double

A double is a Java data type used to store decimal (floating-point) numbers.

double count = 20.1;
double temperature = -9.5;

Memory size and range

A double uses 64 bits (8 bytes) of memory. It can store very small and very large numbers:

• Smallest positive value: 4.9E-324
• Largest value: 1.7976931348623157E308

E means “× 10 to the power of”. Example: 1.5E2 = 150

double vs float

Java has two decimal number types:

• float (32 bits)
• double (64 bits)

double is more precise and is the default choice when working with decimal numbers.

double price = 19.99;   // preferred
float tax = 0.2f;       // less precise

Primitive Type

A double is a primitive data type which means:

• It stores the actual value directly in memory
• It is not an object
• It has no methods of its own

Primitive types do not change internally. When you assign a new value, the old value is replaced.

double a = 15.2;
a = 20.7;  // 15.2 is replaced with 20.7

There is no memory reference to update — just a new value stored.

Double Wrapper Class

Although double has no methods, Java provides a wrapper class called Double. This allows you to:

• Convert values
• Compare values
• Access useful constants

double x = 3.14;
String text = Double.toString(x); // Value of text is "3.14"

Note: These methods must be applied to the Double wrapper class. Applying them to a double primitive will generate an error

Double Constants

Converting values

Converting a numeric String to a Double. A non-numeric String generates an error.

double num = Double.parseDouble("123.5"); // Returns double 123.5
Double.parseDouble("hello"); // Returns NumberFormatException

Converting a Double to a String

String text = Double.toString(42.0); // Returns "42.0"

Comparing double values

compare

Double.compare(1.5, 2.5);

compareTo

Double x = 10.0;
Double y = 12.2;

x.compareTo(y);

In both cases the return values are:

• 1 → if first value is greater
• 0 → if both values are equal
• -1 → if first value is smaller

equals

x.equals(y); // Returns true or false

Math helper methods

Double.max(1.5, 2.5);  // Returns greater figure - 2.5
Double.min(1.5, 2.5);  // Returns lesser figure - 1.5
Double.sum(1.5, 2.5);  // Sums the two doubles - 4.0

Summary

• double stores decimal numbers
• double uses 64 bits / 8 bytes
• double is more precise than float
• Is a primitive type
• It uses the Double wrapper class to access methods

Further Reading

• Java Double class
• Java primitives