Welcome!

Eclipse Authors: Pat Romanski, Elizabeth White, Liz McMillan, David H Deans, JP Morgenthal

Related Topics: Java IoT

Java IoT: Article

Java Exceptions

Lesson 4

Java Exceptions Lesson 4

Let's say a Java class reads a file with the customer's data. What's going to happen if someone deletes this file?

Will the program crash with that scary multi-line error message, or will it stay alive displaying a user friendly message like this one: "Dear friend, for some reason I could not read the file customer.txt. Please make sure that the file exists"? In many programming languages, error processing depends on the skills of a programmer.

Java forces software developers to include error processing code, otherwise the programs will not even compile.

Error processing in the Java is called exception handling.

You have to place code that may produce errors in a so-called try/catch block:

try{
  fileCustomer.read();     
  process(fileCustomer);
}
catch (IOException e){
  System.out.println("Dear friend, I could not read the file customer.txt...");  
}

In case of an error, the method read() throws an exception. In this example the catch clause receives the instance of the class IOException that contains information about input/output errors that have occured. If the catch block exists for this type of error, the exception will be caught and the statements located in a catch block will be executed. The program will not terminate and this exception is considered to be taken care of.

The print statement from the code above will be executed only in case of the file read error. Please note that method process() will not even be called if the read fails.

Reading the Stack Trace

If an unexpected exception occurs that's not handled in the code, the program may print multiple error messages on the screen. Theses messages will help you to trace all method calls that lead to this error. This printout is called a stack trace. If a program performs a number of nested method calls to reach the problematic line, a stack trace can help you trace the workflow of the program, and localize the error.

Let's write a program that will intentionally divide by zero:

class TestStackTrace{    
  TestStackTrace()
  {
    divideByZero();
  }

  int divideByZero()
  {
    return 25/0;
  }

  static void main(String[]args)
  {
    new TestStackTrace();
  }
}

Below is an output of the program - it traced what happened in the program stack before the error had occurred. Start reading it from the last line going up. It reads that the program was executing methods main(), init() (constructor), and divideByZero(). The line numbers 14, 4 and 9 (see below) indicate where in the program these methods were called. After that, the ArithmeticException had been thrown - the line number nine tried to divide by zero.

c:\temp>java TestStackTrace
  Exception in thread "main"  
  java.lang.ArithmeticException: / by zero
     at TestStackTrace.divideByZero(TestStackTrace.java:9)
     at TestStackTrace.(TestStackTrace.java:4)
     at TestStackTrace.main(TestStackTrace.java:14)

Exception Hierarchy

All exceptions in Java are classes implicitely derived from the class Throwable which has immediate subclasses

Error and Exception.

Subclasses of the class Exception are called listed exceptions and have to be handled in your code.

Subclasses of the class Error are fatal JVM errors and your program can't fix them. Programmers also can define their own exceptions.

How you are supposed to know in advance if some Java method may throw a particular exception and the try/catch block should be used? Don't worry - if a method throws an exception and you did not put this method call in a try/catch block, the Java compiler will print an error message similar to this one:

"Tax.java": unreported exception: java.io.IOException; must be caught or declared to be thrown at line 57

Try/Catch Block

There are five Java keywords that could be used for exceptions handling: try, catch, finally, throw, and throws.

By placing a code in a try/catch block, a program says to a JVM: "Try to execute this line of code, and if something goes wrong, and this method throws exceptions, please catch them, so that I could report this situation to a user." One try block can have multiple catch blocks, if more than one problem occurs. For example, when a program tries to read a file, the file may not be there - FileNotFoundException, or it's there, but the code has reached the end of the file - EOFException, etc.

public void getCustomers(){ 
  try{
    fileCustomers.read();
  }catch(FileNotFoundException e){
    System.out.println("Can not find file Customers");
  }catch(EOFException e1){
    System.out.println("Done with file read");
  }catch(IOException e2){
    System.out.println("Problem reading  file " + 
                                 e2.getMessage());
  }
}

If multiple catch blocks are handling exceptions that have a subclass-superclass relationship (i.e. EOFException is a subclass of the IOException), you have to put the catch block for the subclass first as shown in the previous example.

A lazy programmer would not bother with catching multiple exception, but will rather write the above code like this:

public void getCustomers(){ 
  try{
    fileCustomers.read();
  }catch(Exception e){
    System.out.println("Problem reading  file " + 
                        e.getMessage());
  }
}

Catch blocks receive an instance of the Exception object that contains a short explanation of a problem, and its method getMessage() will return this info. If the description of an error returned by the getMessage() is not clear, try the method toString() instead.

If you need more detailed information about the exception, use the method printStackTrace(). It will print all internal method calls that lead to this exception (see the section "Reading Stack Trace" above).

Clause throws

In some cases, it makes more sense to handle an exception not in the method where it happened, but in the calling method. Let's use the same example that reads a file. Since the method read() may throw an IOException, you should either handle it or declare it:

class CustomerList{
  void getAllCustomers() throws IOException{
    file.read(); // Do not use try/catch  if you are not handling exceptions here
  }

  public static void main(String[] args){
    System.out.println("Customer List");

    try{
      // Since the  getAllCustomers()declared exception,       
      // we have to either  handle  it over here, or re- 
      // throw it (see the throw clause explanation below)

     getAllCustomers();  
   }catch(IOException e){
     System.out.println("Sorry, the  Customer List is not 
                                               available");
  }
}

In this case, the IOException has been propagated from the getAllCustomers() to the main() method.

