Floating-point types

A decimal numeral refers generally to the notation of a number in the decimal system, which contains a separator (for example 0.01 or 3.1415). Such numbers represent fractions which are often used in science, statistics, engineering, and many other fields.

Java has two basic types to represent decimal numbers: float and double. They are called floating-point types. In fact, these types cannot represent an arbitrary decimal number, because they support only a limited number of significant decimal digits (6-7 for float, and 14-16 for double). In addition, double can represent a wider range of numbers than float.

In practice, programmers mostly use the double type and we recommend you to use it for our code challenges. But all the information below is valid for the float type as well.

Declaring variables and assigning values

In program, the double literal looks like 5.2 where the dot character separates integer and fractional parts of a number.

There are several initialized double variable below:

double zero = 0.0;
double one = 1.0;
double negNumber = -1.75;
double pi = 3.1415;

System.out.println(pi); // 3.1415 or 3,1415 depending on computer settings

It is also possible to assign an integer value to a double variable.

double one = 1;
System.out.println(one); // 1.0 or 1,0

If you want to declare and initialize a float variable, you should mark the assigned value with the special letter f (float literal).

float pi = 3.1415f;
float negValue = -0.15f;
System.out.println(pi); // 3.1415 or 3,1415 without f

Both types can store the only a limited number of significant decimal digits:

float f = 0.888888888888888888f; // a value with a lot of decimal digits
System.out.println(f);           // it only prints 0.8888889

Floating-point types have one interesting way to mark values using a mantissa:

double eps = 5e-3; // means 5 * 10^(-3) = 0.005
double n = 0.01e2; // means 0.01 * 10^2 = 1.0

Arithmetic operations

You can perform all types of arithmetic operations with floating-point types.

double one = 1.0;
double number = one + 1.5; // 2.5

double a = 1.75;
double b = 5.0;
double c = b - a; // 3.25

double pi = 3.1415;
double squaredPi = pi * pi; // 9.86902225

For double and float operands, the operator / performs a division, not an integer division.

System.out.println(squaredPi / 2); // 4.934511125

Pay attention to the important thing that beginners often do not understand.

double d1 = 5 / 4; // 1.0
double d2 = 5.0 / 4; // 1.25

In the first case, we perform integer division that produces an integer result and then assigns the result to d1. In the second case, we perform a real division that produces double value, and then assign the value to d2.

Errors during computations

Be careful – operations with floating-point numbers can produce an inaccurate result:

System.out.println(3.3 / 3); // prints 1.0999999999999999

Errors can accumulate during computation. In the following example we calculate the sum of ten decimals 0.1:

double d = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1;
System.out.println(d); // it prints 0.9999999999999999

In the following lessons, we will find out how to deal with this issue. For now, just take it into consideration.

Reading floating-point numbers

You can use Scanner to read values of both these types from the standard input.

Scanner scanner = Scanner(System.in);

float f = scanner.nextFloat();
double d = scanner.nextDouble();

We recommend you to use double to solve our programming problems.

As an example, consider a program that calculates the area of a triangle. To find it, the program reads the base and the height from the standard input, then multiplies them, and divide by 2. Note that the base and the height are perpendicular to each other.

import java.util.Scanner;

public class AreaOfTriangle {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        double base = scanner.nextDouble();
        double height = scanner.nextDouble();

        double area = (base * height) / 2;

        System.out.println(area);
    }
}

Let's calculate the area of a triangle with a base of 3.3 meters and a height of 4.5 meters.

Input 1:

3.3 4.5

Output 1:

7.425

As you can see, It has an area of 7.425 square meters!

Keep in mind that the output of this program may have a lot of zeroes like the output below because an operation with floating-point numbers can produce inaccurate results.

Input 2:

2.2 4.01

Output 2:

4.4110000000000005

The decimal separator

If you solve practice lessons locally or trying to repeat our examples, you may encounter a problem that your computer may have different locale settings. In this case, the Scanner cannot read floating-point numbers with the dot character (3.1415). Try to read numbers written with the comma separator (3,1415).

If you want to use the dot character without modifying your local settings, try using the following code to create a scanner:

Scanner scanner = new Scanner(System.in).useLocale(Locale.US);

You do not need to change something when submitting your solutions in our forms.