double Keyword Usage and Examples

Learn how the Java double keyword is used to declare double-precision floating-point variables. Understand syntax, examples, memory usage, and precision tips in Java programming.

This reference explains double Keyword Usage and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

Use this reference
Identify the documented behavior, read a complete example and check its inputs before applying it.

double Keyword in Java

The double keyword in Java is used to declare a variable that can hold a double-precision 64-bit floating-point number. It is a primitive data type and part of the foundational set of types in the Java language.

Think of double as the go-to data type when you need more accuracy and range for decimal numbers than what float provides. It's often used for scientific calculations, measurements, and values that require a higher level of precision.

Syntax of the double Keyword

double variableName = value;

You can assign a literal value directly, or store the result of an expression.

Examples of Using double in Java

1. Declaring and Printing a Double Value

public class Main {
  public static void main(String[] args) {
    double pi = 3.14159;
    System.out.println("Value of pi: " + pi);
  }
}

Value of pi: 3.14159

Here, the variable pi is declared using the double keyword. It can precisely represent decimal values like 3.14159.

2. Performing Arithmetic with Doubles

public class Main {
  public static void main(String[] args) {
    double price = 99.99;
    double taxRate = 0.08;
    double totalPrice = price + (price * taxRate);
    System.out.println("Total Price: " + totalPrice);
  }
}

Total Price: 107.9892

The double type handles calculations involving decimals and gives results with more precision than float.

3. Implicit Type Promotion in Expressions

public class Main {
  public static void main(String[] args) {
    int items = 4;
    double pricePerItem = 12.75;
    double total = items * pricePerItem;
    System.out.println("Total cost: " + total);
  }
}

Total cost: 51.0

Even though items is an integer, Java automatically promotes it to double for the multiplication, maintaining floating-point precision.

Why Use double Instead of float ?

Precision: double is more precise (15–16 digits vs. 6–7 for float).
Default choice: In Java, floating-point literals are treated as double by default.
Scientific needs: When working with real-world measurements or scientific calculations, double is the preferred choice.

Memory Consumption of double

A double occupies 8 bytes (64 bits) of memory. This is double the size of a float which uses 4 bytes. The extra space is worth it when accuracy matters.

Limitations of double

Not suitable for financial calculations where exact precision is needed (use BigDecimal instead).
Floating-point arithmetic can introduce small rounding errors.

Default Values of double

When used as an instance or class variable (i.e., not local), the default value of a double is 0.0.

Example:

public class Example {
  static double rate;
  public static void main(String[] args) {
    System.out.println("Default rate: " + rate);
  }
}

Default rate: 0.0

Using this reference

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Supporting documentation: Official Java learning guide.

Reference checks are optional and do not count toward course progress.

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double Keyword Usage and Examples