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Creating and instantiating custom classes

From: Java Essential Training

Video: Creating and instantiating custom classes

Complete applications in Java typically consist of more than one class. You'll have a starting class that has the main method for a console application, but then you have all sorts of supporting classes that either encapsulates data or functionality or both. For this exercise, I'll show you a simple strategy for taking an application where all the code is in a single class or a single file and extracting it out to multiple files, so it's easier to keep track of and maintain. I'll start with this beginning Calculator application.

Creating and instantiating custom classes

Complete applications in Java typically consist of more than one class. You'll have a starting class that has the main method for a console application, but then you have all sorts of supporting classes that either encapsulates data or functionality or both. For this exercise, I'll show you a simple strategy for taking an application where all the code is in a single class or a single file and extracting it out to multiple files, so it's easier to keep track of and maintain. I'll start with this beginning Calculator application.

This is a version of the Calculator application that I worked on in a previous chapter of the course. It accepts two numeric values and then asks the user what operation they want to do; addition, subtraction, and so on. Right now, all of the code for the application is in a single file. There is code to get the input and to figure out what to do, but then all of the supporting methods are also in the primary class. In a well-structured Java application, you would extract a lot of this code out to separate classes.

It's up to you as the developer and the application architect to figure out where the code goes and how to group it together. I'll show you a strategy for this application, where I take all of the mathematical operations and group them together into one class and then I'll take the functionality that's accepting input from the command line and put it into another class. The goal will be to create classes that are reusable. I'll start in the CustomClasses project and I'm going to create two new classes. I'll right-click on the default package and choose New>Class.

The first class will be SimpleMath. You could name the class anything you want, but make sure you use an uppercase initial character and that you don't put in any spaces or special characters. This will not be the starting class so don't check the option to create a main method. You'll only be able to call this class from other classes of the application. Click Finish, and you end up with a very simple class structure. The class is public and the name is SimpleMath.

When you declare your own classes, the name of the public class must match the name of the Java file and you can only have one public class in each Java file. It is possible to declare multiple classes in a single Java file, but any other classes can't be public and will only be accessible from within the code in this file. I am going to have just one class in this file and I'm going to put all of my mathematical operation methods into it. So I'll go back to the application, I'll scroll down, and I'll locate my divide, multiply, subtract, and add methods.

I'll select all four of them and I'll cut them to the clipboard. Then I'll go to SimpleMath.java, I'll place the cursor inside the class declaration, and paste in the methods. Now because I'm going to be calling these methods from another class, I can no longer have them marked as private. They must be marked public or I can just remove the access modifier entirely. I like to be explicit in my access modifiers though, so I am going to change private to public on all four methods.

I'll save my changes to this file and go back to the application, and now I'll go to the switch statement where I'm calling those methods. I now have errors telling me that these methods are undefined. I'll fix this by adding the name of the class SimpleMath as a prefix to the call to the method. I'll type Simp and press Ctrl+Space and there is my new class, SimpleMath.

I'll separate the name of the class and the method name with a dot. Notice when I type in the dot, the four available methods are shown automatically and I can easily call them where needed. Now I am going to copy SimpleMath and the dot to the clipboard, and I'll paste that string in to each of the other three method calls. I'll save my changes and I'll run the application, I'll enter a value, I'll enter another value, I'll choose subtraction this time, and everything still works.

So now I've successfully refactored my application. I've extracted the code that's not a part of the main operation and put it in its own separate class. Let's do the same thing with the input function. Once again, I'm going to create a new class, I'll right click and choose New>Class, and I'll call this InputHelper. Once again making sure I don't have the main method selected, I'll click Finish. I'll go back to the main application, I'll scroll down to the bottom and I'll grab this getInput method, make sure you get all of the method including the opening and closing braces, I'll cut that code to the clipboard, I'll go to my new class, place the cursor inside the class declaration, and paste.

Notice that when I paste the code into the new class, Eclipse automatically adds import statements for the BufferedReader and InputStreamReader classes that I need. Just as I did with the methods in the SimpleMath class, I'll change this access modifier from private to public. I'll save my changes to this file and go back to the main application, and I'll go up to the calls to the getInput method. I'll type in input and press Ctrl+ Space and I'll choose my new class, InputHelper and add the dot.

I'll select and copy the InputHelper .string and paste that in twice and I'll save my changes. Down to the bottom, I get a warning, The import java.io is never used. That import statement was handling the classes that deal with input, but because I've moved the usage of those classes into my own separate custom class, I no longer need the import statement. I can either delete it explicitly or I can let Eclipse do it for me.

To do it in Eclipse, hold down the Ctrl and Shift keys on Windows or the Command and Shift keys on Mac and press the letter O, and that is something called organizing imports. That's a great trick, because it will not only get rid of the imports that you don't need, but it will also add or propose the imports that you do need and alphabetize and otherwise organize them for you. I'll save my changes and run the application again and I'll test it again with values of 10, 3 and this time I'll multiply and everything still works.

By organizing your application in smaller classes, it makes it easier to maintain your work over time, and you'll be tempted to tune and refactor your application as you go along. Eclipse will help you with this in many ways. Let's say, for example, that you end up not liking the name of this class SimpleMath, and you think well, I'd like to change the name of that class so that it matches InputHelper, I'd like it really to be named MathHelper. But it would be a real pain to go through and try to search and replace for every occurrence of this class name.

I have it here and I have it four times in the main method. Well, this kind of class renaming is called refactoring and it's incredibly simple in Eclipse. Go to the class in your package Explorer, right click, choose Refactor and then Rename. Assign the new name of the class and I'm going to call it MathHelper. Then click Finish and you'll see that the name of the class is updated in all the calls in the main application, the name of the file is changed, and the name of the class in the class declaration is changed as well.

At the minimum, you can use your custom classes to organize the code, bringing together methods that execute similar operations and moving them out of the main application file that has your main method. How you design the application and how you separate the code is completely up to you, but Java gives you all the tools to do it.

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This video is part of

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Java Essential Training

71 video lessons · 81191 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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