Exception -exception

== = = = = = = = abnormal - Exception = = = = = = = ==

One 、 Basic concepts
Java In language , The abnormal situation in program execution is called “ abnormal ”.( Syntax and logic errors in the development process are not exceptions )
Abnormal events during execution can be divided into two categories
1)Error( error ):Java Serious problems that virtual machines can't solve .
Such as :JVM System internal error 、 Serious situations such as resource exhaustion .
such as :StackOverflowError[ Stack overflow ] and OOM(out of memory).
Error It's a serious mistake , The program will crash .
2) Exception: Other general problems caused by programming errors or accidental external factors , You can use targeted code for processing .
For example, null pointer access , Trying to read a file that doesn't exist , Network connection interruption, etc ,Exception There are two main categories :
Runtime exception [ Program runtime , Abnormal occurrence ] and
Compile time exception [ When programming , Exception detected by compiler ].

Two 、 Exception system diagram

Summary of exception system diagram
1． Exceptions fall into two categories , Runtime and compile time exceptions .
2 Runtime exception , The compiler can't check . It generally refers to the logic error in programming , Is the exception that programmers should avoid .java.lang.RuntimeException Class and its subclasses are runtime exceptions
3. For runtime exceptions , You don't have to deal with it , Because this kind of anomaly is very common , If the whole process may affect the readability and efficiency of the program
4. Compile time exception , Is an exception that the compiler requires to handle .

3、 ... and 、 Common runtime exceptions

1. NullPointerException Null pointer exception
2. ArithmeticException The mathematical operation is abnormal
3. ArrayIndexOutOfBoundsException Array subscript out of bounds exception
4. ClassCastException Type conversion exception
5. NumberFormatException Incorrect number format, exception []

3、 ... and 、 Common compilation exceptions
Compilation exception means that during compilation , An exception that must be handled , Otherwise, the code cannot be compiled
Common compilation exceptions are ：
1、SQLException// When operating the database , An exception may occur in the query table
2、IOException// When operating files , Abnormal occurrence
3、FileNotFoundException T// When operating on a nonexistent file , Something goes wrong
4、ClassNotFoundException // Load class , When this class does not exist , abnormal
5、EOFException// Operation file , To the end of the file , Something goes wrong
6、lllegalArguementException // Parameter exception

Four 、 exception handling
Exception handling is when an exception occurs , How to handle exceptions .

How to handle exceptions ：
1) try-catch-finally
The programmer catches the exception in the code , Do it yourself
2) throws
Throw the exception that occurs , Give it to the caller ( Method ) To deal with it , The top processor is JVM
① try-catch-finally Processing mechanism
try {
The code may have exceptions
}catch(Exception e) {
// Exception caught
//1. When an exception occurs
2. The system encapsulates exceptions into Exception object e, Pass to catch
//3 After getting the exception object , The programmer , Do it yourself
//4. Be careful , If nothing unusual happens
catch Code blocks do not execute
}finally{
//1. No matter try Whether there are exceptions in the code block , Always carry out finally
//2. therefore , Code that will usually release resources , Put it in finally
}

matters needing attention ：
1) If something goes wrong , The code behind the exception will not execute , Direct access to catch block .
2) If the exception does not occur , Then execute in sequence try Code block for , Will not enter into catch.
3) If you want to, whether or not an exception occurs , All execute a piece of code ( For example, close the connection , Release resources, etc )
Then use the code - finally

②throws Processing mechanism

1) If there's a way ( When the statement in is executed ) It may generate some kind of exception , But I'm not sure how
Handle this exception , This method should explicitly declare that an exception is thrown , Indicates that this method will not handle these exceptions , It's up to the caller of the method to handle .
2) Use... In the method declaration throws Statement can declare a list of exceptions thrown ,throws Difference behind
A constant type can be an exception type generated in a method , It can also be its parent class .

matters needing attention
1) For compilation exceptions , Must be handled in the program , such as try-catch perhaps throws
2) For runtime exceptions , If there is no processing in the program , The default is throws How to deal with [ give an example ]
3) When a subclass overrides a method of a parent class , Rules for throwing exceptions : Subclass override method , The type of exception thrown is either the same as the exception thrown by the parent class , Or the subtype of the type of exception thrown for the parent class [ give an example ]
4) stay throws In the process , If there is a way try-catch, It's equivalent to handling exceptions , You don't have to
throws

5、 ... and 、 Custom exception
1、 Basic concepts
When something appears in the program “ error ”, But the error message is not in Throwable Subclass describes the handling of , You can design your own exception class at this time , Used to describe the error message .
2、 To customize an exception
1) Defining classes : Custom exception class name ( The programmer writes it himself ) Inherit Exception or RuntimeException
2) If you inherit Exception, This is a compilation exception
3) If you inherit RuntimeException, It belongs to operation exception ( Generally speaking , Inherit RuntimeException)

6、 ... and 、throw and throws The difference between