Java - RandomAccessFile readDouble() method

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Description

The Java RandomAccessFile readDouble() method reads a double from this file. This method reads a long value, starting at the current file pointer, as if by the readLong method and then converts that long to a double using the longBitsToDouble method in class Double.

Declaration

Following is the declaration for java.io.RandomAccessFile.readDouble() method.

public final double readDouble()

Parameters

NA

Return Value

This method returns the next eight bytes of this file, interpreted as a double.

Exception

• IOException − If an I/O error occurs.Not thrown if end-of-file has been reached.

• EOFException − If this file reaches the end before reading eight bytes.

Example - Usage of RandomAccessFile readDouble() method

The following example shows the usage of RandomAccessFile readDouble() 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 = 1.5987475;
         
         // create a new RandomAccessFile with filename test
         RandomAccessFile raf = new RandomAccessFile("test.txt", "rw");

         // write something in the file
         raf.writeDouble(765.497634);

         // 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(d);

         // 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 −

765.497634
1.5987475

Example - Writing and Reading Double Values

The following example shows the usage of RandomAccessFile readDouble() 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("doubles.dat", "rw");

         // Write two double values (each takes 8 bytes)
         raf.writeDouble(3.14);
         raf.writeDouble(42.0);

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

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

         System.out.println("First double: " + d1); // 3.14
         System.out.println("Second double: " + d2); // 42.0

         raf.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

First double: 3.14
Second double: 42.0

Explanation

• writeDouble(double d) writes 8 bytes (64 bits) to store the double.

• readDouble() reads 8 bytes and returns a double value.

• You must read values in the same order and type as they were written.

Example - Random Access to Specific Double Value

The following example shows the usage of RandomAccessFile readDouble() 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("doubles2.dat", "rw");

         // Write 3 doubles
         raf.writeDouble(10.5);  // Position 0
         raf.writeDouble(20.75); // Position 8
         raf.writeDouble(30.25); // Position 16

         // Jump to the third double (each double = 8 bytes)
         raf.seek(16);
         double third = raf.readDouble();

         System.out.println("Third double: " + third); // 30.25

         raf.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Third double: 30.25

Explanation

• Each double occupies 8 bytes.

• To read the third double, we seek(16) to skip the first two (2 x 8).

• readDouble() fetches the value at that position.

• This is useful in binary files where fixed-size records are stored and you want to access specific data efficiently.