Buffer arrayOffset() method in Java with Examples

The arrayOffset() method of java.nio.Buffer class is used to return the offset within the given buffer’s backing array of the first element of the buffer.

If this buffer is backed by an array then buffer position p corresponds to array index p + arrayOffset().

Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

Syntax:

public abstract int arrayOffset()

Return Value: This method returns the offset within this buffer’s array of the first element of the buffer

Exception:: This method throws the ReadOnlyBufferException, If this buffer is backed by an array but is read-only.

Below are the examples to illustrate the arrayOffset() method:

Example 1:

// Java program to demonstrate
// arrayOffset() method

import java.nio.*;
import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // Declaring the capacity of the ByteBuffer
        int capacity = 4;

        // Creating the ByteBuffer
        try {

            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.allocate(capacity);

            // putting the int to byte
            // typecast value in ByteBuffer
            bb.put((byte)20);
            bb.put((byte)30);
            bb.put((byte)40);
            bb.put((byte)50);

            // Typecasting ByteBuffer into Buffer
            Buffer buffer = (Buffer)bb;

            // offset within this buffer's array
            // of the first element of the buffer
            // using arrayOffset() method
            int offset = buffer.arrayOffset();

            // print the array
            System.out.println("arrayOffset is : "
                               + offset);
        }

        catch (ReadOnlyBufferException e) {

            System.out.println("buffer is backed by an "
                               + "array but is read-only");
            System.out.println("Exception throws: " + e);
        }
    }
}

Output:

arrayOffset is : 0

Example 2: For ReadOnlyBufferException

// Java program to demonstrate
// arrayOffset() method

import java.nio.*;
import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // Declaring the capacity of the ByteBuffer
        int capacity = 4;

        // Creating the ByteBuffer
        try {

            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.allocate(capacity);

            // putting the int to byte typecast
            // value in ByteBuffer
            bb.put((byte)20);
            bb.put((byte)30);
            bb.put((byte)40);
            bb.put((byte)50);

            // Creating a read-only copy of  ByteBuffer
            // using asReadOnlyBuffer() method
            ByteBuffer bb1 = bb.asReadOnlyBuffer();

            // Typecasting read-only ByteBuffer
            // into read-only Buffer
            Buffer buffer = (Buffer)bb1;

            // offset within this buffer's array
            // of the first element of the buffer
            // using arrayOffset() method
            int offset = buffer.arrayOffset();

            // print the array
            System.out.println("arrayOffset is : "
                               + offset);
        }

        catch (ReadOnlyBufferException e) {

            System.out.println("buffer is backed by "
                               + "an array but is read-only");
            System.out.println("Exception throws: " + e);
        }
    }
}

Output:

buffer is backed by an array but is read-only
Exception throws: java.nio.ReadOnlyBufferException

Reference: https://docs.oracle.com/javase/9/docs/api/java/nio/Buffer.html#arrayOffset–