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Using abstract classes and methods

From: Java Essential Training

Video: Using abstract classes and methods

In previous videos, I showed examples of creating inheritance patterns with concrete superclasses and with interfaces. There is another option in Java called an abstract class. Abstract classes combine the best of the other two options. Like concrete superclasses, they can have fully implemented methods that they can share with their subclasses, but like interfaces, they typically aren't instantiated directly, so they can create a better programming contract. I'm starting in a version of the Olive Press application that has a concrete superclass named Olive. To change it to an abstract class I'll add the keyword abstract after the public access modifier, but before the keyword class. Nothing has changed so far in the functionality of this application. I'll save and run the application, and show that it still runs. But the reason there are no functional changes yet is because I'm not trying to instantiate the Olive class directly. Going back to the Main class, I'll show that I am instantiating the subclasses: Kalamata, Ligurian, and Kalamata again. Watch what happens when I try to instantiate Olive directly. I will move the cursor down to the end of the Main method, and I will add a line of code that declares a new variable. I will set the data type as Olive, and I will name it olive2, and I will instantiate it with new Olive, the constructor method. When I save my changes, I get the error message, Cannot instantiate the type Olive. So when you declare a class as abstract, you can't instantiate it directly, except under certain conditions. I am just going to comment-out this code for the moment, I will come back to it later. The point of using an abstract class as a superclass is that you can create a contract that says all of this abstract classes' subclasses promise to implement some sort of functionality. Here is an example, I will go back to the Olive class and show that it has a method called getOrigin(), which returns a string of Greece. Now because this class is abstract, it can have this concrete implementation, but the reality is that not all olives come from Greece. Some have an origin of Greece, some of Italy, and some of other countries. So I might make the decision that I'm going to implement this method in each of the subclasses; not in the superclass. But I want to make sure that all of the subclasses implement it. So I will change this method to an abstract method. The syntax is just like the class itself, you place the word abstract after the access modifier, and in this case, before the return type. And then, you use the same kind of syntax you would in an interface. I will delete the implementation and add the semicolon at the end of the declaration. I'm now saying that this method must be implemented in all classes that extend this class. Watch what happens when I save my changes. I get errors. The classes Kalamata and Ligurian are extending the abstract class and they must implement all abstract methods. So let's go to each of these classes and fix the problem. I will go to Kalamata, I will place the cursor below all of the other methods but within the class declaration. I will right-click, and choose Source > Override/Implement Methods. This is a trick that most good Java editors will be able to do for you. It will look at the superclass and say, here are the methods that you have not implemented yet. And then it will generate the stub for the method for you. I'll click OK, and I get the new method getOrigin(). The Override directive above the method can be retained if you like. It is optional, but it is a good way of noting that you are implementing a method that was declared in the superclass. I will delete the TODO comment and I will replace the value null with a string of Greece. I will save those changes and now I will go to Ligurian, and I will do the same thing. I will once again make a little bit of space, right-click in the open space, and choose Source > Override/ Implement Methods, once again getOrigin() is checked, and I will click OK. Then I will delete the TODO comment and replace null with Italy, again as a string. I'll save my changes, and all of the errors go away. Now to prove this works correctly, I will go back to Main.java. At the bottom of this code I'm already declaring a variable named olive1, data-typed as Kalamata. I am going to change that so that the data type is the superclass of Olive, and because Olives is a collection of olive objects, I can remove the casting syntax. Then I will copy and paste that code, both the declaration and the output of the getOrigin() method. And I will change the second set to olive2, the name of the variable is olive2. I will change index that I am looking for from 0 to 1, and I will change the string. I will save the changes and run the application, and now I am told that Olive 1 is from Greece, and Olive 2 is from Italy. The magic of this inheritance architecture is that at the top level, I am only designating the type once, when I instantiate each object. But later when I call the method getOrigin(), I know that that method will be there because of the contract created by the abstract class, and I called the correct method because it is implemented as a concrete method in the appropriate subclass. Finally, let's go back to the declaration where I tried to instantiate the superclass directly. I mentioned that you typically don't instantiate abstract classes directly, it is possible to do it though, as long as you satisfy the contract. I will remove this comment, and I am going to recode this. I will backspace over the semicolon and the parenthesis, and then press Control+Space, and I will choose Olive as the constructor method. Notice the notation Anonymous Inner Type. I will double-click to select, and take a look at what Eclipse generates. It allows me to instantiate the Olive class directly, but only if I implement the abstract method getOrigin(). So I am satisfying the contract again, but in a different way. Instead of an explicit subclass, in its own class file, now I am creating a subclass inline as something called an anonymous inner type. I am not actually going to do this in the application, so I will select that code, and then comment it out, save my changes, and run the application one last time. So that is a look at how you can use abstract classes in your inheritance architecture. Again, they combine the best features of interfaces and of concrete superclasses. They support a better programming contract then the concrete superclasses, but have a lot more flexibility and power than interfaces.

