finally)

public class ExceptionTest {
	public static void main(String[] args) {
		Scanner input=new Scanner(System.in);  
        try{ //監(jiān)聽代碼塊  
        int a=input.nextInt();  
        int b=input.nextInt();  
        double sum=a/b;   
        System.out.println(sum);  
        }  
        catch(InputMismatchException e){  
            System.out.println("只能輸入數(shù)字");  
        }  
        catch(ArithmeticException e){  
            System.out.println("分母不能為0");  
        }  
        catch(Exception e){ //Exception是所有異常的父類  
            System.out.println("發(fā)生了其他異常");  
        }  
        finally{ //不管是否出現(xiàn)異常，finally一定會被執(zhí)行  
            System.out.println("程序結(jié)束");  
        }  
	}
}

Sample code: (throw keyword)

import java.util.InputMismatchException;
import java.util.Scanner;
 
public class ExceptionTest {
	public static void main(String[] args) {
		Scanner input=new Scanner(System.in);  
        try{ //監(jiān)聽代碼塊  
        int a=input.nextInt();  
        int b=input.nextInt();  
        double sum=a/b;   
        System.out.println(sum);  
        }  
        catch(InputMismatchException e){ //catch（異常類型 異常名稱）  
            System.out.println("只能輸入數(shù)字");  
            throw e; //拋出catch捕捉到的異常  
            //throw new InputMismatchException(); 同上  
        }  
        catch(ArithmeticException e){  
            System.out.println("分母不能為0");  
            throw new ArithmeticException("分母為0拋出異常"); //拋出ArithmeticException異常  
        }  
        catch(Exception e){ //Exception是所有異常的父類  
            System.out.println("發(fā)生了其他異常");  
        }  
        finally{ //不管是否出現(xiàn)異常，finally一定會被執(zhí)行  
            System.out.println("程序結(jié)束");  
        }   
	}
}

Sample code: (throws)

public class Throws {
	int a=1;
	int b=0;
	public void out() throws ArithmeticException{ //聲明可能要拋出的異常，可以有多個異常,逗號隔開
		try{ //監(jiān)聽代碼塊
		int sum=a/b;
		System.out.println(sum);
		}
		catch(ArithmeticException e){
			System.out.println("分母不能為0");
		}
		finally{ //不管是否出現(xiàn)異常，finally一定會被執(zhí)行
			System.out.println("程序結(jié)束");
		}
	}
	public static void main(String[] args){
		Throws t=new Throws();
			t.out(); //調(diào)用方法
			throw new ArithmeticException("分母為0拋出異常"); //由調(diào)用的方法決定是否要拋出異常
			/*
			 * 第二種拋出方式
			 */
//			ArithmeticException a=new ArithmeticException("分母為0拋出異常");
//			throw a;
	}
}

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