Exception Handling in Java

1: Concept of the anomaly (Exception)

Java exception when Java provides a mechanism for handling errors in a program

2: Processing mode

Java uses an object-oriented approach to handling exceptions.

• Throw exception: If an exception occurs, the method generates an object representing the exception, stops the current execution path, and submits the exception object to the JRE
• Catch exception: After the JRE gets the exception, it looks for the appropriate code to handle the exception, and the JRE looks in the method's call stack, starting from the generated exception method until the appropriate exception-handling code is found.

3: Exception Classification:

Many exception classes are defined in the JDK, which correspond to a variety of possible exception events, all of which are derived from an instance of the thorwable class. if the built-in exception class does not meet your needs, you can also create your own exception class

　　

Error class---Do not need our---the only way: restart

Exception in Checked Exception is the compiler will check

The RuntimeException compiler does not check, for example:

1:int a = 1/0;

2: null pointer exception (NULLPOINTEREXCEPTION):, normally the object is empty

3: Array out of bounds (arrayindexoutofboundsexception)

4: Object conversion error (ClassCastException) to Judge E.g:obj instanceof

5: Number format exception (NumberFormatException)

e.g String str = "153153AG";

Integer i = new integer (str);

4: Exception handling method First: Catch exception (try Catch finally)

Try

• A try statement must have at least one catch statement block or a finally statement block:
• After the execution of the exception-handling code is completed, it is not returned to the try statement to execute the code that has not been executed

Catch:

• Each try statement block can accompany one or more catch statements to handle the different types of exception objects that may be produced
• Common methods:

ToString () Method: Displays the class name of the exception and the cause of the exception

GetMessage () Method: Displays only the cause of the exception, but does not display the class name.

Printstacktrace () Method: Used to track the contents of the stack when an exception event occurs.

--These methods are inherited from the Throwable class

• If there is an inheritance relationship between the exception classes, it is important to note in order that the more top-level classes are placed below , then the extra catch is omitted directly.

Finally

• It is common to close the open resources of the program block in finally, such as: file stream, release database connection, and so on.

Instance:

File operation:

　　　　

This problem is because the file may not exist

Code:

Importjava.io.FileNotFoundException;ImportJava.io.FileReader;Importjava.io.IOException; Public classTestfilereader { Public Static voidMain (String args[]) {FileReader reader=NULL; Try{Reader=NewFileReader ("D:/lp.txt"); Charc = (Char) Reader.read (); CharC2 = (Char) Reader.read (); System.out.println ("" + C +C2); } Catch(FileNotFoundException e) {e.printstacktrace (); }        //the two catch here cannot be swapped because IOException is the parent of FileNotFoundException        Catch(IOException e) {e.printstacktrace (); } finally {            Try{reader.close (); } Catch(IOException e) {e.printstacktrace (); }        }    }}

file Operations

　Try-catch-finally-return Order of execution:

Code:

ImportJava.io.FileInputStream;Importjava.io.FileNotFoundException;Importjava.io.IOException; Public classTestfilereader { Public Static voidMain (String args[]) {string str=NewTestfilereader (). OpenFile ();    System.out.println (str); }    PrivateString OpenFile () {System.out.println ("AAA"); Try{FileInputStream fis=NewFileInputStream ("D:/lp.txt"); intA =Fis.read (); System.out.println ("BBB"); return"Step1"; } Catch(FileNotFoundException e) {System.out.println ("catching!!");            E.printstacktrace (); return"Step2"; } Catch(IOException e) {e.printstacktrace (); return"Step3"; }finally{System.out.println ("Finally"); }    }    }

no exception code

Results:

ImportJava.io.FileInputStream;Importjava.io.FileNotFoundException;Importjava.io.IOException; Public classTestfilereader { Public Static voidMain (String args[]) {string str=NewTestfilereader (). OpenFile ();    System.out.println (str); }    PrivateString OpenFile () {System.out.println ("AAA"); Try{FileInputStream fis=NewFileInputStream ("D:/wu.txt"); intA =Fis.read (); System.out.println ("BBB"); return"Step1"; } Catch(FileNotFoundException e) {System.out.println ("catching!!");            E.printstacktrace (); return"Step2"; } Catch(IOException e) {e.printstacktrace (); return"Step3"; }finally{System.out.println ("Finally"); }    }    }

there is an exception code

Results:

　　

　　Conclusion: 1: Execute a try catch to assign a value to the return value

2: Execute finally

3:return

5: Exception Handling method The second type: throws sentence

　　 　　　　

Exception Handling in Java