Experiment 8    exception handling

The experiment purpose

1. Understand the concept of exception and exception handling mechanism
2. Master the method of catching exceptions
3. Master creating custom exceptions

Experimental hours 2 Class hours

Experimental content

1. Write a program , It is required to input the radius of a circle from the keyboard （double type ）, Calculate and output the area of the circle . When no exception handling mechanism is added , The input data is not double Type data （ Such as a string “abc”） What will happen ？ After adding the exception handling mechanism , Let the program give an error prompt when entering incorrect type data and ask for re-entry . 

package code81;
import java.util.InputMismatchException;  // Import input mismatch exception 
import java.util.Scanner;
public class code81 {

	public static void main(String[] args) {
		// TODO  Automatically generated method stubs 
		input:
		while(true) {  // Loop input 
		try {  // Select the range to catch exceptions 
			Scanner in = new Scanner(System.in);  
			double r,area;
			System.out.println(" Please enter the radius of the circle :");
			r=in.nextDouble();
			area=r*r*3.14;
			System.out.println(" The area of the circle is :"+area);
		  }catch(InputMismatchException e) {  // Handle input mismatch exceptions 
			  System.out.println(" The input data does not meet the requirements !");
			  System.out.println(" Please re-enter ");
			  e.printStackTrace();  // Print the exception information on the command line, the location and cause of the error in the program 
			  continue input;  // Keep typing , Make sure the input data is correct 
		  }
		  }
	}

}

2. Analyze the following procedure .

（1） What exceptions will occur when the program runs ？ How to catch and handle exceptions ？

（2） Modify the code ： No matter whether the program will produce exceptions during execution , Finally, it outputs “ End of program running ”

import java.util.Scanner;

public class ExceptionSample {

public static void main(String[] args) {

    Scanner sc=new Scanner(System.in);

    int a[]=new int[5];

    int n,sum=0;

    float average;

              n=sc.nextInt();

              for(int i=0;i<n;i++){

                 a[i]=sc.nextInt();

                 sum=sum+a[i];

              }

              average=(float)sum/n;

              System.out.println(average);

      }

}

1. An input data mismatch exception may occur 、 except 0 abnormal 、 Array out of bounds exception ;

package code82;
import java.util.Scanner;
import java.util.InputMismatchException;  // Import input mismatch exception 

public class ExceptionSample1 {

	public static void main(String[] args) {
		// TODO  Automatically generated method stubs 
		Scanner sc=new Scanner(System.in);
		int a[]=new int[5];
		int n,sum=0;
		float average;
		n=sc.nextInt();
		for(int i=0;i<n;i++) {
			try {  // Select the range to catch exceptions 
			a[i]=sc.nextInt();
			sum=sum+a[i];
			average=(float)sum/n;
			System.out.println(average);
		}catch(InputMismatchException e) {  // Handle input mismatch exceptions 
			System.out.println(" The input data format does not meet the requirements !");
		}catch(ArithmeticException e) {  // Processing division 0 abnormal 
			System.out.println(" except 0 abnormal !");
		}catch(ArrayIndexOutOfBoundsException e) {  // Handle array out of bounds exception 
			System.out.println(" Array out of bounds exception !");
		}
		}
	}
}

2. Modify the code ： No matter whether the program will produce exceptions during execution , Finally, it outputs “ End of program running ”

package code82;
import java.util.Scanner;
import java.util.InputMismatchException;  // Import input mismatch exception 
public class ExceptionSample2 {

	public static void main(String[] args) {
		// TODO  Automatically generated method stubs 
		Scanner sc=new Scanner(System.in);
		int a[]=new int[5];
		int n,sum=0;
		int average;
		try {  // Select the range to catch exceptions 
			n=sc.nextInt();
			for(int i=0;i<n;i++) {
				a[i]=sc.nextInt();
				sum=sum+a[i];
			}
			average=sum/n;
			System.out.println(average);
		}catch(InputMismatchException e) {  // Handle input mismatch exceptions 
			System.out.println(" The input data format does not meet the requirements !");
		}catch(ArithmeticException e) {  // Processing division 0 abnormal 
			System.out.println(" except 0 abnormal !");
		}finally {  // Ensure that the final program is output as required 
			System.out.println(" End of program running !");
		}

