double type — real numbers

1. double type

Java provides the double type for working with real (fractional) numbers. It occupies 8 bytes in memory (twice as many as the int type) and can store values in the range from -1.7*10308 to +1.7*10308. For comparison: the int type can store a value in the range from -2*109 to +2*109.

In real numbers, the fractional part is written after a decimal point. For example, 123.456, or 2.5, or 100.00, or 0.01. When computers deal with such numbers, we call them floating point numbers.

By the way, in addition to the double type, we also have the float type (which is only 4 bytes in size). Its name comes from floating point. And the name double comes from double float. A double is twice as large as a float: 8 bytes versus 4. It is also called a double precision floating-point number.

2. Creating a double type

The double type is used to store real numbers. To create a variable in code that can store real numbers, you need to use a statement like this:

double name;

Where name is the name of the variable. Examples:

double price;

double weight;

double lightSpeed;

As with the int type, you can use shorthand to create multiple double variables simultaneously:

double name 1, name 2, name 3;

And even immediately assign values to them:

double name 1 = value 1, name 2 = value 2, name 3 = value 3;

Examples:

double price = 5.0;

double weight = 2;

double x = 1.0, y = 2.0, z = 3.0;

3. Assigning integers and real numbers

It would be bad if integers could be assigned only to int variables, and real numbers — only to double variables. We want to be able to convert between the two kinds of numbers. And Java provides this ability.

First, both real and integer numbers can be assigned to double variables. When assigning integers, they are simply converted to real numbers. Of course, some accuracy may be lost in the process.

double price = 5.0;

double weight = 2;

int t = 1000;
double x =  t * t;

Second, if an integer and a real number are involved in some expression, then the integer is first converted to a real number and only then is the operation with the other real number performed.

int t = 1000;
double x = t * 5.0;

System.out.println(5 * 2);

System.out.println(5 * 2.0);

And finally, it is also possible to assign real numbers to int variables. When we do this, the fractional part of the real number is discarded — the number is rounded down to the nearest whole number.

Additionally, the compiler requires the programmer to explicitly document what is happening (to be sure that other programmers understand that the fractional part is being dropped). In general, such a conversion looks like this in code:

integer_variable = (int)(real_number);

Examples:

int x = (int)(5.5);

double a = 5.999;
int x = (int)(a);

double a = 5.999;
int b = 2;
int x = (int)(a * b);

4. Dividing integers and real numbers in Java

When dividing an integer by an integer, the remainder is always discarded. How then can we divide 5 by 2 to get 2.5?

At first, it seems like the correct option is:

double d = 5 / 2;

But it's not so simple. The problem here is that the Java machine first calculates the value of 5 / 2 and only then assigns the result to the d variable. And the 5 / 2 operation is integer division. That means d will contain 2 or, to be more precise, 2.0

The correct solution is to write at least one of the numbers involved in the division as a real number (i.e. with a decimal point):

double d = 5.0 / 2;

double d = 5 / 2.0;

double d = 5.0 / 2.0;

In each of expressions, d will contain 2.5

But what if we are working with variables? What if we have code like this?:

int a = 5;
int b = 2;
double d = a / b;

There is a slick (and obvious) solution here — force the Java machine to convert variables to real numbers by multiplying them by one as a real number (1.0)

int a = 5;
int b = 2;
double d = a * 1.0 / b;

Note that multiplication and division have equal precedence, and are performed from left to right. That means that it matters where we multiply the 1.0.

Examples:

int a = 5;
int b = 2;
double d = 1.0 * a / b;

int a = 5;
int b = 2;
double d = a * 1.0 / b;

int a = 5;
int b = 2;
double d = a / b * 1.0;