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Everything is an object

From: Java Essential Training

Video: Everything is an object

As you learn how to program in Java, keep this idea always in mind, everything is an object. As you design your applications, you'll be placing your code inside class definitions and wrapping that code inside functions known as methods. Let's take a look at a very simple application the Hello World! application I've shown previously. This time I've given the starting class a different name SimpleApplication. Notice that the main method though looks the same as before. That syntax is required, and as stated previously, the executable code is inside the main method.

Everything is an object

As you learn how to program in Java, keep this idea always in mind, everything is an object. As you design your applications, you'll be placing your code inside class definitions and wrapping that code inside functions known as methods. Let's take a look at a very simple application the Hello World! application I've shown previously. This time I've given the starting class a different name SimpleApplication. Notice that the main method though looks the same as before. That syntax is required, and as stated previously, the executable code is inside the main method.

It outputs the string "Hello World!" to the console. You can diagram your Java applications using a syntax known as UML or Unified Modeling Language. This diagram shows that the name of the class is SimpleApplication and that it has a single method or function named main and it returns void which means it returns nothing. It doesn't return any value at all. Now, from this simple class, you might not get the idea that everything is an object. So let's expand the application a little bit to two classes.

In this version of the application there are two classes, the starting class is still called SimpleApplication. It's the starting class because it's the one with the main method, and as the application starts up, the main method is called and the code in that method creates an instance of another class called Welcomer. That class is shown at the bottom of this screen. Shift your attention down to the welcome request. It declares a variable named welcome outside of any methods. We call those either instance variables or fields.

In this case, there is an instance variable named welcome, it's a string, and it's private to the class which means that it can only be read by the code within that class. Its value is Hello. When, the function sayHello is called from that class, that class is responsible for outputting the string to the console. From the main application we create an instance of that class using the syntax Welcomer, that's the uppercase version, that's the data type and then the lowercase version is the name of the variable and we say that it's equal to the new instance of the Welcomer class that's an object and the object can have methods and properties.

We call the object sayHello method and the result is to output the string to the command line. Once again, you can diagram this application using UML, in this case there are two classes SimpleApplication and Welcomer. This first class simply has the main method; the second class has both an instance variable or a field and a method named sayHello. Once again sayHello returns void or nothing, but it takes the action of outputting a string to the console. In Java, we say that everything is an object because it's very common to create an instance of class to execute some code and that code will be wrapped inside the class.

As we actually get into coding some classes, I'll describe what's an object, what's a variable, and what's a field. Here is another way in which Java sees everything as objects. You can declare a String variable by declaring the data type String and then the variable name, in this case the variable name is welcome and then you can say that that variable has a value of a string wrapped in double codes, but in fact, this is shorthand. Here is the bit of code that's the longhand version. We declare the data type, we say what the variable name is, but now we say that we're creating a new instance of the String class and we pass in the string that we want.

This is the longhand version and these two bits of code are exactly functionally identical. They do the same thing. They create an instance of the String class and so welcome is an object. But you can go further with this idea, because within a string object there is actually an array of individual objects, one for each character. So you can say that welcome is a String, Hello or you can say that welcome is an array of six object, H-e-l-l-o-! And here is a bit of code that breaks it down even further.

I'll start off with an array of characters. char, bracket, bracket means create an array of characters. The name of the array is chars, C-H-A-R-S, and then the following syntax with the individual characters each wrapped in single quotes and then all then all six characters wrapped inside braces, means create an array. I can take that array of characters and wrap them inside a new string called s and now once again I have either an array of six objects or a single object the String s. When you start seeing everything as an object, you see that you can break things down to their smallest components in Java; a String is really an array of characters.

A primitive numeric value is an individual value, but a complex object can reference many objects. By breaking things down into small pieces in Java, it makes it easy to find the code that you want to expand on or fix. The output of this code would be simply Hello! Because the String class is a complex object, it can encapsulate or offer all sorts of functionality and in fact the String class, which in longhand is java.lang.String includes many, many methods.

This is a short list of just a few of the methods that are a part of the String class. The charAt method lets you pass in a numeric value and get back the character in the position in the array that's a part of the String class, you can use compareTo to compare one String to another, the concat to concatenate Strings together and so on. We'll be looking at these and many more methods of the String class later on, but my point is that a string is not simply a string, it's an instance of a class and that gives you access to all the functionality that's built into that class as a part of the Java Programming Language and its core runtime, the Java Class Library.

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This video is part of

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Java Essential Training

71 video lessons · 69843 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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