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As another aside, member variables in Java are automatically initialized to zero or the equivalent of zero. However, local variables , of which ch2 is an example, are not automatically initialized.
Why declare the variables as type int?
It was necessary to declare these variables as type int because the statement in Listing 4 (more specifically, the call to the System.in.read method) returns a value of type int .
| Listing 4 . Beginning of a while loop. |
|---|
while( (ch1 = System.in.read() ) != '#') ch2 = ch1; |
Java provides very strict type checking and generally refuses to compile statements with type mismatches.
(There is a lot of complicated code in Listing 4 that I haven't previously explained. I will explain thatcode later in this and future modules.)
Another variable declaration
The program in Listing 1 also makes another variable declaration shown by the statement in Listing 5 .
| Listing 5 . Beginning of the main method. |
|---|
public static void main(String[] args) //define main method |
An array of String references
In Listing 5 , the formal argument list of the main method declares an argument named args (first cousin to a variable) as a reference to an array object of type String .
Capturing command-line arguments in Java
As you learned in an earlier module, this is the feature of Java that is used to capture arguments entered on the command line, and is required whetherarguments are entered or not. In this case, no command-line arguments were entered, and the variable named args is simply ignored by the remainder of the program.
The purpose of the type of a variable
The type determines the set of values that can be stored in the variable and the operations that can be performed on the variable.
For example, the int type can only contain whole numbers (integers) . A whole host of operations are possible with an int variable including add, subtract, divide, etc.
Signed vs. unsigned variables
Unlike C++, all variables of type int in Java contain signed values. In fact, with the exception of type char , all primitive numeric types in Java contain signed values.
Platform independence
At this point in the history of Java, a variable of a specified type is represented exactly the same way regardless of the platform on which theapplication or applet is being executed.
This is one of the features that causes compiled Java programs to be platform-independent.
In Java, there are two major categories of data types:
Primitive variables contain a single value of one of the eight primitive types shown in Listing 2 .
Reference variables contain references to objects (or null, meaning that they don't refer to anything) .
The eight primitive types in Java?
The table in Figure 2 lists all of the primitive types in Java along with their size and format, and a brief description of each.
| Figure 2 . Information about the primitive types in Java. |
|---|
Type Size/Format Description
byte 8-bit two's complement Byte-length integershort 16-bit two's complement Short integer
int 32-bit two's complement Integerlong 64-bit two's complement Long Integer
float 32-bit IEEE 754 format Single-precisionfloating point
double 64-bit IEEE 754 format Double-precisionfloating point
char 16-bit Unicode character Single characterboolean true or false True or False |
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