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Understanding interfaces and implementing classes

From: Java Essential Training

Video: Understanding interfaces and implementing classes

In previous videos, I've describe how to create an inheritance relationship between Java classes, superclasses that contain functionality, and subclasses that inherit that functionality. There is another way of creating inheritance relationship using an interface. An interface in Java lets you define the structure of a class including the names and signatures of the classes methods and any final fields that is class or instance variables that will always have the same values. You can then implement that interface with a class that you can actually instantiate.

Understanding interfaces and implementing classes

In previous videos, I've describe how to create an inheritance relationship between Java classes, superclasses that contain functionality, and subclasses that inherit that functionality. There is another way of creating inheritance relationship using an interface. An interface in Java lets you define the structure of a class including the names and signatures of the classes methods and any final fields that is class or instance variables that will always have the same values. You can then implement that interface with a class that you can actually instantiate.

The Java class library uses interfaces extensively. Here is an example of an interface you probably will use in your own applications, the collection interface. The collection interface is the high- level definition of objects that are a part of the Java collections framework and we've only used a number of classes in this framework including the array list. The collection interface defines the names and signatures of methods that are a part of all the classes in the list of implementing classes. I'll go down to the method list and you'll see that they look familiar.

These are the same methods that are a part of the array list class. The idea of an interface is that you're defining a contract. If you say that a class implements an interface, it must implement all of these classes and it must use the exact method names and the same number and data types of arguments and the same return data type. This then allows you to use the concept of polymorphism in your code. An array list can be seen either as an array list or as the interface that it implements, collection.

Let's see how you might use this in an actual application. I am in a project named UsingInterfaces that's a part of his chapter's exercise files. In this version of the application I have a call to a method named getOil that's defined in the class OlivePress. I am passing in an object as an array and this object is declared as an array list containing instances of the Olive class. In the OlivePress class, where I've defined that method, I'm using exactly the same data type array list. Now because array list implements the collection interface I can actually make this method a lot more flexible.

Instead of requiring an instance of the array list class, I can instead require an instance of the collection interface. So I'll change the data type in the method signature from array list to collection. After typing the name of the interface, I'll press Ctrl+Space and choose the collection interface and that causes the required import statement to be added to the top of the code. I'll then clean up my imports by going to Source>Organize Imports and that removes the import for the array list, which is no longer referred to in this class.

I'll save my changes and that rebuilds the project and you'll see that there are no programming problems as a result. I am up casting from array list to collection, from the more specific type to the more general type. And now this method can accept any instance of any class that implements the collection interface. In the Main class, I'm going to still keep the array list declaration here. I'm not going to change its collection and the reason is because the array list class has other methods that are not part of the contract that are not described in the collection interface.

One of the most important of these is the get method. This method is a specific part of array list and is not part of the contract and if I data typed this object as collection the compiler would not accept my call to get. But with that change I'll save and run the application and show you that it still works exactly as it did before. But now my getOil method is more flexible. I can pass in any instance of any class that implements the collection interface. Now you can create your own custom interfaces in your applications and your own implementing classes and I'll show you how to do that in another video.

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This video is part of

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Java Essential Training

71 video lessons · 69495 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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