	}

}

3. Design a program , Enter two integers and an arithmetic operator （+、-、*、/）, According to the operator , Calculate the result of two integers . Considering that the data entered by the user is illegal , Need to catch exception （ Data format is abnormal 、 Abnormal division by zero 、 Illegal operator exception ）.

package code83;
import java.util.Scanner;
import java.util.InputMismatchException;  // Import input mismatch exception 
public class Calculate {

	public static void main(String[] args) {
		// TODO  Automatically generated method stubs 
		Scanner in = new Scanner(System.in);
		int a,b;
		String c;
		try {  // Select the range to catch exceptions 
			System.out.println(" Please enter two integers to be calculated :");
			a=in.nextInt();
			b=in.nextInt();
			System.out.println(" Please enter operator :");
			c=in.next();
			switch(c) {
			case "+":
				System.out.println(a + c + b +"="+(a+b));
				break;
			case "-":
				System.out.println(a + c + b +"="+(a-b));
				break;
			case "*":
				System.out.println(a + c + b +"="+(a*b));
				break;
			case "/":
				System.out.println(a + c + b +"="+(a/b));
				break;
			default:  
				throw new Exception();  // Declare discard exception 
			}
		}catch(InputMismatchException e) {  // Handle data mismatch exceptions 
			System.out.println(" Data format is abnormal !");
		}catch(ArithmeticException e) {  // Processing division 0 abnormal 
			System.out.println(" except 0 abnormal !");
		}catch(Exception e) {  // Handle default Illegal operator exception discarded in statement 
			System.out.println(" Illegal operator exception !");
		}
			

	}

}

4. Design a program , Calculate the area of a triangle according to its three sides . It is required to customize an exception class IllegaException, In the method of finding the area area() Declare to throw this exception type , An exception is thrown when the data of three sides input from the keyboard cannot form a triangle .

package code84;
import java.util.Scanner;
import java.util.InputMismatchException;  // Import input mismatch exception 
public class Triangle {
	public static double area(int a,int b,int c) throws IllegaException {  // Construct a method to calculate the area of a triangle 
		if(a+b<=c || a+c<=b || b+c<=a || a<=0 || b<=0 || c<=0) {
			throw new IllegaException();  // The type of exception thrown 
		}
			double s=(a+b+c)/2.0;
			double area=Math.sqrt(s*(s-a)*(s-b)*(s-c));
			return area;  // Find the area of the triangle 
		}
	public static void main(String[] args) throws IllegaException {
		// TODO  Automatically generated method stubs 
		Scanner in = new Scanner(System.in);
		int a,b,c;
		System.out.println(" Please enter the length of three sides of the triangle :");
		try {  // Select the range to catch exceptions 
			a=in.nextInt();
			b=in.nextInt();
			c=in.nextInt();
			System.out.println(" The area of the triangle is :"+area(a,b,c));  // Call the method of finding the area , The value of the output area 
		}catch(InputMismatchException e) {  // Handle input mismatch exceptions 
			System.out.println(" The input data format does not match !");
		}

	}

}
package code84;

public class IllegaException extends Exception {  // Custom exception classes IllegaException
	IllegaException(){  // Construction method 
		System.out.println(" Such three sides cannot form a triangle !");
	}

}

Summary of experiments

1. Using exception handling has more advantages than traditional error management techniques ;
2. Using exception handling can separate error handling code from normal code ;
3. If a method does not know how to handle the exception that occurs , You can declare to discard the exception when declaring the method ;
4. Inheritance other than runtime exceptions is from Exception Subclasses of class are collectively referred to as non runtime exceptions , For this kind of exception ,Java The compiler requires that the program must catch or declare to discard exceptions ;
5. Capture exception ：try...catch...finally;
6. Regardless of try Whether there is an exception in the statement block ,finally Statement blocks are executed ;
7. If Java The system exception type provided cannot meet the needs of program design , You can design your own exception types .Java Recommend the user's exception type to Exception Is a direct parent class .