Section 4.7
The Truth About Declarations
Names are fundamental to programming, as I said a few chapters ago. There are a lot of details involved in declaring and using names. I have been avoiding some of those details. In this section, I'll reveal most of the truth (although still not the full truth) about declaring and using variables in Java. The material in the subsections "Initialization in Declarations" and "Named Constants" is particularly important, since I will be using it regularly from now on.
4.7.1 Initialization in Declarations
When a variable declaration is executed, memory is allocated for the variable. This memory must be initialized to contain some definite value before the variable can be used in an expression. In the case of a local variable, the declaration is often followed closely by an assignment statement that does the initialization. For example,
int count; // Declare a variable named count. count = 0; // Give count its initial value.
However, the truth about declaration statements is that it is legal to include the initialization of the variable in the declaration statement. The two statements above can therefore be abbreviated as
int count = 0; // Declare count and give it an initial value.
The computer still executes this statement in two steps: Declare the variable count, then assign the value 0 to the newly created variable. The initial value does not have to be a constant. It can be any expression. It is legal to initialize several variables in one declaration statement. For example,
char firstInitial = 'D', secondInitial = 'E';
int x, y = 1; // OK, but only y has been initialized!
int N = 3, M = N+2; // OK, N is initialized
// before its value is used.
This feature is especially common in for loops, since it makes it possible to declare a loop control variable at the same point in the loop where it is initialized. Since the loop control variable generally has nothing to do with the rest of the program outside the loop, it's reasonable to have its declaration in the part of the program where it's actually used. For example:
for ( int i = 0; i < 10; i++ ) {
System.out.println(i);
}
You should remember that this is simply an abbreviation for the following, where I've added an extra pair of braces to show that i is considered to be local to the for statement and no longer exists after the for loop ends:
{
int i;
for ( i = 0; i < 10; i++ ) {
System.out.println(i);
}
}
A member variable can also be initialized at the point where it is declared, just as for a local variable. For example:
public class Bank {
private static double interestRate = 0.05;
private static int maxWithdrawal = 200;
.
. // More variables and subroutines.
.
}
A static member variable is created as soon as the class is loaded by the Java interpreter, and the initialization is also done at that time. In the case of member variables, this is not simply an abbreviation for a declaration followed by an assignment statement. Declaration statements are the only type of statement that can occur outside of a subroutine. Assignment statements cannot, so the following is illegal:
public class Bank {
private static double interestRate;
interestRate = 0.05; // ILLEGAL:
. // Can't be outside a subroutine!:
.
.
Because of this, declarations of member variables often include initial values. In fact, as mentioned in Subsection 4.2.4, if no initial value is provided for a member variable, then a default initial value is used. For example, when declaring an integer member variable, count, "static int count;" is equivalent to "static int count = 0;".
Even array variables can be initialized. An array contains several elements, not just a single value. To initialize an array variable, you can provide a list of values, separated by commas, and enclosed between a pair of braces. For example:
int[] smallPrimes = { 2, 3, 5, 7, 11, 13, 17, 23, 29 };
In this statement, an array of int of length 9 is created and filled with the values in the list. The length of the array is determined by the number of items in the list.
Note that this syntax for initializing arrays cannot be used in assignment statements. It can only be used in a declaration statement at the time when the array variable is declared.
It is also possible to initialize an array variable with an array created using the new operator (which can also be used in assignment statements). For example:
String[] namelist = new String[100];
but in that case, of course, all the array elements will have their default value.
4.7.2 Named Constants
Sometimes, the value of a variable is not supposed to change after it is initialized. For example, in the above example where interestRate is initialized to the value 0.05, it's quite possible that 0.05 is meant to be the value throughout the entire program. In that case, the programmer is probably defining the variable, interestRate, to give a meaningful name to the otherwise meaningless number, 0.05. It's easier to understand what's going on when a program says "principal += principal*interestRate;" rather than "principal += principal*0.05;".
In Java, the modifier "final" can be applied to a variable declaration to ensure that the value stored in the variable cannot be changed after the variable has been initialized. For example, if the member variable interestRate is declared with
public final static double interestRate = 0.05;
then it would be impossible for the value of interestRate to change anywhere else in the program. Any assignment statement that tries to assign a value to interestRate will be rejected by the computer as a syntax error when the program is compiled. (A "final" modifier on a public interest rate makes a lot of sense -- a bank might want to publish its interest rate, but it certainly wouldn't want to let random people make changes to it!)