Clause finally

A try/catch block could be completed in different ways

  1. the code inside the try block successfully ended and the program continues,
  2. the code inside the try block ran into a return statement and the method is exited,
  3. an exception has been thrown and code goes to the catch block, which handles it
  4. an exception has been thrown and code goes to the catch block, which throws another exception to the calling method.
If there is a piece of code that must be executed regardless of the success or failure of the code, put it under the clause finally:

try{
  file.read();
}catch(Exception e){
  printStackTrace();
}
finally{
 file.close();
}

The code above will definitely close the file regardless of the success of the read operation. The finally clause is usually used for the cleanup/release of the system resources such as files, database connections, etc..

If you are not planning to handle exceptions in the current method, they will be propagated to the calling method. In this case, you can use the finally clause without the catch clause:

void myMethod () throws IOException{
  try{
    file.read();
  }
  finally{
    file.close();
  }
}

Clause throw

If an exception has occurred in a method, you may want to catch it and re-throw it to the calling method. Sometimes, you might want to catch one exception but re-throw another one that has a different description of the error (see the code snippet below).

The throw statement is used to throw Java objects. The object that a program throws must be Throwable (you can throw a ball, but you can't throw a grand piano). This technically means that you can only throw subclasses of the Throwable class, and all Java exceptions are its subclasses:

 
class CustomerList{

  void getAllCustomers() throws Exception{
    try{
      file.read(); // this line may throw an exception
    } catch (IOException e) {
      // Perform some internal processing of this error, and _
      throw new  Exception (
        "Dear Friend, the file has problems..."+ 
                                     e.getMessage());
      }
    }

    public static void main(String[] args){
      System.out.println("Customer List");

      try{
        // Since the  getAllCustomers() declares an  
        // exception, wehave  to either  handle  it, or re-throw it
         getAllCustomers();
      }catch(Exception e){
        System.out.println(e.getMessage());
      }
   }
}

User-Defined Exceptions

Programmers could also create user-defined exceptions, specific to their business. Let's say you are in business selling bikes and need to validate a customers order. Create a new class TooManyBikesException that is derived from the class Exception or Throwable, and if someone tries to order more bikes than you can ship - just throw this exception:

class TooManyBikesException extends Exception{
  TooManyBikesException (String msgText){
     super(msgText);      
  }  
}

class BikeOrder{
  static  void validateOrder(String bikeModel,int quantity) throws TooManyBikesException{

  // perform  some data validation, and if you do not like 
  // the quantity for the specified model, do  the following: 
 
  throw new TooManyBikesException("Can not ship" + 
    quantity+" bikes of the model " + bikeModel +);
  }
}

class Order{
   try{   
     bikeOrder.validateOrder("Model-123", 50);

     // the next line will be skipped in case of an exception
     System.out.println("The order is valid");   
   } catch(TooManyBikes e){
      txtResult.setText(e.getMessage());
   }
}

In general, to make your programs robust and easy to debug, you should always use the exception mechanism to report and handle exceptional situations in your program. Be specific when writing your catch clauses - catch as many exceptional situations as you can predict. Just having one catch (Exception e) statements is not a good idea.

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (1) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Virendra Mehta 04/03/05 05:25:58 PM EDT

It is also easy to abuse this facility (see my article at http://www.informit.com/articles/article.asp?p=337136) and that can cause serious performance bottlenecks!

