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Handling exceptions with try/catch

From: Java Essential Training

Video: Handling exceptions with try/catch

If you know in your application that you might generate a runtime exception, you can anticipate and handle it using a syntax called try and catch. The first thing you have to know is how exceptions behave. I'm working in a project named exceptions and it has a couple of lines of code that will generate a runtime error. I've declared a strings variable which is an array of string objects and I've initialized it with one item, a string named welcome. And then in the next line, I try to output the second item in the array, which doesn't exist.

Handling exceptions with try/catch

If you know in your application that you might generate a runtime exception, you can anticipate and handle it using a syntax called try and catch. The first thing you have to know is how exceptions behave. I'm working in a project named exceptions and it has a couple of lines of code that will generate a runtime error. I've declared a strings variable which is an array of string objects and I've initialized it with one item, a string named welcome. And then in the next line, I try to output the second item in the array, which doesn't exist.

When I run the application, I get an exception. When these kinds of exceptions happen in Java it stops your application cold, it crashes the application. I can show you that in fact this application is crashing and is not continuing with its execution. I'll add another line of code down here System.out.println and I'll output, "The application is still running1" I'll save and run the application, I'll see my error and I don't see anything after it.

So how do you handle a potential runtime error? The first thing you need to know is that when an exception occurs, a variable is generated, an exception object. The exception object will be an instance of a class named exception or of another class that's a subclass of exception. If you want to wrap your code and deal with a possible exception, the first thing to do is put the code into a try block. The try block will be followed by a catch block or more than one catch block and the code in the catch block will deal with the exception in some way.

Here is a very simple way to add a try catch block around some code that might generate a run-time error. I'm going to select that code and then I'll right click on it and I'll choose Surround With, Try/catch block. The syntax of the Try/catch block is that you start off with the word try and then you put the code that might generate the exception inside the first set of braces, then you follow that with a catch block starting with the keyword catch and then a variable declaration which is of data type exception or one of its subclasses.

Then within the catch block, you can handle the exception in some fashion. If your code execution gets into the catch block that will clear the error and your application will continue running. I'm just going to get rid of the TODO comment and save my changes and then I'll run the application again. And I still see that there is an error but then I get this extra little bit of code. The application is still running. And so I know that my try catch block has handled the exception. Within the catch block right now, I'm executing a method of the exception class called printStackTrace and that's why in the console block, you're seeing this output.

java.lang.ArrayIndexOutOfBoundsException: 1.I could take that little bit of code out, I am just going to comment it and I'll put in a little bit of my own custom code, System.out.println, there was an error. So I'm telling the user that something went wrong, but I'm being a little more vague. Stack traces tend to scare users, because they don't know what's going on inside the program and they shouldn't have to. I'll run the application again and now I get clean output.

There was an error, but the application is still running. In this situation, the particular type of exception that was thrown was not just an instance of the exception class but of one of its subclasses. You can be more specific in your catch block by knowing exactly which type of exception you are anticipating. To find out what that class is, I'm going to remove the comments and run the application again and I see that the exception object is an instance of this one, ArrayIndexOutOfBoundsException.

To be more specific about this, I'll go into the console and I'll copy the name of that exception class to the clipboard, then I'll go back to my code and I'll change the data type of the exception object. I'm declaring from exception to a rate index out of bounds exception, just by pasting that in. Then I once again comment out the printStackTrace method and run the application. And I'll see that everything is working the way I wanted it to. But now I set up a situation where as long as they get this particular exception, I'll be able to keep running the application but if any other exception happens the application will stop.

The compiler will tell me what went wrong and I'll be able to figure out what that exception was about and fix it. So that's a look at how to use the try catch structure, to anticipate errors that you know might happen in your application and then elegantly deal with them.

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This video is part of

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Java Essential Training

71 video lessons · 71971 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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