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Managing instance data with getter and setter methods

From: Java Essential Training

Video: Managing instance data with getter and setter methods

When you're working with object- oriented development patterns a lot of developers feel that fields should always be private and their data should only be accessed through what we call getter and setter methods. In some languages there is a port for implicit setters and getters. For example, in both C# and in ActionScript 3, you can use the words set and get before method names as separate words and then you can refer to those methods as properties. In the Java language, in the absence of some other framework, you can only use explicit setters and getters, that is methods that start with the word get as part of the method name, but these are so common that there is support for generating these kinds of methods in the development environment such as Eclipse.

Managing instance data with getter and setter methods

When you're working with object- oriented development patterns a lot of developers feel that fields should always be private and their data should only be accessed through what we call getter and setter methods. In some languages there is a port for implicit setters and getters. For example, in both C# and in ActionScript 3, you can use the words set and get before method names as separate words and then you can refer to those methods as properties. In the Java language, in the absence of some other framework, you can only use explicit setters and getters, that is methods that start with the word get as part of the method name, but these are so common that there is support for generating these kinds of methods in the development environment such as Eclipse.

I'm working in the class Olive which is a member of the project getters and setters. This is class has 4 fields, 2 strings, one long in one end, and 3 of the fields are public and one private. To generate getter and setter methods, place the cursor anywhere in the class. You don't have to select the code as I've shown here, and then you can right click and choose Source or you can go to the main Menu and choose Source. From there choose Generate Getters and Setters.

In this dialog box choose the field for which you want to Generate Getter and Setter methods. For this first example, I'll only choose the oilfield. Next, determined the insertion point, where do you want to put the new code. If you choose first member, the code will go right after the class declaration, and last member will put it at the end. I typically like to put my getters and setters after the field declarations. So I'll choose after oil; that's the last field declaration and before the methods.

Next, indicate how you want to Sort. Do you want to put the Fields in alphabetical order or do you want to do all the getters first and then all the setters. I'm generating only getter and setter for one field initially, so it doesn't matter which I choose, but typically, I like the first option, Fields in getter/setter pairs. Next, set the Access modifier, are your getters and setters going to be public, protected, default, or private? I'm going to set them as public, so that they can be called from anywhere in the application. I'll click OK, and that generates two methods named getOil and setOil.

Here is the syntax of an explicit getter method, it always has a return data type, which matches the original fields' data type, the name of the method matches the name of the field with a prefix of get, and the name of the field is changed to have an uppercase initial character. So oil, all lowercase, becomes oil with an uppercase O, and then within the getter method, there is a return statement that returns the value of the field. A setter method always has a void return type, and it has a prefix of set before the field name, it receives a value that matches the data type of the field, and then it sets the field using this, to distinguish the field from the argument.

Once you've created these methods, you can we refactor the rest of the application to always set the fields through the methods. So for example in my public constructor down here the one who receives an int value, instead of saying this.oil=oil, I'll call the method setOil and pass in oil, the argument. Following this pattern results in centralizing the processing of the argument, later on if you want to validate, modify, or otherwise secure the data; it makes it very easy to do.

Now let's do something similar in the olivepress class. Currently this getOil method in this class loops through an array list of olive objects, crushes each of them, then calculates the amount of olive oil, but it doesn't actually return the olive oil to the application. You could change this method so that it have a return type say of int, and then add a return statement, or another approach, would be to create a field of the class, so that once the getOil method was done, it would set the amount of oil as a member of the class and then the rest of the application could access that oil.

So I'm going to place the cursor right after the class declaration and before the constructor method. I'll declare a private integer variable and I'll name it totalOil and I'll initialize it to a value of 0. When the class is instantiated, this private field will have a value of 0, but then I can add values to it each time the getOil method is called. Next, I'll add a getter and a setter for this field. I'll right click and choose Source>Generate getters and setters.

I'll check the only field, I'll place the insertion point after totalOil, and I'll set the access modifier to public and click okay. So there's my getter and setter. The rule that I want to follow is the only code within the class is allowed to set the totalOil, but code anywhere in the application can access it. So I am going to change the access modifier for setTotalOil to private. So now from the point of view of the rest of the application, it's a read-only value.

Within the getOil method I'll move the cursor down here, and I'll call the method setTotalOil and pass in the value that was calculated. I'll take this line of code that's outputting the total amount of oil and I'm going to place it in the main application. I'll go to main.java and place the cursor after the call to the OlivePresses get oil method, I'll paste in that line of code and now instead of outputting the value of an oil variable, which is no longer in the scope of this class, I'll get that value from press.getTotalOil, because it's a public accessor method, a public getter method, I can call it from anywhere in the application.

It's up to the olivepress to set the oil, but my main application code can get to it. I'll run the application as a Java application and there is the result. Everything still works exactly the same as before. Finally, let's say that in this application I wanted to reuse the press object, so that I could add olives at one point in the application and then at another point. As long as I create a single instance of the olivepress class and keep it in memory for the duration of the application session, I should be able to do that, but right now my setTotalOil method is rewriting the value of total oil every time it's called.

Let's say that instead, I wanted to incrementally add to it. I could go to the setTotalOil method and change the operator from equals to plus equals, and now I'm adding to the total. I'll Save my changes. I'll go back to main.java and I'm going to copy these lines of code starting with press.getOil. I'll paste those at the end and so now I am going to crush those olives once, and then I am going to crush them again, because I'm now incrementally adding a value, I should be multiplying the amount of olive oil each time I go through the loop.

I'll Save and Run, and the first time through I got the 5 units and after the second time through I have 10, and so on. This illustrates the value of getters and setters, when you wrap the access of the data in a setter; it lets you change the rules very, very easily. So before every time the method was called, I was just assigning the value, but then with one little change, now I'm incrementally adding to the value, and you can design and code any other sort of business rule you like once you have the getters and setters in place.

So this is a major part of what we talked about when we were discussing encapsulation. Hiding data refers to the practice of setting fields as private and then only allowing access to those fields through getter and setter methods. Again in Java, in the pure programming language, you don't have implicit setters and getters, like you do in some other languages, but there is a very strong convention for how you design setters and getters and then call them from within your application.

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This video is part of

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Java Essential Training

71 video lessons · 75773 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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