Chapter05

Chapter 5: Numbers

5.1 Checking Whether a String Is a Valid Number

Problem

You need to check whether a given string contains a valid number, and, if so, convert it to binary (internal) form.

Solution

To accomplish this, use the appropriate wrapper class’s conversion routine and catch the NumberFormatException.

5.3 Taking a Fraction of an Integer Without Using Floating Point

Problem

You want to multiply an integer by a fraction without converting the fraction to a floating-point number.

Solution

Multiply the integer by the numerator and divide by the denominator.

This technique should be used only when efficiency is more important than clarity because it tends to detract from the readability—and therefore the maintainability—of your code.

You should beware of the possibility of numeric overflow and avoid this optimization if you cannot guarantee that the multiplication by the numerator will not overflow.

5.4 Working with Floating-Point Numbers

Problem

You want to be able to compare and round floating-point numbers.

Solution

Compare with the INFINITY constants, and use isNaN() to check for NaN (not a number).

Compare floating values with an epsilon value.

Round floating point values with Math.round() or custom code.

5.5 Formatting Numbers

Problem

You need to format numbers.

Solution

Use a NumberFormat subclass.

5.6 Converting Among Binary, Octal, Decimal, and Hexadecimal

Problem

You want to display an integer as a series of bits—for example, when interacting with certain hardware devices—or in some alternative number base (binary is base 2, octal is base 8, decimal is 10, hexadecimal is 16). You want to convert a binary number or a hexadecimal value into an integer.

Solution

The class java.lang.Integer provides the solutions. Most of the time you can use Integer.parseInt(String input, int radix) to convert from any type of number to an Integer, and Integer.toString(int input, int radix) to go the other way.

5.7 Operating on a Series of Integers

Problem

You need to work on a range of integers.

Solution

For a contiguous set, use IntStream::range and rangeClosed, or the older for loop.

For discontinuous ranges of numbers, use a java.util.BitSet.

5.8 Formatting with Correct Plurals

Problem

You’re printing something like "We used " + n + " items", but in English, “We used 1 items” is ungrammatical. You want *“We used 1 item”.

Solution

Use a ChoiceFormat or a conditional statement.

Use Java’s ternary operator (condition ? trueValue : falseValue) in a string concatenation. Both zero and plurals get an “s” appended to the noun in English (“no books, one book, two books”), so we test for n == 1.

i.e.:

Given

double[] limits = { 0, 1, 2 };
int[] data = { -1, 0, 1, 2, 3 };

ChoiceFormat can be used to get a plurized format:

String[] formats = { "reviews", "review", "reviews"};
ChoiceFormat pluralizedFormat = new ChoiceFormat(limits, formats);

for (int i : data) {
    System.out.println("Found " + i + " " + pluralizedFormat.format(i));  // Found -1 reviews
                                                                          // Found 0 reviews
                                                                          // Found 1 review
                                                                          // Found 2 reviews
                                                                          // Found 3 reviews
}

and to get a quantized format:

ChoiceFormat quantizedFormat = new ChoiceFormat("0#no reviews|1#one review|1<many reviews");

for (int i : data) {
    System.out.println("Found " + quantizedFormat.format(i));             // Found no reviews
                                                                          // Found no reviews
                                                                          // Found one review
                                                                          // Found many reviews
                                                                          // Found many reviews
}

In addition to ChoiceFormat, the same result can be achieved with a MessageFormat.

5.9 Generating Random Numbers

Problem

You need to generate pseudorandom numbers in a hurry.

Solution

Use java.lang.Math.random() to generate random numbers. There is no claim that the random values it returns are very good random numbers, however. Like most software-only implementations, these are Pseudorandom Number Generators (PRNGs), meaning that the numbers are not totally random, but devised from an algorithm. That said, they are adequate for casual use.

This method only generates double values. If you need integers, construct a java.util.Random (or ThreadLocalRandom or SecureRandom) object and call its nextInt() method; if you pass it an integer value, this will become the upper bound.

5.12 Handling Very Large Numbers

Problem

You need to handle integer numbers larger than Long.MAX_VALUE or floating-point values larger than Double.MAX_VALUE.

Solution

Use the BigInteger or BigDecimal values in package java.math.