It is legal to apply the final modifier to local variables and even to formal parameters, but it is most useful for member variables. I will often refer to a static member variable that is declared to be final as a named constant, since its value remains constant for the whole time that the program is running. The readability of a program can be greatly enhanced by using named constants to give meaningful names to important quantities in the program. A recommended style rule for named constants is to give them names that consist entirely of upper case letters, with underscore characters to separate words if necessary. For example, the preferred style for the interest rate constant would be
public final static double INTEREST_RATE = 0.05;
This is the style that is generally used in Java's standard classes, which define many named constants. For example, we have already seen that the Math class contains a variable Math.PI. This variable is declared in the Math class as a "public final static" variable of type double. Similarly, the Color class contains named constants such as Color.RED and Color.YELLOW which are public final static variables of type Color. Many named constants are created just to give meaningful names to be used as parameters in subroutine calls. For example, the standard class named Font contains named constants Font.PLAIN, Font.BOLD, and Font.ITALIC. These constants are used for specifying different styles of text when calling various subroutines in the Font class.
Enumerated type constants (see Subsection 2.3.3) are also examples of named constants. The enumerated type definition
enum Alignment { LEFT, RIGHT, CENTER }
defines the constants Alignment.LEFT, Alignment.RIGHT, and Alignment.CENTER. Technically, Alignment is a class, and the three constants are public final static members of that class. Defining the enumerated type is similar to defining three constants of type, say, int:
public static final int ALIGNMENT_LEFT = 0; public static final int ALIGNMNENT_RIGHT = 1; public static final int ALIGNMENT_CENTER = 2;
In fact, this is how things were generally done before the introduction of enumerated types, and it is what is done with the constants Font.PLAIN, Font.BOLD, and Font.ITALIC mentioned above. Using the integer constants, you could define a variable of type int and assign it the values ALIGNMENT_LEFT, ALIGNMENT_RIGHT, or ALIGNMENT_CENTER to represent different types of alignment. The only problem with this is that the computer has no way of knowing that you intend the value of the variable to represent an alignment, and it will not raise any objection if the value that is assigned to the variable is not one of the three valid alignment values. With the enumerated type, on the other hand, the only values that can be assigned to a variable of type Alignment are the constant values that are listed in the definition of the enumerated type. Any attempt to assign an invalid value to the variable is a syntax error which the computer will detect when the program is compiled. This extra safety is one of the major advantages of enumerated types.
Curiously enough, one of the major reasons to use named constants is that it's easy to change the value of a named constant. Of course, the value can't change while the program is running. But between runs of the program, it's easy to change the value in the source code and recompile the program. Consider the interest rate example. It's quite possible that the value of the interest rate is used many times throughout the program. Suppose that the bank changes the interest rate and the program has to be modified. If the literal number 0.05 were used throughout the program, the programmer would have to track down each place where the interest rate is used in the program and change the rate to the new value. (This is made even harder by the fact that the number 0.05 might occur in the program with other meanings besides the interest rate, as well as by the fact that someone might have, say, used 0.025 to represent half the interest rate.) On the other hand, if the named constant INTEREST_RATE is declared and used consistently throughout the program, then only the single line where the constant is initialized needs to be changed.
As an extended example, I will give a new version of the RandomMosaicWalk program from the previous section. This version uses named constants to represent the number of rows in the mosaic, the number of columns, and the size of each little square. The three constants are declared as final static member variables with the lines:
final static int ROWS = 20; // Number of rows in mosaic. final static int COLUMNS = 30; // Number of columns in mosaic. final static int SQUARE_SIZE = 15; // Size of each square in mosaic.
The rest of the program is carefully modified to use the named constants. For example, in the new version of the program, the Mosaic window is opened with the statement
Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE);
Sometimes, it's not easy to find all the places where a named constant needs to be used. If you don't use the named constant consistently, you've more or less defeated the purpose. It's always a good idea to run a program using several different values for any named constant, to test that it works properly in all cases.
Here is the complete new program, RandomMosaicWalk2, with all modifications from the previous version shown in red. I've left out some of the comments to save space.
public class RandomMosaicWalk2 {
final static int ROWS = 20; // Number of rows in mosaic.
final static int COLUMNS = 30; // Number of columns in mosaic.
final static int SQUARE_SIZE = 15; // Size of each square in mosaic.
static int currentRow; // Row currently containing the disturbance.
static int currentColumn; // Column currently containing the disturbance.
public static void main(String[] args) {
Mosaic.open( ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE );
fillWithRandomColors();
currentRow = ROWS / 2; // start at center of window
currentColumn = COLUMNS / 2;
while (Mosaic.isOpen()) {
changeToRandomColor(currentRow, currentColumn);
randomMove();
Mosaic.delay(1);
}
} // end main
static void fillWithRandomColors() {
for (int row=0; row < ROWS; row++) {
for (int column=0; column < COLUMNS; column++) {
changeToRandomColor(row, column);
}
}
} // end fillWithRandomColors
static void changeToRandomColor(int rowNum, int colNum) {
int red = (int)(256*Math.random()); // Choose random levels in range
int green = (int)(256*Math.random()); // 0 to 255 for red, green,
int blue = (int)(256*Math.random()); // and blue color components.