@ThingsExpo Stories
Explosive growth in connected devices. Enormous amounts of data for collection and analysis. Critical use of data for split-second decision making and actionable information. All three are factors in making the Internet of Things a reality. Yet, any one factor would have an IT organization pondering its infrastructure strategy. How should your organization enhance its IT framework to enable an Internet of Things implementation? In his session at @ThingsExpo, James Kirkland, Red Hat's Chief Archi...
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...
Personalization has long been the holy grail of marketing. Simply stated, communicate the most relevant offer to the right person and you will increase sales. To achieve this, you must understand the individual. Consequently, digital marketers developed many ways to gather and leverage customer information to deliver targeted experiences. In his session at @ThingsExpo, Lou Casal, Founder and Principal Consultant at Practicala, discussed how the Internet of Things (IoT) has accelerated our abilit...
Organizations planning enterprise data center consolidation and modernization projects are faced with a challenging, costly reality. Requirements to deploy modern, cloud-native applications simultaneously with traditional client/server applications are almost impossible to achieve with hardware-centric enterprise infrastructure. Compute and network infrastructure are fast moving down a software-defined path, but storage has been a laggard. Until now.
Digital Transformation is much more than a buzzword. The radical shift to digital mechanisms for almost every process is evident across all industries and verticals. This is often especially true in financial services, where the legacy environment is many times unable to keep up with the rapidly shifting demands of the consumer. The constant pressure to provide complete, omnichannel delivery of customer-facing solutions to meet both regulatory and customer demands is putting enormous pressure on...
The best way to leverage your CloudEXPO | DXWorldEXPO presence as a sponsor and exhibitor is to plan your news announcements around our events. The press covering CloudEXPO | DXWorldEXPO will have access to these releases and will amplify your news announcements. More than two dozen Cloud companies either set deals at our shows or have announced their mergers and acquisitions at CloudEXPO. Product announcements during our show provide your company with the most reach through our targeted audienc...
JETRO showcased Japan Digital Transformation Pavilion at SYS-CON's 21st International Cloud Expo® at the Santa Clara Convention Center in Santa Clara, CA. The Japan External Trade Organization (JETRO) is a non-profit organization that provides business support services to companies expanding to Japan. With the support of JETRO's dedicated staff, clients can incorporate their business; receive visa, immigration, and HR support; find dedicated office space; identify local government subsidies; get...
DXWorldEXPO LLC announced today that ICC-USA, a computer systems integrator and server manufacturing company focused on developing products and product appliances, will exhibit at the 22nd International CloudEXPO | DXWorldEXPO. DXWordEXPO New York 2018, colocated with CloudEXPO New York 2018 will be held November 11-13, 2018, in New York City. ICC is a computer systems integrator and server manufacturing company focused on developing products and product appliances to meet a wide range of ...
@DevOpsSummit at Cloud Expo, taking place November 12-13 in New York City, NY, is co-located with 22nd international CloudEXPO | first international DXWorldEXPO and will feature technical sessions from a rock star conference faculty and the leading industry players in the world.
DXWorldEXPO LLC announced today that the upcoming DXWorldEXPO | CloudEXPO New York event will feature 10 companies from Poland to participate at the "Poland Digital Transformation Pavilion" on November 12-13, 2018.
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
In his keynote at 19th Cloud Expo, Sheng Liang, co-founder and CEO of Rancher Labs, discussed the technological advances and new business opportunities created by the rapid adoption of containers. With the success of Amazon Web Services (AWS) and various open source technologies used to build private clouds, cloud computing has become an essential component of IT strategy. However, users continue to face challenges in implementing clouds, as older technologies evolve and newer ones like Docker c...
Business professionals no longer wonder if they'll migrate to the cloud; it's now a matter of when. The cloud environment has proved to be a major force in transitioning to an agile business model that enables quick decisions and fast implementation that solidify customer relationships. And when the cloud is combined with the power of cognitive computing, it drives innovation and transformation that achieves astounding competitive advantage.
Cloud-enabled transformation has evolved from cost saving measure to business innovation strategy -- one that combines the cloud with cognitive capabilities to drive market disruption. Learn how you can achieve the insight and agility you need to gain a competitive advantage. Industry-acclaimed CTO and cloud expert, Shankar Kalyana presents. Only the most exceptional IBMers are appointed with the rare distinction of IBM Fellow, the highest technical honor in the company. Shankar has also receive...
Michael Maximilien, better known as max or Dr. Max, is a computer scientist with IBM. At IBM Research Triangle Park, he was a principal engineer for the worldwide industry point-of-sale standard: JavaPOS. At IBM Research, some highlights include pioneering research on semantic Web services, mashups, and cloud computing, and platform-as-a-service. He joined the IBM Cloud Labs in 2014 and works closely with Pivotal Inc., to help make the Cloud Found the best PaaS.
In his Opening Keynote at 21st Cloud Expo, John Considine, General Manager of IBM Cloud Infrastructure, led attendees through the exciting evolution of the cloud. He looked at this major disruption from the perspective of technology, business models, and what this means for enterprises of all sizes. John Considine is General Manager of Cloud Infrastructure Services at IBM. In that role he is responsible for leading IBM’s public cloud infrastructure including strategy, development, and offering m...
DXWorldEXPO LLC announced today that All in Mobile, a mobile app development company from Poland, will exhibit at the 22nd International CloudEXPO | DXWorldEXPO. All In Mobile is a mobile app development company from Poland. Since 2014, they maintain passion for developing mobile applications for enterprises and startups worldwide.
We are seeing a major migration of enterprises applications to the cloud. As cloud and business use of real time applications accelerate, legacy networks are no longer able to architecturally support cloud adoption and deliver the performance and security required by highly distributed enterprises. These outdated solutions have become more costly and complicated to implement, install, manage, and maintain.SD-WAN offers unlimited capabilities for accessing the benefits of the cloud and Internet. ...
Headquartered in Plainsboro, NJ, Synametrics Technologies has provided IT professionals and computer systems developers since 1997. Based on the success of their initial product offerings (WinSQL and DeltaCopy), the company continues to create and hone innovative products that help its customers get more from their computer applications, databases and infrastructure. To date, over one million users around the world have chosen Synametrics solutions to help power their accelerated business or per...
Dion Hinchcliffe is an internationally recognized digital expert, bestselling book author, frequent keynote speaker, analyst, futurist, and transformation expert based in Washington, DC. He is currently Chief Strategy Officer at the industry-leading digital strategy and online community solutions firm, 7Summits.