1. Rounding Numbers

    In Chapter 2 and Appendix B, you learned about the imprecision of floating-point numbers. For example, if you write a program that subtracts 2.00 from 2.20, the result is not 0.20—it is 0.20000000000000018. To eliminate odd-looking output and nonintuitive comparisons caused by imprecise calculations in floating-point numbers, you can take the approach shown in the class in Figure: • Multiply the value by 100. So, for example: ° 0.416 would become 41.6. ° 0.20000000000000018 becomes 20.000000000000018. • Add 0.5. This increases a value’s whole number part by 1 if the fractional part is 0.5 or greater. For example: ° 41.6 would become 42.1. ° 20.000000000000018 becomes 20.500000000000018. • Cast the value to an integer: ° 42.1 would become 42. ° 20.500000000000018 becomes 20. • Divide by 100: ° If the original number was 0.416, it is now 0.42. ° If the original number was 0.20000000000000018, it becomes 0.20.
    public class RoundingDemo1 { public static void main(String[] args) { double answer = 2.20 - 2.00; boolean isEqual; isEqual = answer == 0.20; System.out.println("Before conversion"); System.out.println("answer is " + answer); System.out.println("isEqual is " + isEqual); answer = answer * 100; // Together, these four statements round the number. answer = answer + 0.5; answer = (int) answer; answer = answer / 100; isEqual = answer == 0.20; System.out.println("After conversion"); System.out.println("answer is " + answer); System.out.println("isEqual is " + isEqual); } }
    As an alternative, you can use the round() method that is supplied with Java’s Math class. The round() method returns the nearest long value. Next Figure shows a program that multiplies the double answer by 100, rounds it, and then divides by 100.0.
    public class RoundingDemo2 { public static void main(String[] args) { double answer = 2.20 - 2.00; boolean isEqual; isEqual = answer == 0.20; System.out.println("Before conversion"); System.out.println("answer is " + answer); System.out.println("isEqual is " + isEqual); answer = answer * 100; // These three statements produce the rounded number long roundedAnswer = Math.round(answer); answer = roundedAnswer / 100.0; isEqual = answer == 0.20; System.out.println("After conversion"); System.out.println("answer is " + answer); System.out.println("isEqual is " + isEqual); } }
  2. Using the printf() Method
    When you display numbers using the println() method in Java applications, it sometimes is difficult to make numeric values appear as you want. For example, in the output in previous Figure, the difference between 2.20 and 2.00 is displayed as 0.2. By default, Java eliminates trailing zeros when floating-point numbers are displayed because they do not add any mathematical information. You might prefer to see 0.20 because both original numbers were expressed to two decimal places, or if the values represent currency. Additionally, you frequently want to align columns of numeric values. For example, Figure shows a NumberList application that contains an array of floating-point values. The application displays the values using a for loop, but because the println() method displays values as Strings, the displayed values are left-aligned, just as series of words would be.
    public class NumberList { public static void main(String[] args) { double[] list = {0.20, 2. 00, 2.20, 22.22, 22.20, 222.00, 222.22}; int x; for(x = 0; x < list.length; ++x) System.out.println(list[x]); } }
    When creating numeric output, you can specify a number of decimal places to display by using the printf() method with two types of arguments that represent the following: • A format string • A list of arguments A format string is a string of characters; it includes optional text (that is displayed literally) and one or more format specifiers. A format specifier is a placeholder for a numeric value. Within a call to printf(), you include one argument (either a variable or a constant) for each format specifier. The format specifiers for general, character, and numeric types contain the following elements, in order: • A percent sign (%). The percent sign starts every format specifier. • An optional argument index followed by a dollar sign ($). The argument index is an integer that represents the position of the argument within the argument list. You will learn more about this option later in this appendix. • Optional flags that modify the output format. The set of valid flags depends on the data type being formatted. You can find more details about this feature at the Java website. • An optional field width. This integer indicates the minimum number of characters to be written to the output. You will learn more about this option later in this appendix. • An optional precision factor. The precision factor consists of a decimal point followed by a number. It typically is used to control the number of decimal places displayed. You will learn more about this option in the next section. • The required conversion character. This character indicates how its corresponding argument should be formatted. Java supports a variety of conversion characters, but the three you want to use most frequently are d, f, and s—the characters that represent decimal (base 10 integer) values, floating-point (float and double) values, and string values, respectively. Other conversion characters include those used to display hexadecimal numbers and scientific notation. If you need these display formats, you can find more details at the Java website. For example, you can use the ConversionCharacterExamples class in previous Figure to display a declared integer and double. The main() method of the class contains three printf() statements. The three calls to printf() in this class each contain a format string; the first two calls contain a single additional argument, and the last printf() statement contains two arguments after the string. None of the format specifiers in this class uses any of the optional parameters—only the required percent sign and conversion character. The first printf() statement uses %d in its format string as a placeholder for the integer argument at the end. The second printf() statement uses %f as a placeholder for the floating-point argument at the end. The last printf() statement uses both a %d and %f to indicate the positions of the integer and floating-point values at the end, respectively.
