RandomAccessFile in Java

Most input/output streams in Java read or write sequentially from start to end. Sometimes, however, you need direct access to a specific location in a file—for example, updating a record in a database-like file or reading a fixed-length record from the middle of a large file. RandomAccessFile provides this capability: it allows both reading and writing at arbitrary byte positions without reopening the file.

Key Characteristics

• A random access file behaves like a large array of bytes stored on disk.
• The file pointer marks the current position for the next read or write.
• Reads and writes advance the pointer automatically.
• The pointer can be moved explicitly with seek(long position).
• Writing beyond the current end of file extends the file.
• Files may be opened in "r" (read-only) or "rw" (read/write) mode.

Class Definition

  public class RandomAccessFile
      extends Object
      implements DataInput, DataOutput, Closeable
  

Constructors

  public RandomAccessFile(String filename, String mode) 
      throws FileNotFoundException

  public RandomAccessFile(File file, String mode) 
      throws IOException
  

The constructor immediately opens the file. If the file does not exist (and cannot be created in write mode), an exception is thrown.

Common Methods

Reading

• int read() — reads a single byte and returns its value (0–255) or -1 if end of file is reached.
• int read(byte[] b) — fills the array with bytes and returns how many were actually read (may be less than b.length).
• int read(byte[] b, int off, int len) — reads up to len bytes into a subsection of the array.
• void readFully(byte[] b) — blocks until the array is completely filled; throws EOFException if not enough bytes remain.
• void readFully(byte[] b, int off, int len) — same as above but for a subsection of the array.
• Typed reads: readBoolean(), readByte(), readChar(), readInt(), readLong(), readFloat(), readDouble(), readUTF().

Writing

• void write(int b) — writes a single byte (lowest 8 bits of b).
• void write(byte[] b) — writes the entire array to the file.
• void write(byte[] b, int off, int len) — writes a subsection of the array.
• Typed writes: writeBoolean(), writeByte(), writeChar(), writeInt(), writeLong(), writeFloat(), writeDouble(), writeUTF(String s).

Pointer Control

• long getFilePointer() — returns the current position.
• void seek(long pos) — repositions the pointer to a specific location.
• int skipBytes(int n) — moves the pointer forward by n bytes.

Other

• FileDescriptor getFD() — returns the underlying file descriptor.
• void close() — closes the file, releasing system resources.

Example: Updating a Record

  try (RandomAccessFile raf = new RandomAccessFile("records.dat", "rw")) {
      long recordSize = 128; // fixed-length records
      int recordNumber = 10;

      // Move to start of the 10th record
      raf.seek(recordNumber * recordSize);

      // Read an integer field
      int id = raf.readInt();

      // Update a value
      raf.writeUTF("Updated Name");
  }
  

Exceptions

EOFException is thrown if end-of-file is reached unexpectedly (e.g., when using readFully()). Other IOExceptions occur if the file is closed or another I/O error happens.

Notes and Best Practices

• RandomAccessFile is useful for legacy binary file formats and record-based storage.
• For new development, the Java NIO FileChannel and MappedByteBuffer APIs often provide better performance and flexibility.
• Always use try-with-resources to ensure files are closed properly and prevent leaks.