visual basic 2008 barcode generator Data Types and Variables in Java

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CHAPTER 2 Data Types and Variables
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You can temporarily change the data type returned by the Systeminread() method from an integer to a char data type by casting the value this method returns Casting temporarily changes a value from one data type to another data type in order to carry out an operation In the previous example, the operation is to store the character entered into the keyboard in memory You cast a value by placing the temporary data type within parentheses, as shown in the next example:
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class Demo { public static void main( String args[]) throws javaioIOException { char choice = (char) Systeminread(); } }
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Here s what happens when a value is cast: 1 The method Systeminread() returns an integer representing the character entered into the keyboard 2 The return value (integer) is temporarily converted to (cast as) a char data type, making it the same data type as the choice variable 3 The converted integer is assigned to the choice variable You probably noticed something a little different in the previous example Alongside the main() method name is throws javaioIOException This is telling Java that an IOException will be created by the method if something doesn t go right when a character is read from the keyboard You ll learn about exceptions in 9
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Variables
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Previously in this chapter, you learned that computer memory is divided into chucks of eight bits (one byte) Each byte is identified by a unique memory address similar to how each house in your town in identified by a unique address However, you might be more comfortable thinking of each chuck of memory as an empty box that is identified by a unique number Using numbers to identify boxes (memory location) isn t the most intuitive way for you to keep track of them within your program A more appropriate way is to assign a name to the boxes that is meaningful to you and that you tell Java to reserve for your program
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In the previous example, we use the word choice for the name of the box (memory location) that Java reserved to store the character entered at the keyboard We refer to the word choice in the program whenever the program needs to access data stored in the corresponding box (memory location) Java is smart enough to translate the word choice into the actual address of the box (memory address) The memory location that a program reserves is referred to as a variable, and the name you give to the variable is called an identifier Therefore, choice is an identifier that refers to a memory location called a variable Java automatically relates the identifier to the variable s memory address Each time you refer to the identifier in your program, Java looks up the memory address that corresponds to the identifier and accesses that memory address This process is called resolving
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Declaring a Variable
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Your program must tell Java to reserve memory before your program can store data in memory The instruction to reserve memory is called declaring a variable You declare a variable by using a declaration statement within your program A declaration statement consists of three components:
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Data type Identifier Semicolon
The data type is one of the keywords described in Table 2-3, and it tells Java the kind of data your program will be storing in memory Java then knows how much memory to reserve The identifier is the name you will use within your program to refer to that memory location The semicolon is Java punctuation that tells Java it has reached the end of the statement Here s how to declare a variable In this example, Java is told to reserve enough memory to store an int and that you will use the identifier grade within the program to refer to that memory location:
class Demo { public static void main( String args[]) { int grade; } }
The name you select as the identifier for the variable should be meaningful and represent the nature of data that is stored in the corresponding memory location The
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