Array Elements as Actual Parameters
Array elements, like other variables, can store values of primitive data types or reference values of objects. In the latter case, they can also be arrays—that is, arrays of arrays (p. 124). If an array element is of a primitive data type, its data value is passed; if it is a reference to an object, the reference value is passed. The method invocation conversions apply to the values of array elements as well.
Example 3.13 Array Elements as Primitive Data Values
public class FindMinimum {
public static void main(String[] args) {
int[] dataSeq = {6,4,8,2,1};
int minValue = dataSeq[0];
for (int index = 1; index < dataSeq.length; ++index)
minValue = minimum(minValue, dataSeq[index]); // (1)
System.out.println(“Minimum value: ” + minValue);
}
public static int minimum(int i, int j) { // (2)
return (i <= j) ? i : j;
}
}
Output from the program:
Minimum value: 1
In Example 3.13, the value of all but one element of the array dataSeq is retrieved and passed consecutively at (1) to the formal parameter j of the minimum() method defined at (2). The discussion on passing primitive values (p. 129) also applies to array elements that have primitive values.
In Example 3.14, the formal parameter seq of the findMinimum() method defined at (4) is an array variable. The variable matrix denotes an array of arrays declared at (1) simulating a multidimensional array that has three rows, where each row is a simple array. The first row, denoted by matrix[0], is passed to the findMinimum() method in the call at (2). Each remaining row is passed by its reference value in the call to the findMinimum() method at (3).
Example 3.14 Array Elements as Reference Values
public class FindMinimumMxN {
public static void main(String[] args) {
int[][] matrix = { {8,4},{6,3,2},{7} }; // (1)
int min = findMinimum(matrix[0]); // (2)
for (int i = 1; i < matrix.length; ++i) {
int minInRow = findMinimum(matrix[i]); // (3)
min = Math.min(min, minInRow);
}
System.out.println(“Minimum value in matrix: ” + min);
}
public static int findMinimum(int[] seq) { // (4)
int min = seq[0];
for (int i = 1; i < seq.length; ++i)
min = Math.min(min, seq[i]);
return min;
}
}
Output from the program:
Minimum value in matrix: 2
final Parameters
A formal parameter can be declared with the keyword final preceding the parameter declaration in the method declaration. A final parameter is also known as a blank final variable; that is, it is blank (uninitialized) until a value is assigned to it, (e.g., at method invocation) and then the value in the variable cannot be changed during the lifetime of the variable (see also the discussion in §6.6, p. 352). The compiler can treat final variables as constants for code optimization purposes. Declaring parameters as final prevents their values from being changed inadvertently. A formal parameter’s declaration as final does not affect the caller’s code.
The declaration of the method calcPrice() from Example 3.10 is shown next, with the formal parameter pizzaPrice declared as final:
public double calcPrice(int numberOfPizzas, final double pizzaPrice) { // (2′)
pizzaPrice = pizzaPrice/2.0; // (3) Not allowed. Compile-time error!
return numberOfPizzas * pizzaPrice;
}
If this declaration of the calcPrice() method is compiled, the compiler will not allow the value of the final parameter pizzaPrice to be changed at (3) in the body of the method.
As another example, the declaration of the method bake() from Example 3.11 is shown here, with the formal parameter pizzaToBeBaked declared as final:
public static void bake(final Pizza pizzaToBeBaked) { // (3)
pizzaToBeBaked.meat = “chicken”; // (3a) Allowed
pizzaToBeBaked = null; // (4) Not allowed. Compile-time error!
}
If this declaration of the bake() method is compiled, the compiler will not allow the reference value of the final parameter pizzaToBeBaked to be changed at (4) in the body of the method. Note that this applies to the reference value in the final parameter, but not to the object denoted by this parameter. The state of the object can be changed as before, as shown at (3a).
For use of the final keyword in other contexts, see §5.5, p. 225.