JAVA fundamentals of exception handling mechanism

Agenda

• Three Categories Of Exceptions
• Exceptions Hierarchy
• try-catch-finally block
• The try-with-resources
• User-defined Exceptions
• The Throws/Throw Keywords

Three Categories Of Exceptions

There have various of reasons cause different exceptions during execution or compilation, some of them are caused by user's invalid operation, others are caused by program error/bug, and others are caused by environmental/physical issue ( file resource missing,
network unavailable, e.g.). here we have three categories of exceptions. Checked exceptions, Unchecked exception and Errors.

• Checked exceptions - A checked exception is an exception occurs at the compile time, which is also called as compile time exception. The compiler will automatically prompts to programmer to handle these exceptions.

Example

import java.io.File;
import java.io.FileReader;

public class FilenotFound_Demo {

   public static void main(String args[]) {
      File file = new File("E://file.txt");
      FileReader fr = new FileReader(file);
   }
}

If you try to compile above code, you will get below exceptions.

C:\>javac FilenotFound_Demo.java
FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
      FileReader fr = new FileReader(file);
                      ^
1 error

• Uncheck exceptions - An uncheck exception is an exception occurs at the time of execution, these exceptions are also calledRuntime Exceptions. These includes programing bugs, such as logic errors or improper usage of API.
  For example,ArrayIndexOutOfBoundsExceptionexception.
• Errors - Error is not  Exception at all, but problems that arise beyond the control of user of the program, they are invariably ignored in your code and the compiler as well, because you can rarely do anything, for example, an stack overflow
  occurs, an error will arise.

Exception Hierarchy

All exceptions/errors inherit from the base class Throwable class, it contains two implortant subclass, Errors and Exception. as you see, all exception in Java are class.

• Errors- Errors indicate the pro they are serious problems that can not be handled by the program such asJVM stack overflow error, out of memory error, etc., Most of them are not caused by Java programs code, instead, caused
  by the JVM, which is the programs runtime environment. Normally, programs should not try to recover Errors because it should be handled by JVM by stopping the thread. Errors also can not be traced in program as it is out of the program control.
• Exceptions-Exception class has two main subclass,  IOException class and RuntimeException Class. RuntimeException means validate JVM operations such as NullPointerException, ArithmeticException, ArrayIndexOutOfBoundException,etc., they
  are uncheck exceptions that occurs at the time of execution. other exceptions such as IOException, SQLException are checked Exceptions.

Here is the graph to show you the Java Exception Hierarchy,

P1 Java Exception Hierarchy

the try-catch-finally block

The exception object will be created if exception occurs in try block during execution and threw out to JVM,  JVM will try to look for a catch block witch is matched with the exception to handle the exception, then try-catch block ends.

The rules to match the type of exceptions: if the exception which is threw in try block, is the same class of each catch block, or the subclass of the catch block, it is considered to be matched.

that means we'd better put the underlying exceptions in more front place for high priority handling, put the business exceptions in more back place for low priority handling, otherwise the underlying exceptions may be masked by the business exceptions.

Example

public class TestException {
    public static void main(String[] args) {
        int a = 6;
        int b = 0;
        try { // try监控区域  

            if (b == 0) throw new ArithmeticException(); // 通过throw语句抛出异常
            System.out.println("a/b的值是：" + a / b);
        }
        catch (ArithmeticException e) { // catch捕捉异常
            System.out.println("程序出现异常，变量b不能为0。");
        }
        System.out.println("程序正常结束。");
    }
} 

ps: actually the ArithmeticException is the subclass of RuntimeException, will be threw out automatically by JVM without throw keyword explicitly in try block.

The finally block

the statement in the finally block will always be executed no matter whether exception occurs or not. if the return statement appears in try-catch block, the finally block will be executed before the return back statement and it would not be executed for
following 4 cases.

• If exceptions occur in finally block
• Have used System.exit() in preceding codes.(JVM stopping too early)
• The thread of the program has dead.
• CPU has been closed.(hardware issue)

The sequence of execution of statements in finally block

• if no exceptions are thrown by try block-statements in try block will be executed in orders one by one, the program will skip the statements in catch block and continue to execute the statements in finally block and following statements.
• if exceptions are thrown by try block but no catch block can catch the them-the exceptions will be handled by JVM, the statements in finally block will be executed but the statements following with finally will
  not be executed.
• if exceptions are thrown by try block and caught by catch block-the exception will be just caught by one catch block in catch block orders and other catch blocks will be skipped, in try block, the statements following with the exception
  line will not be executed, after the catch block, the statements in finally block will be executed and the following codes as well.

The try-with-resources

Generally, we declare resources like streams, connections, etc. in try block then you have to explicitlyclose them in finally block. Here is a typically example when we use resources instance in try-catch-finally block.

