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Declaring and initializing variables

From: Java Essential Training

Video: Declaring and initializing variables

As with all good programming languages, Java supports the use of variables to store data in memory. There are two major types of variables in Java known as primitives and complex objects. Primitive variables are stored in the fastest possible memory, but there is a very small number of data types that you can store this way. They include numeric values such as integers and floating decimals, single characters not complete strings, and Boolean values, variables that equate to either true or false.

Declaring and initializing variables

As with all good programming languages, Java supports the use of variables to store data in memory. There are two major types of variables in Java known as primitives and complex objects. Primitive variables are stored in the fastest possible memory, but there is a very small number of data types that you can store this way. They include numeric values such as integers and floating decimals, single characters not complete strings, and Boolean values, variables that equate to either true or false.

These values can be stored as primitive values and so they are placed in memory and they can be accessed very, very fast. Everything else that you might want to store in memory in a Java application is going to be represented as a complex object. You have to clear a complex object as an instance of a Java class. Here are some data types that you might think of as simple values depending on what languages you might have used before, but in Java, are complex. These include strings. Any string value longer than a single character must be stored as a complex object and not as a primitive.

Date values which can be interpreted as numeric values, the number of ticks from a particular date, but in terms of Java memory management are stored as complex objects and everything else that you might want to work with. As I have mentioned previously, pretty much everything in Java is an object of some kind. So how do you actually declare a variable in your code? The syntax looks very similar regardless of whether you are working with a primitive or a complex object, but there is a significant syntactical difference.

Let's look at primitives first. You declare your variables in three parts. The first part is the data type. Every variable that you declare in Java has to be declared with its data type and once the data type is set, it can't be changed. This is how the Java virtual machine determines how much memory needs to be allocated. The second part is the variable name. Variable names in Java always start with the lower case initial character which can be an alphabetical value or an underscore, but not a numeric.

These two parts are required. The third part is optional and it's the initial value of the variable. Here is a classic primitive variable declaration int newVariable = 10. The first part is the data type, int stands for integer, so you are declaring the variables in integer and you will only be allocating enough memory to store an integer. You can't just magically change this variable at runtime and turn it into say a double or a float. It's always going to be an integer.

The second part of the variable declaration is the name. The name of the variable can't have any spaces or other special characters in it and the initial character has to be either alphabetical or an underscore character, it can't be a number. The length of the variable name is up to you, but I recommend that you name the variables so that they mean something to you as a developer. You'll see variable named i or x in varied limited scopes, but if a variable is going to stick around and be useful in your application, give it a name that matters.

The last part of the declaration is optional. This is this assignment of the initial value. You use the equals operator which is the assignment operator and you pass in a value that's compatible with the data type that you declared. The value 10, in this context, is a literal integer. So I am assigning the value of 10 to the integer and while I can change the value of new variable later on the value I assign later also has to be an integer. If you leave the assignment off, then the primitive variable will have its default value.

For numeric, that's always zero. For Booleans, it's false. Now let's take a look at how you declare a complex object. The declaration of a variable as a complex object once again is an instance of a class. As with primitives, you declare them in three parts; the required data type and required variable name and then the optional assignment of the initial value. When you set the value of a complex object you use the new keyword and a particular kind of function or method called the constructor method of the class.

The name of the constructor method matches the name of the class. So if you are declaring something is a string, then you are going to use a method name String. Here is what the code looks like. Once again it will have the data type and the variable name and then optionally the initialization. The first part of this declaration is the data type. I am referring to a class. How do I know this is a class and not a primitive? Well, primitive data types are always all lower case. So int double float Boolean and so on, they will be spelled all lowercase.

When you see a data type within an initial uppercase character, now you are talking about a class and you know you are working with a complex object. The variable name once again goes second and the conventions for naming complex object variables, is the same as for primitives. The initial character must be an alphabetical character or an underscore, you can't use numbers for the first character and by very strong convention it's lowercase. You can use uppercase characters in the middle, a syntax, we sometimes call camel case to make it more readable.

The optional assignment looks like this. As with primitives you use the equals assignment operator, but now because you are creating an instance of a class, you use the new keyword and you call the constructor method of the class which matches the name of the class. Some constructor methods are called without passing any values in and some constructor methods are called and you have to pass values in and in some cases it's optional. You'll look at the API documentation for the class you want to use to figure out what the right syntax is.

For complex object variables, if you eliminate the assignment, the variable name will exist, but it won't point to anything in memory and we say that that variable has a value of null, n-u-l- l and I'll show how to deal with null values, later in the course. When you need to clear a variable, it's important to know what its scope and lifetime is. You can either declare a variable inside a function or method or you can declare it outside the function as a part of the class. When you declare the variable inside the function, the variable is local to the function and when the function is finished executing, the variable goes away.

It's no longer available. We call it being dereferenced. Here is an example of a string variable which is declared inside a function or method called doSomething. The data type is String. The name of the variable is sayHello and it's an instance of this String class with the value Hello!. This code works fine because I am referencing a variable that was declared inside the same function in which I am trying to output it, but let's take a look at a bit of code that doesn't work so well. In the function, doSomethingElse, I am once again declaring a variable and giving it an initial value of Hello! But then I try to reference that variable in code that's outside the function.

Because the variable was declared inside the function, it is not available to code outside the function, it can only be referenced inside the function and when you type this code and try to save your application, you'll get a Compile-time error!!. If you are working in an Eclipse you'll see something in the Problems view that says I don't know what that variable is, it's not in scope. There are times when you want variables to be available to more than one function or method at the same time and there are a few different ways of doing this. One of the most popular is to use something called a class variable known officially in Java as a field.

When you declare a variable outside of the function, it belongs to the whole class and it can be referenced by any function within the class. Here is an example. I am declaring a class called MyClass. Notice that this time I don't have a main method. So this is not a class that's starting up the whole application. Within the class, I am declaring a variable and giving it a data type of String, a name of sayHello and once again an initial value. The function doSomething can reference that variable because the function and the variable are both members of the class.

As long as the method and the variable are both part of the same class, the method can reference the variable and use its value. So once again this kind of variable is officially called a field in Java. You will also hear it referred to sometimes as a class variable, but the two phrases mean the same thing, class variable and field. And if you look at the documentation for the classes in the Java Class Library, you'll see that this type of variable is referred to as a field in the API docs.

So that's a brief look at how to declare variables and a little bit about the differences between primitives and complex objects. In the next video, we'll do some exercises in Eclipse where we declare some variables and output their values using the System.out.println syntax.

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This video is part of

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Java Essential Training

71 video lessons · 69495 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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