The exception of java learning

Life's in danger 2021-01-20 22:45:50
exception java learning


One 、 What's unusual .

Two 、 How to handle exceptions

-try-catch-finally

-throw

-throws

- Custom exception

- Abnormal chain

3、 ... and 、 Exceptions are essentially procedural errors , Mistakes often happen when we write programs , Includes errors during compilation and runtime .

Brackets don't pair properly ; The semicolon is missing at the end of the statement ; Keyword writing error ;

① Calling methods with a reference to an empty object . Null pointer exception .

 

② An array

 

③ Divisor is 0

These are all manifestations of errors that occur during runtime .

 

Four 、 While the program is running , What happened by accident , A departure from the intent of our program itself , Can be understood as an exception . Through reasonable exception handling , Can better improve the robustness of the program .

5、 ... and 、Throwable yes java The root class of exception in , He has two important subclasses :Error and Exception

Error It's a bug that the program can't handle , Represents a more serious problem in running an application .java Some of the problems with virtual machines .

Virtual machine error 、 Memory overflow error 、 Thread deadlock error . They are beyond the control and processing power of the application , And most of them are not allowed when the program is running . For well-designed applications , Just in time, there was a mistake , In essence, we should not try to deal with the abnormal situation caused by it .

Exception It's an exception that can be handled by the program itself . Exception handling usually refers to the handling of this type of exception .

 

6、 ... and 、 stay java In the application , The exception handling mechanism is :

Throw an exception 、 The catching .

When an error occurs in a method and an exception is thrown , Method will create an exception object and deliver it to the runtime system for processing . Then the exception object will contain the exception type or The state of the program when an exception occurs .

For runtime exceptions 、 Error or checkable exception ,Java The exception handling methods required by the technology are different .

  Throwing and catching exceptions are done through 5 Keywords to achieve , They are :try、catch、finally、throw、throws

 

 

 

try Block can be connected to zero or more catch block , without catch block , You have to be with a finally block .

or try catch or try finally Must be used in combination .

Example : When main Error reporting during the execution of the method ,

import java.util.Scanner;
public class ErrorTest {
public static void main(String[] args) {
// // Define two variables export merchant 
// int one = 12;
// int two = 2;
// System.out.println(one/two); //6
// When these two variables are changed to keyboard input 
Scanner inputScanner = new Scanner(System.in);
System.out.println("=== The operation begins ===");
System.out.println(" Please enter the first integer :");
int one = inputScanner.nextInt();
System.out.println(" Please enter the second integer :");
int two = inputScanner.nextInt();
System.out.println(one/two);
System.out.println("=== End of calculation ===");
}
}

When the divisor enters 0 when , The program will report the following exception

When the divisor enters a letter , The following error is reported .( Input format error exception )

 

Although it's a simple factor to get along with , After human input, there are many situations , So you need to handle exceptions properly .

Will use the program that may go wrong try When it's wrapped up ,

import java.util.Scanner;
public class ErrorTest {
public static void main(String[] args) {
// // Define two variables export merchant 
// int one = 12;
// int two = 2;
// System.out.println(one/two); //6
// When these two variables are changed to keyboard input 
Scanner inputScanner = new Scanner(System.in);
System.out.println("=== The operation begins ===");
try {
System.out.println(" Please enter the first integer :");
int one = inputScanner.nextInt();
System.out.println(" Please enter the second integer :");
int two = inputScanner.nextInt();
System.out.println(one/two);
} catch (Exception e) {
System.out.println(e);
}
System.out.println("=== End of calculation ===");
}
}

It will print out

 

stay catch Through e.printStackTrace() Catch trace exception

 

finally Packages are code that will be executed no matter what .

import java.util.Scanner;
public class ErrorTest {
public static void main(String[] args) {
// // Define two variables export merchant 
// int one = 12;
// int two = 2;
// System.out.println(one/two); //6
// When these two variables are changed to keyboard input 
Scanner inputScanner = new Scanner(System.in);
System.out.println("=== The operation begins ===");
try {
System.out.println(" Please enter the first integer :");
int one = inputScanner.nextInt();
System.out.println(" Please enter the second integer :");
int two = inputScanner.nextInt();
System.out.println(one/two);
} catch (Exception e) {
System.out.println(e);
e.printStackTrace();
}finally {
System.out.println("=== End of calculation ===");
}
}
}

multiple catch: Transform the code into multiple catch block , Handle according to different types of error reports .

import java.util.InputMismatchException;
import java.util.Scanner;
public class ErrorTest {
public static void main(String[] args) {
// // Define two variables export merchant 
// int one = 12;
// int two = 2;
// System.out.println(one/two); //6
// When these two variables are changed to keyboard input 
Scanner inputScanner = new Scanner(System.in);
System.out.println("=== The operation begins ===");
try {
System.out.println(" Please enter the first integer :");
int one = inputScanner.nextInt();
System.out.println(" Please enter the second integer :");
int two = inputScanner.nextInt();
System.out.println(one/two);
} catch (ArithmeticException e) {
System.out.println(" Divisor is not allowed to be 0");
e.printStackTrace();
} catch(InputMismatchException e) {
System.out.println(" Please enter a number ");
e.printStackTrace();
}
finally {
System.out.println("=== End of calculation ===");
}
}
}

 catch No matter how many exceptions there are, they are not covered completely , So the last one catch in Delivery required Exception class . It's also a security mechanism , Be sure to put it on the last one catch

import java.util.InputMismatchException;
import java.util.Scanner;
public class ErrorTest {
public static void main(String[] args) {
// // Define two variables export merchant 
// int one = 12;
// int two = 2;
// System.out.println(one/two); //6
// When these two variables are changed to keyboard input 
Scanner inputScanner = new Scanner(System.in);
System.out.println("=== The operation begins ===");
try {
System.out.println(" Please enter the first integer :");
int one = inputScanner.nextInt();
System.out.println(" Please enter the second integer :");
int two = inputScanner.nextInt();
System.out.println(one/two);
} catch (ArithmeticException e) {
System.out.println(" Divisor is not allowed to be 0");
e.printStackTrace();
} catch(InputMismatchException e) {
System.out.println(" Please enter a number ");
e.printStackTrace();
} catch(Exception e) {
System.out.println(e);
e.printStackTrace();
}
finally {
System.out.println("=== End of calculation ===");
}
}
}

7、 ... and 、System.exit(1)// Terminate program operation Right and wrong in brackets 0 Can

 

stay finally Don't write return xxx. Because when catch and finally There are return When , No matter which one you enter catch, all return Yes. finally The value of the inside .

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