Example

import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadData_Demo {

   public static void main(String args[]) {
      FileReader fr = null;
      try {
         File file = new File("file.txt");
         fr = new FileReader(file); char [] a = new char[50];
         fr.read(a);   // reads the content to the array
         for(char c : a)
         System.out.print(c);   // prints the characters one by one
      }catch(IOException e) {
         e.printStackTrace();
      }finally {
         try {
            fr.close();
         }catch(IOException ex) {
            ex.printStackTrace();
         }
      }
   }
}

now you can use resources without explicitly to close them in try-with-resources block, because it can be automaticallyimplicitlyclosed in final block, it is a new feature in Java 7 referred asautomatic resource management.

to use this statement, you just need to declare resource within parenthesis as below syntax, the resource will be closed at the end of the block.

Syntax

try(FileReader fr = new FileReader("file path")) {
   // use the resource
   }catch() {
      // body of catch
   }
}

Example

import java.io.FileReader;
import java.io.IOException;

public class Try_withDemo {

   public static void main(String args[]) {
      try(<strong><span style="color:#FF0000;">FileReader fr = new FileReader("E://file.txt")</span></strong>) {
         char [] a = new char[50];
         fr.read(a);   // reads the contentto the array
         for(char c : a)
         System.out.print(c);   // prints the characters one by one
      }catch(IOException e) {
         e.printStackTrace();
      }
   }
}

Keep in mind below points when using try-with-resources statement.

• it should implement AutoCloseable interface, the close() method will get invoked automatically at runtime.
• you can declare multiple resource in parenthesis and there will be closed in reverse order.
• the resource gets instance just before the start of try block.

User-Defined Exceptions

An user-defined exception class is like any other class, containing useful methods and fields. you can create user-defined Exceptions just keep following points in mind -

• All exceptions must be a child of Throwable class.
• You need to extend the Exception class if you want to create a checked exception that is enforced by the Handle or Declare rule.
• You need to extend the RuntimeException class in case you want to create a run time exception (unchecked exception)

here is a sample of user-defined exception that extends the Exception class, making it a checked exception.

Example

// File Name InsufficientFundsException.java
import java.io.*;

public class InsufficientFundsException <strong><span style="color:#ff0000;">extends Exception</span></strong> {
   private double amount;

   public InsufficientFundsException(double amount) {
      this.amount = amount;
   }

   public double getAmount() {
      return amount;
   }
}

Following withdraw() method in CheckingAcount class demonstrates the usage of above user-defined exception. here we will throw out an exception object that contains its own fields and method to process the parameters we pass in and the methods can be used in
higher lever class to show as format what we want, it is cool right?!

// File Name CheckingAccount.java
import java.io.*;

public class CheckingAccount {
   private double balance;
   private int number;

   public CheckingAccount(int number) {
      this.number = number;
   }

   public void deposit(double amount) {
      balance += amount;
   }

   public void withdraw(double amount) <strong><span style="color:#ff0000;">throws InsufficientFundsException</span></strong> {
      if(amount <= balance) {
         balance -= amount;
      }else {
         double needs = amount - balance;
         <strong><span style="color:#ff0000;">throw new InsufficientFundsException(needs);</span></strong>
      }
   }

   public double getBalance() {
      return balance;
   }

   public int getNumber() {
      return number;
   }
}

now we create a higher level class which is calling above CheckingAccount class and also will catch the exceptions thrown by CheckingAccount class. then we can call the user-defined exception class to process the checked exception.

// File Name BankDemo.java
public class BankDemo {

   public static void main(String [] args) {
      CheckingAccount c = new CheckingAccount(101);
      System.out.println("Depositing $500...");
      c.deposit(500.00);

      try {
         System.out.println("\nWithdrawing $100...");
         c.withdraw(100.00);
         System.out.println("\nWithdrawing $600...");
         c.withdraw(600.00);
      }<strong><span style="color:#ff0000;">catch(InsufficientFundsException e) {
         System.out.println("Sorry, but you are short $" + e.getAmount());</span></strong>
         e.printStackTrace();
      }
   }
}

Output

Depositing $500...

Withdrawing $100...

Withdrawing $600...
Sorry, but you are short $200.0
<strong><span style="color:#ff0000;">InsufficientFundsException
         at CheckingAccount.withdraw(CheckingAccount.java:25)
         at BankDemo.main(BankDemo.java:13)</span></strong>

The Throws/Throw Keywords

difference

• If a method does not handle a checked exception, the method must declare it using the throws keyword, thethrows
  keyword appears at the end of a method's signature.
• you can throw a exception, either a newly instantiated one or an exception that you just caught, by usingthrow
  keyword.

functionality

the throws is used to postpone the handling of a checked exception (to a high level class or method) and the throw is used to invoke an exception explicitly.

example

import java.io.*;
public class className {

   public void deposit(double amount) <span style="color:#FF0000;">throws </span>RemoteException {
      // Method implementation
      <span style="color:#FF0000;">throw </span>new RemoteException();
   }
   // Remainder of class definition
}

Reference

深入理解java异常处理机制
http://blog.csdn.net/hguisu/article/details/6155636

Java - Exceptions --- http://www.tutorialspoint.com/java/java_exceptions.htm

Lesson: Exceptions (The Java™ Tutorials > Essential Classes)

http://docs.oracle.com/javase/tutorial/essential/exceptions