    public class ConversionCharacterExamples { public static void main(String[] args) { int age = 23; double money = 123.45; System.out.printf("Age is %d\n", age); System.out.printf("Money is $%f\n", money); System.out.printf ("Age is %d and money is $%f\n", age, money); } }
    Specifying a Number of Decimal Places to Display with printf()
    You can control the number of decimal places displayed when you use a floating-point value in a printf() statement by adding the optional precision factor to the format specifier. Between the percent sign and the conversion character, you can add a decimal point and the number of decimal positions to display. For example, the following statements produce the output Money is $123.45, displaying the money value with just two decimal places instead of six, which would occur without the precision factor: double money = 123.45; System.out.printf("Money is $%.2f\n", money); Similarly, the following statements display 8.10. If you use the println() equivalent with amount, only 8.1 is displayed. If you use the printf() statement without inserting the .2 precision factor, 8.100000 is displayed. double amount = 8.1; System.out.printf("%.2f", amount); When you use a precision factor on a value that contains more decimal positions than you want to display, the result is rounded. For example, the following statements produce 100.457 (not 100.456), displaying three decimals because of the precision factor. double value = 100.45678; System.out.printf("%.3f", value); You cannot use the precision factor with an integer value; if you do, your program will throw an IllegalFormatConversionException.
    Specifying a Field Size with printf()
    public class NumberList2 { public static void main(String[] args) { double[] list = {0.20, 2.00, 2.20, 22.22, 22.20, 222.00, 222.22}; int x; for(x = 0; x < list.length; ++x) System.out.printf("%6.2f\n", list[x]); } }
    Throughout this book, you have been encouraged to use named constants for numeric values instead of literal constants, so that your programs are clearer. In the program in Figure C-8 you could define constants such as: final int DISPLAY_WIDTH = 6; final int DISPLAY_DECIMALS = 2; Then the printf() statement would be: System.out.printf("%" + DISPLAY_WIDTH + "." + DISPLAY_DECIMALS + "f\n", list[x]); Another, perhaps clearer alternative is to define a format string such as the following: final String FORMAT = "%6.2f\n"; Then the printf() statement would be: System.out.printf(FORMAT, list[x]); You can specify that a value be left-aligned in a field instead of right-aligned by inserting a negative sign in front of the width. Although you can do this with numbers, most often you choose to left-align strings. For example, the following code displays five spaces followed by hello and then five spaces followed by there. Each string is left-aligned in a field with a size of 10. String string1 = "hello"; String string2 = "there"; System.out.printf("%-10s%-10s", string1, string2);
    Using the Optional Argument Index with printf()
    The argument index is an integer that indicates the position of an argument in the argument list of a printf() statement. To separate it from other formatting options, the argument index is followed by a dollar sign ($). The first argument is referenced by "1$", the second by "2$", and so on. For example, the printf() statement in the following code contains four format specifiers but only two variables in the argument list: int x = 56; double y = 78.9; System.out.printf("%1$6d%2$6.2f%1$6d%2$6.2f", x, y); The printf() statement displays the value of the first argument, x, in a field with a size of 6, and then it displays the second argument, y, in a field with a size of 6 with two decimal places. Then, the value of x is displayed again, followed by the value of y. The output appears as follows: 56 78.90 56 78.90
  3. Using the DecimalFormat Class
    The DecimalFormat class provides ways to easily convert numbers into strings, allowing you to control the display of leading and trailing zeros, prefixes and suffixes, grouping (thousands) separators, and the decimal separator. You specify the formatting properties of DecimalFormat with a pattern String. The pattern String is composed of symbols that determine what the formatted number looks like; it is passed to the DecimalFormat class constructor. The symbols you can use in a pattern String include: • A pound sign ( # ), which represents a digit • A period ( . ), which represents a decimal point • A comma ( , ), which represents a thousands separator • A zero ( 0 ), which represents leading and trailing zeros when it replaces the pound sign For example, the following lines of code result in value being displayed as 12,345,678.90. double value = 12345678.9; DecimalFormat aFormat = new DecimalFormat("#,###,###,###.00"); System.out.printf("%s\n", aFormat.format(value)); A DecimalFormat object is created using the pattern #,###,###,###.00. When the object’s format() method is used in the printf() statement, the first two pound signs and the comma between them are not used because value is not large enough to require those positions. The value is displayed with commas inserted where needed, and the decimal portion is displayed with a trailing 0 because the 0s at the end of the pattern indicate that they should be used to fill out the number to two places. When you use the DecimalFormat class, you must use the following import statement: import java.text.*; Figure shows a class that creates a String pattern that it passes to the DecimalFormat constructor to create a moneyFormat object. The class displays an array of values, each in a field that is 10 characters wide. Some of the values require commas, and some do not.
    import java.text.*; public class DecimalFormatTest { public static void main(String[]args) { String pattern = "###,###.00"; DecimalFormat moneyFormat = new DecimalFormat(pattern); double[] list = {1.1, 23.23, 456.249, 7890.1, 987.5678, 65.0}; int x; for(x = 0; x < list.length; ++x) System.out.printf("%10s\n", moneyFormat.format(list[x])); } }