Sometimes, a variable should only hold a restricted set of values. For example, you may sell clothes or pizza in four sizes: small, medium, large, and extra large. Of course, you could encode these sizes as integers 1, 2, 3, 4 or characters S, M, L, and X. But that is an error-prone setup. It is too easy for a variable to hold a wrong value (such as 0 or m). You can define your own enumerated type whenever such a situation arises. An enumerated type has a finite number of named values. For example,
enum Size { SMALL, MEDIUM, LARGE, EXTRA_LARGE };Now you can declare variables of this type:
Size s = Size.MEDIUM;The Math class contains an assortment of mathematical functions that you may occasionally need, depending on the kind of programming that you do. To take the square root of a number, use the sqrt method:
double x = 4;
double y = Math.sqrt(x);
System.out.println(y); // prints 2.0Numeric conversions are possible in Java, but of course information may be lost. Conversions in which loss of information is possible are done by means of casts. The syntax for casting is to give the target type in parentheses, followed by the variable name. For example:
double x = 9.997;
int nx = (int) x;Now, the variable nx has the value 9 because casting a floating-point value to an integer discards the fractional part. If you want to round a floating-point number to the nearest integer (which in most cases is a more useful operation), use the Math.round method:
double x = 9.997;
int nx = (int) Math.round(x);Now the variable nx has the value 10. You still need to use the cast (int) when you call round.
Conceptually, Java strings are sequences of Unicode characters. For example, the string "Java\u2122" consists of the five Unicode characters J, a, v, a, and ™. Java does not have a built-in string type. Instead, the standard Java library contains a predefined class called, naturally enough, String. Each quoted string is an instance of the String class:
String e = ""; // an empty string
String greeting = "Hello";- Substrings
You can extract a substring from a larger string with the substring method of the String class. For example,
String greeting = "Hello";
String s = greeting.substring(0, 3);creates a string consisting of the characters "Hel".
- Empty and Null Strings
The empty string "" is a string of length 0. You can test whether a string is empty by calling
if (str.length() == 0)or
if (str.equals(""))The if/else construct can be cumbersome when you have to deal with multiple selections with many alternatives. Java has a switch statement that is exactly like the switch statement in C and C++, warts and all. For example, if you set up a menu system with four alternatives like that in Figure 3.13, you could use code that looks like this:
Scanner in = new Scanner(System.in);
System.out.print("Select an option (1, 2, 3, 4) ");
int choice = in.nextInt();
switch (choice)
{
case 1:
. . .
break;
case 2:
. . .
break;
case 3:
. . .
break;
case 4:
. . .
break;
default:
// bad input
. . .
break;
}You can do the initialization as follows:
balances = new double[NYEARS][NRATES];In other cases, if you know the array elements, you can use a shorthand notation for initializing a multidimensional array without a call to new. For example:
int[][] magicSquare =
{
{16, 3, 2, 13},
{5, 10, 11, 8},
{9, 6, 7, 12},
{4, 15, 14, 1}
};Once the array is initialized, you can access individual elements by supplying
two pairs of brackets—for example, balances[i][j].
The example program stores a one-dimensional array interest of interest rates
and a two-dimensional array balances of account balances, one for each year
and interest rate. We initialize the first row of the array with the initial
balance:
for (int j = 0; j < balances[0].length; j++)
balances[0][j] = 10000;Then we compute the other rows, as follows:
for (int i = 1; i < balances.length; i++)
{
for (int j = 0; j < balances[i].length; j++)
{
double oldBalance = balances[i - 1][j];
double interest = . . .;
balances[i][j] = oldBalance + interest;
}
}