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Creating and using instance methods

From: Java Essential Training

Video: Creating and using instance methods

When you declare a method in a Java application, you can declare it either as a class method or an instance method. A class method is a method like the ones I've been using so far, it's called from the definition of the class. An instance method in contrast is called from an instance of the class or an object. Typically, you can use class methods when you're building up a library, say, of utility functions and you are going to pass in all the values that the method is going to operate on as arguments passed into the method when it's called. But when you start working with more complex objects, objects that have persistent data in them, you are going to want to learn how to use instance methods.

Creating and using instance methods

When you declare a method in a Java application, you can declare it either as a class method or an instance method. A class method is a method like the ones I've been using so far, it's called from the definition of the class. An instance method in contrast is called from an instance of the class or an object. Typically, you can use class methods when you're building up a library, say, of utility functions and you are going to pass in all the values that the method is going to operate on as arguments passed into the method when it's called. But when you start working with more complex objects, objects that have persistent data in them, you are going to want to learn how to use instance methods.

For this exercise, I'm working a project named InstanceMethods that has three classes. All three classes are starting off pretty empty. The Main class has a main method that doesn't do anything yet and there's an OlivePress class and an Olive class, and again, they don't have any functionality yet. I'm going to design this Olive class so that it can be instantiated and then its methods can be called. I'll place the cursor inside the class declaration. To define in the instance method, you use the same sort of signature as you do with a class method, but you don't put in the static keyword.

Remember, the Main method has this word, static, and all of the methods that I've defined throughout this course up to this point have also used the word static. The word static means this is a class method that can be called from the class definition. If you drop out the word, static, it's an instance method. Just as with class method you start with an access modifier; public if you want to method to be called from anywhere in the application; private if it's only available within the class; and protected if it's available within this class or any of its subclasses. I'll declare this method as public.

Next you need a return type. Initially, this method will return void or nothing and then the method name which I'll call crush. Add in the parentheses at the end of the method name and a pair of braces and now you can add code to implement the method. When an Olive is crushed, we'll say it hurts, so the Olive says ouch. So I'll output using system.out.println the word ouch and I'll save my changes and that's my first instance method. Now I'll go back to the main application.

Within the main application, in order to call that method, I need to create an instance of the Olive class. So I'll declare a variable. I'll assign a data type of Olive, the name of the class. Notice that as I type the name of the class, I get the little squiggly line underneath the class name and that tells me that the class currently is being seen as undefined. The compiler doesn't know where it is. So I'll press Ctrl+Space, I'll see a listing of all the classes that start with the word Olive and I'll choose Olive. An Eclipse adds a required import statement at the top of the code.

Even though this is my own class and my own application, the rules for imports are the same as with the Java class library. If the class that I'm using is in a package other than the one that contains the current class, I must import it. Next, I'll declare the variable name which I'll set as olive, all lowercase, and I'll instantiate using new Olive. Now you may be thinking where is that method. If I go back to Olive.java notice that there is no definition of a constructor method or any other method that matches the class name.

The answer is that the Java compiler always adds a no arguments constructor method, a method that can be used to create an instance of the class. If you don't define one, one is defined for you and that's what I'm calling here. So I have created an instance of the Olive class and now I'll call its method using olive., and when I type the dot, Eclipse shows me the list of methods that are available. The crush method is at the top of the list. All of the other methods that you see here are defined as a part of the class object. I'll talk later in the course about inheritance, but basically all these methods are coming from a class name object.

That is the super class of other classes in Java. I'll call the crush method, I'll save my changes and run the application and there is the output. I call the objects instance method and it was responsible for outputting that string to the console. Now I'll create one more instance method. So far my application knows how to crush a single olive, but clearly that's not enough to get enough olive oil. So now, I'm going to create an OlivePress class and once again, just like the olive, it will be designed to be instantiated.

I'll go to OlivePress.java and I'll create a public method named getOil. Once again, it will return void. The getOil method will receive an array of olive objects and it will loop through the olives and crush them one at a time. So I'll go to my method getOil and I'll design it to receive an array of olives. I'll type in the word, olive, I'll press Ctrl+Space and choose olive, that will add the required import statement. I'll add the opening and closing brackets to indicate that I'm receiving an array of olives and I'll name the array, Olives.

Now within the method, I'll loop through the array. I'll type for and press Ctrl+Space. I'll choose the for each syntax and I'm saying for each olive in the olives array do something. And once again just like I'm doing in the main application right now, I'll call olive.crush. I'll save those changes; I'll go back to the main application class and change it so that it's now going to crush a whole bunch of olives. I'll go back to main.java and I'm going to take out this code. I don't need that anymore and I'm going to declare an array of olives.

Once again, I'll press Ctrl+Space and choose olive, I already have my import statement at the top so I don't need re-declare it. I'll declare this as an array and name it olives, and then I'll use the syntax to initialize the array with values. I'll start with a brace and then newOlive(), newOlive(), and newOlive(). So I am creating three anonymous objects. I am not assigning variable names to them; I'm just creating them and passing them into the array, and then immediately assigning the entire array to the olives array.

Next, I'll create an instance of the OlivePress. I'll type in OlivePress, I'll press Ctrl+ Space that adds an import statement for that class, I'll name it press and I'll instantiate it, once again using the constructor method that the Java compiler adds for me, and finally, I'll call press.getOil and I'll pass in the olives array. I'll make sure to add the semicolon at the end of the line and I'll run the application and because I created three olives in the array, I get the word, ouch, three times.

So this is how instance methods work. Don't put in the static keyword and it becomes an instance method that's called from an instance of the class rather than from the class definition. Once you know how to create and use instance methods, you are on your way to creating full-blown objects that incorporate both methods that is functionality and data.

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This video is part of

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Java Essential Training

71 video lessons · 75479 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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