Java self-study notes (17): Exception

The basic concept of abnormal

The program is running, there is a program operation can not be performed while the cause of the interruption.

An exception is an event occur during the execution of a program, he interrupted the normal operation command,

　　Blocker is a normal operation of the error

　　It is a special object, Exception subclass

Java errors

Serious problem a reasonable application does not intercepted, such as a failure of VM

Error are also objects is a subclass of Error

 

Abnormal program itself can be processed by the program and the external environment is caused by

Originally built-in error system is generally unable to handle the programmer does not need to deal with

 

Runtime exception

RuntimeException class and its subclasses are exceptions that are typically exempt abnormalities, such as ArithmeticException, NullPointException, IndexOutOfBoundsException

Abnormal non-runtime

Exception belong to the type class and its subclasses, need to be processed. Such as: abnormal IOException, SQLException and user-defined

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Exception Handling

step:

1, throw an exception: when semantic constraints are violated will throw (the throw) abnormalities, i.e. an exception event, an exception object generated, and submit it to a running system, and then look for the corresponding code from the operating system to handle exceptions

2, catch the exception: the exception is thrown, the runtime system begins to generate exception object code, the method call stack eye start looking until you find appropriate treatment method code indulgence, and the exception object to the method so far

java.util.Scanner Import; 

public  class StringDemo { 

    public  static  void main (String [] args) { 
        Scanner INPUT = new new Scanner (the System. in ); 
        the System. OUT .println ( " Please enter the dividend: " );
         int X = input.nextInt (); 
        the System. OUT .println ( " Please enter the divisor: " );
         int Y = input.nextInt ();               // Y = 0 exception occurs 
        the System. OUT .println (Divide (X, Y)) ; 
    }
    public static int divide(int x,int y) {
        return x/y; 
    }
}

//y=0 时，抛出异常
/* 
Exception in thread "main" java.lang.ArithmeticException: / by zero
    at StringDemo.divide(StringDemo.java:14)
    at StringDemo.main(StringDemo.java:11)
*/

Abnormal, the program is still running arithmetic exception operation belongs

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The inevitable exceptions, such as a network outage, memory overflow, how to deal with?

Disposition:

1, using the try ... catch ... finally statement to handle exceptions

　　The first step in using try {....} catch exceptions sentence block selected range, that is, the exception may trigger code included in the try block

　　A try block may be accompanied by one or more catch statement, a plurality of catch blocks, treated by a different exception

　　finally block: whether or not an exception occurs, will be executed

Import java.util.InputMismatchException;
 Import java.util.Scanner; 

public  class StringDemo { 

    public  static  void main (String [] args) { 
        
        the try { 
            Scanner INPUT = new new Scanner (the System.in); 
            System.out.println ( "Please enter the dividend: " );
             int the X-= input.nextInt (); 
            System.out.println ( " Please enter the divisor: " );
             int the y-input.nextInt = (); // the y-appear abnormal = 0   
            int RES = 0 ; 
            RES =Divide (X, Y); 
            System.out.println (RES); 
        } the catch (an ArithmeticException E) { 
            System.out.println ( "Exception divisor" ); 
        } the catch (a InputMismatchException E) { 
            System.out.println ( "Enter exception " ); 
        } the finally { 
　　　　　　　System.out.println (" whether abnormality will be executed ") 
　　　　　} 
        
    } 
    public  static  int Divide ( int X, int Y) {
         return X / Y; 
    } 
}

2, using the keyword throws an exception transfer

Import a java.io.FileInputStream;
 Import java.io.FileNotFoundException; 

public  class StringDemo { 

    public  static  void main (String [] args) throws a FileNotFoundException { 
        testThrows (); 
    } 
        
    public  static  void testThrows () throws a FileNotFoundException { 
        the FileInputStream FIS = new new the FileInputStream ( "test.txt" ); 
    } 
    
} 

// throws an exception, throw up, who caused an exception, who will be responsible