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Throwing exceptions in methods

From: Java Essential Training

Video: Throwing exceptions in methods

As I showed in a previous video, a runtime exception generates an instance of a class called an exception class, either the exception class itself or one of its subclasses. I've shown how to deal with this with a try catch block around the possibly offending code. But I'd also like to show you how to explicitly declare some code that might throw an exception and wrap it in a method that has a keyword called throws, that tells the rest of the application what might happen. Here is the same starting code as in a previous video, instead of wrapping this directly inside try catch, I'm going to take this code and put it into its own method.

Throwing exceptions in methods

As I showed in a previous video, a runtime exception generates an instance of a class called an exception class, either the exception class itself or one of its subclasses. I've shown how to deal with this with a try catch block around the possibly offending code. But I'd also like to show you how to explicitly declare some code that might throw an exception and wrap it in a method that has a keyword called throws, that tells the rest of the application what might happen. Here is the same starting code as in a previous video, instead of wrapping this directly inside try catch, I'm going to take this code and put it into its own method.

I'll select these two lines of code right click on the selected code and choose Refactor, Extract Method. I'll give the method a name of getArrayItem, and right here I'm going to select an option of Declare thrown runtime exceptions. This sort of refactoring, taking code that might generate a runtime exception and putting it into its own separate method is such a common practice that Eclipse has a special option for it. I'll click OK and that creates the new method.

Now notice, it didn't add any additional code and the reason is because in this situation none of the code that I executed already told Eclipse what kind of runtime exception might happen. So I'm going to put in the code myself, I'll place the cursor after the name of the method, now at this point, the code is going to get a little bit wide. So I'm going to go to a new line here and I'll put in the keyword throws and then add Array press control space and choose the exception class ArrayIndexOutOfBoundsException.

This now tells the rest of the application that when this particular code is running, that is the code within the getArrayItem method, that it could throw this specific kind of exception object. Now, I'll go to the method getArrayItem, I'll select that, I'll right click and I'll select Surround With, Try/catch Block and now Eclipse is smart enough to know that because I'm calling a method that could throw this object, that's the one it's looking for. I'll get rid of the TODO comment, I'll replace e.printStraceTrace with my own custom message, System.out.println " Array item was out of bounds" and I'll run the application.

And now I'm elegantly catching the error, handling it myself, and the rest of the application can keep on running. So again, the common practice is when you have a bit of code that might generate an exception, put it in its own special method and mark the method with the keyword throws and the particular kind of exception class that might be thrown.

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This video is part of

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Java Essential Training

71 video lessons · 72679 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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