Java - RandomAccessFile writeDouble(double v) method

Description

The Java RandomAccessFile writeDouble(double v) method converts the double argument to a long using the doubleToLongBits method in class Double, and then writes that long value to the file as an eight-byte quantity, high byte first. The write starts at the current position of the file pointer.

Declaration

Following is the declaration for java.io.RandomAccessFile.writeDouble(double v) method.

public final void writeDouble(double v)

Parameters

v − a double value to be written.

Return Value

This method does not return a value.

Exception

• IOException − If an I/O error occurs.

Example - Usage of RandomAccessFile writeDouble(double v) method

The following example shows the usage of RandomAccessFile writeDouble(double v) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.RandomAccessFile;
import java.io.IOException;

public class RandomAccessFileDemo {
   public static void main(String[] args) {
   
      try {
         double d = 1847.4986;

         // create a new RandomAccessFile with filename test
         RandomAccessFile raf = new RandomAccessFile("test.txt", "rw");

         // write a double in the file
         raf.writeDouble(d);

         // set the file pointer at 0 position
         raf.seek(0);

         // read double
         System.out.println(raf.readDouble());

         // set the file pointer at 0 position
         raf.seek(0);

         // write a double at the start
         raf.writeDouble(473.5645);

         // set the file pointer at 0 position
         raf.seek(0);

         // read double
         System.out.println(raf.readDouble());
         
      } catch (IOException ex) {
         ex.printStackTrace();
      }
   }
}

Output

Assuming we have a text file test.txt in current directory which has the following content. This file will be used as an input for our example program −

ABCDE

Let us compile and run the above program, this will produce the following result −

1847.4986
473.5645

Example - Write a Single Double to a File

The following example shows the usage of RandomAccessFile writeDouble(double v) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.RandomAccessFile;
import java.io.IOException;

public class RandomAccessFileDemo {
   public static void main(String[] args) {
      try (RandomAccessFile raf = new RandomAccessFile("double1.dat", "rw")) {
         raf.writeDouble(123.456);
         System.out.println("Double written successfully.");
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Double written successfully.

Explanation

• 123.456 is written as an 8-byte double.

• File double1.dat will contain 8 bytes representing the binary form of 123.456.

• You can later read it back using readDouble().

Example - Write Multiple Doubles and Read Them Back

The following example shows the usage of RandomAccessFile writeDouble(double v) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.RandomAccessFile;
import java.io.IOException;

public class RandomAccessFileDemo {
   public static void main(String[] args) {
      try (RandomAccessFile raf = new RandomAccessFile("double2.dat", "rw")) {
         // Write multiple doubles
         raf.writeDouble(3.14159);
         raf.writeDouble(2.71828);
         raf.writeDouble(1.61803);

         // Move pointer to beginning
         raf.seek(0);

         // Read back values
         double d1 = raf.readDouble();
         double d2 = raf.readDouble();
         double d3 = raf.readDouble();

         System.out.printf("Read values: %.5f, %.5f, %.5f%n", d1, d2, d3);
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Read values: 3.14159, 2.71828, 1.61803

Explanation

• Writes three double values: 3.14159, 2.71828 and 1.61803.

• Each takes 8 bytes → total file size = 24 bytes.

• seek(0) resets the file pointer before reading.

• Values are read back exactly as written.