Using abstract classes and methods

In previous videos, I showed examples of creating inheritance patterns with concrete superclasses and with interfaces. There is another option in Java called an abstract class. Abstract classes combine the best of the other two options. Like concrete superclasses, they can have fully implemented methods that they can share with their subclasses, but like interfaces, they typically aren't instantiated directly, so they can create a better programming contract. I'm starting in a version of the Olive Press application that has a concrete superclass named Olive. To change it to an abstract class I'll add the keyword abstract after the public access modifier, but before the keyword class. Nothing has changed so far in the functionality of this application. I'll save and run the application, and show that it still runs. But the reason there are no functional changes yet is because I'm not trying to instantiate the Olive class directly. Going back to the Main class, I'll show that I am instantiating the subclasses: Kalamata, Ligurian, and Kalamata again. Watch what happens when I try to instantiate Olive directly. I will move the cursor down to the end of the Main method, and I will add a line of code that declares a new variable. I will set the data type as Olive, and I will name it olive2, and I will instantiate it with new Olive, the constructor method. When I save my changes, I get the error message, Cannot instantiate the type Olive. So when you declare a class as abstract, you can't instantiate it directly, except under certain conditions. I am just going to comment-out this code for the moment, I will come back to it later. The point of using an abstract class as a superclass is that you can create a contract that says all of this abstract classes' subclasses promise to implement some sort of functionality. Here is an example, I will go back to the Olive class and show that it has a method called getOrigin(), which returns a string of Greece. Now because this class is abstract, it can have this concrete implementation, but the reality is that not all olives come from Greece. Some have an origin of Greece, some of Italy, and some of other countries. So I might make the decision that I'm going to implement this method in each of the subclasses; not in the superclass. But I want to make sure that all of the subclasses implement it. So I will change this method to an abstract method. The syntax is just like the class itself, you place the word abstract after the access modifier, and in this case, before the return type. And then, you use the same kind of syntax you would in an interface. I will delete the implementation and add the semicolon at the end of the declaration. I'm now saying that this method must be implemented in all classes that extend this class. Watch what happens when I save my changes. I get errors. The classes Kalamata and Ligurian are extending the abstract class and they must implement all abstract methods. So let's go to each of these classes and fix the problem. I will go to Kalamata, I will place the cursor below all of the other methods but within the class declaration. I will right-click, and choose Source > Override/Implement Methods. This is a trick that most good Java editors will be able to do for you. It will look at the superclass and say, here are the methods that you have not implemented yet. And then it will generate the stub for the method for you. I'll click OK, and I get the new method getOrigin(). The Override directive above the method can be retained if you like. It is optional, but it is a good way of noting that you are implementing a method that was declared in the superclass. I will delete the TODO comment and I will replace the value null with a string of Greece. I will save those changes and now I will go to Ligurian, and I will do the same thing. I will once again make a little bit of space, right-click in the open space, and choose Source > Override/ Implement Methods, once again getOrigin() is checked, and I will click OK. Then I will delete the TODO comment and replace null with Italy, again as a string. I'll save my changes, and all of the errors go away. Now to prove this works correctly, I will go back to Main.java. At the bottom of this code I'm already declaring a variable named olive1, data-typed as Kalamata. I am going to change that so that the data type is the superclass of Olive, and because Olives is a collection of olive objects, I can remove the casting syntax. Then I will copy and paste that code, both the declaration and the output of the getOrigin() method. And I will change the second set to olive2, the name of the variable is olive2. I will change index that I am looking for from 0 to 1, and I will change the string. I will save the changes and run the application, and now I am told that Olive 1 is from Greece, and Olive 2 is from Italy. The magic of this inheritance architecture is that at the top level, I am only designating the type once, when I instantiate each object. But later when I call the method getOrigin(), I know that that method will be there because of the contract created by the abstract class, and I called the correct method because it is implemented as a concrete method in the appropriate subclass. Finally, let's go back to the declaration where I tried to instantiate the superclass directly. I mentioned that you typically don't instantiate abstract classes directly, it is possible to do it though, as long as you satisfy the contract. I will remove this comment, and I am going to recode this. I will backspace over the semicolon and the parenthesis, and then press Control+Space, and I will choose Olive as the constructor method. Notice the notation Anonymous Inner Type. I will double-click to select, and take a look at what Eclipse generates. It allows me to instantiate the Olive class directly, but only if I implement the abstract method getOrigin(). So I am satisfying the contract again, but in a different way. Instead of an explicit subclass, in its own class file, now I am creating a subclass inline as something called an anonymous inner type. I am not actually going to do this in the application, so I will select that code, and then comment it out, save my changes, and run the application one last time. So that is a look at how you can use abstract classes in your inheritance architecture. Again, they combine the best features of interfaces and of concrete superclasses. They support a better programming contract then the concrete superclasses, but have a lot more flexibility and power than interfaces.

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This video is part of

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Java Essential Training

71 video lessons · 69975 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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