Mosaic.setColor(rowNum,colNum,red,green,blue);
} // end changeToRandomColor
static void randomMove() {
int directionNum; // Randomly set to 0, 1, 2, or 3 to choose direction.
directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0: // move up
currentRow--;
if (currentRow < 0)
currentRow = ROWS - 1;
break;
case 1: // move right
currentColumn++;
if (currentColumn >= COLUMNS)
currentColumn = 0;
break;
case 2: // move down
currentRow++;
if (currentRow >= ROWS)
currentRow = 0;
break;
case 3: // move left
currentColumn--;
if (currentColumn < 0)
currentColumn = COLUMNS - 1;
break;
}
} // end randomMove
} // end class RandomMosaicWalk2
4.7.3 Naming and Scope Rules
When a variable declaration is executed, memory is allocated for that variable. The variable name can be used in at least some part of the program source code to refer to that memory or to the data that is stored in the memory. The portion of the program source code where the variable is valid is called the scope of the variable. Similarly, we can refer to the scope of subroutine names and formal parameter names.
For static member subroutines, scope is straightforward. The scope of a static subroutine is the entire source code of the class in which it is defined. That is, it is possible to call the subroutine from any point in the class, including at a point in the source code before the point where the definition of the subroutine appears. It is even possible to call a subroutine from within itself. This is an example of something called "recursion," a fairly advanced topic that we will return to in Chapter 9. If the subroutine is not private, it can also be accessed from outside the class where it is defined, using its full name.
For a variable that is declared as a static member variable in a class, the situation is similar, but with one complication. It is legal to have a local variable or a formal parameter that has the same name as a member variable. In that case, within the scope of the local variable or parameter, the member variable is hidden. Consider, for example, a class named Game that has the form:
public class Game {
static int count; // member variable
static void playGame() {
int count; // local variable
.
. // Some statements to define playGame()
.
}
.
. // More variables and subroutines.
.
} // end Game
In the statements that make up the body of the playGame() subroutine, the name "count" refers to the local variable. In the rest of the Game class, "count" refers to the member variable (unless hidden by other local variables or parameters named count). However, the member variable named count can also be referred to by the full name Game.count. Usually, the full name is only used outside the class where count is defined. However, there is no rule against using it inside the class. The full name, Game.count, can be used inside the playGame() subroutine to refer to the member variable instead of the local variable. So, the full scope rule is that the scope of a static member variable includes the entire class in which it is defined, but where the simple name of the member variable is hidden by a local variable or formal parameter name, the member variable must be referred to by its full name of the form className.variableName. (Scope rules for non-static members are similar to those for static members, except that, as we shall see, non-static members cannot be used in static subroutines.)
The scope of a formal parameter of a subroutine is the block that makes up the body of the subroutine. The scope of a local variable extends from the declaration statement that defines the variable to the end of the block in which the declaration occurs. As noted above, it is possible to declare a loop control variable of a for loop in the for statement, as in "for (int i=0; i < 10; i++)". The scope of such a declaration is considered as a special case: It is valid only within the for statement and does not extend to the remainder of the block that contains the for statement.
It is not legal to redefine the name of a formal parameter or local variable within its scope, even in a nested block. For example, this is not allowed:
void badSub(int y) {
int x;
while (y > 0) {
int x; // ERROR: x is already defined.
.
.
.
}
}
In many languages, this would be legal; the declaration of x in the while loop would hide the original declaration. It is not legal in Java; however, once the block in which a variable is declared ends, its name does become available for reuse in Java. For example:
void goodSub(int y) {
while (y > 10) {
int x;
.
.
.
// The scope of x ends here.
}
while (y > 0) {
int x; // OK: Previous declaration of x has expired.
.
.
.
}
}
You might wonder whether local variable names can hide subroutine names. This can't happen, for a reason that might be surprising. There is no rule that variables and subroutines have to have different names. The computer can always tell whether a name refers to a variable or to a subroutine, because a subroutine name is always followed by a left parenthesis. It's perfectly legal to have a variable called count and a subroutine called count in the same class. (This is one reason why I often write subroutine names with parentheses, as when I talk about the main() routine. It's a good idea to think of the parentheses as part of the name.) Even more is true: It's legal to reuse class names to name variables and subroutines. The syntax rules of Java guarantee that the computer can always tell when a name is being used as a class name. A class name is a type, and so it can be used to declare variables and formal parameters and to specify the return type of a function. This means that you could legally have a class called Insanity in which you declare a function
static Insanity Insanity( Insanity Insanity ) { ... }
The first Insanity is the return type of the function. The second is the function name, the third is the type of the formal parameter, and the fourth is the name of the formal parameter. However, please remember that not everything that is possible is a good idea!