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Storing data in instance variables

From: Java Essential Training

Video: Storing data in instance variables

When you declare your own custom classes you can assign them instance variables. Instance variables in Java are known as fields and they are variables that are assigned at the class level and are available to the entire class and optionally to the rest of the application. I'm working in a version of the application named InstanceVariables that's in this chapter's exercise files. It starts within Olive class that has a public method named crush. Right now it's just outputting a simple string, but I'm going to define a set of instance variables in this class that can be used later on.

Storing data in instance variables

When you declare your own custom classes you can assign them instance variables. Instance variables in Java are known as fields and they are variables that are assigned at the class level and are available to the entire class and optionally to the rest of the application. I'm working in a version of the application named InstanceVariables that's in this chapter's exercise files. It starts within Olive class that has a public method named crush. Right now it's just outputting a simple string, but I'm going to define a set of instance variables in this class that can be used later on.

To declare an instance variable or a field you place the cursor inside the class definition and within the class definition, you declare each variable with an access modifier, a data type, and a name and optionally, you can set an initial value. So for example, I'll start with public string, meaning I am declaring a variable or a field that can be accessed from anywhere in the application and that will contain a string. I'll give the field a name of name, and I'll set it initially to a value of Kalamata. So that's the type of olive that I'm working with.

I'll let another string variable, this one will be named flavor and it will be a flavor of grassy. According to Wikipedia, Kalamata olives have a grassy flavor. Next, I'll also declare a variable named color. Now rather than setting this as a string, I'll use long integer. Colors can be translated into long integers using the same hexadecimal values that you might use in HTML for cascading style sheets. So I'll call this public, long, color and I'll set it to a value of 0x000000 or Black, and finally, I'll create a variable called oil.

I'll set this as an integer and I'll give it a value of 3. This is random, what I'm saying is that when I crush a Kalamata olive, I'm getting 3 ml of oil, that's almost certainly incorrect in the real world, but it will work for this example. So now these variables or fields are available to the entire application. Eventually, I'm going to hide the actual data within the application, but for now this will work. I'll save my changes and go to the main application where I'm creating an array of olives.

Now I'm going to re-factor or change the structure of the application so that instead of creating an array of Olives that's fixed at runtime I'm going to instead use an array list and this will allow me to resize the array as needed. I'll go into the Main method and add some extra whitespace. I'll create an ArrayList after typing ArrayList, I'll press Ctrl+Space and that will add the required import, and I'll set the array list as containing instances of the Olive class, and I'll name it olives.

There'll be a conflict because I'm already using that name further down, but that's okay, I'll take care of that in a moment. Next, I'll instantiate the array list. Now, I'm going to create three instances of the Olive class. I'll start by declaring a single variable, Olive olive. I'm not instantiating it yet; I'm declaring it once so that I can reuse that reference three times. Next, I'll say olive = new Olive and I'll output the name of the olive that I just created using system.out.println( olive.name) and then I'll add the olive to the array list using olives.add(olive).

I'm done with this array, I'll get rid of it and then I'll copy these three lines of code and paste them in twice. Now I'm obviously making this code more verbose than it really needs to be, but my goal here is to show you that because I declared a public field in the olive class, I can reference it from within the main application. I'll save my changes and run the application and I see some errors. I'll go down to my errors list and double-click and it tells me that the getOil method is expecting an array, but I'm trying to pass in array list. So I'll jump to that method and here's a great little Eclipse shortcut.

Now that my methods are being scattered across multiple classes, I need to be able to move among them more easily. Eclipse lets you click on a method name and jump to the reference and here's the shortcut. Hold down the Ctrl key on Windows or the Command key on Mac, move the cursor over the method, and choose Open Declaration and that will take you to the method itself. Now change the method declaration so that it's receiving an array list of Olives. Save and run the application and you'll see that you're looping through the array list using the same syntax as you did with the array earlier.

You don't need to create an iterator for this; if all you need to do is look through, you can simply use it for each command. So as I create the Kalamata olives, I am outputting the names of the olives and as I crush them, I'm still getting ouch. Now let's go ahead and fix that. We need to be more specific. I'll go to the Olive class and I'm going to change how the crush method behaves. I'm going to change it to return the amount of oil that just got crushed. I'll change the data type from void to int and I'll add return oil.

I'll save the changes there and go to OlivePress. Within the OlivePress class and within its getOil method, I'll declare an integer variable named oil and I'll set it a value of 0. Within the for loop, I'll increment the amount of oil that I've received using oil += Olive.crush. I'll jump to the crush method. Remember that I'm returning the total amount of oil which is this integer value. I'll come back to the OlivePress and after the for loop, I'll output the amount of oil that I just received.

The string will be, You got, and then the amount of oil and then units of oil. I'll save and run the application and it tells me, I got 9 units of oil three times three. Finally I'll go back to Olive. You might have noticed that at no point was I actually accessing the oil field from the main application. So I don't need to make this field public and this is the first time I'm going to talk about hiding data within a Java application. In a Java application, you should only make data accessible that must be accessible to the rest of the application.

If you can hide the data within a class, you should do it. So the only method that's actually accessing this value at this point is a method within the class and I can make this field private. I'll save and run the application again and it still works. So that's a look at how to declare instance variables or fields in a Java class. These are persistent data values. When you create the instance of the class, the object, these data values will persist as long as the object persists in memory. I've shown you how to make those variables either public or private.

If you make them public, if you need to be able to access those values from the rest of the application or private, if they're only used within the class, and there is also the access modifier protected, but we won't need that until we start dealing with inheritance.

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This video is part of

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Java Essential Training

71 video lessons · 75713 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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