Input and Output: Reading and Writing Binary Data in Java

Text format is convenient for testing and debugging because it is humanly readable, but it is not as efficient as transmitting data in binary format. In the following sections, you will learn how to perform input and output with binary data.

1. The DataInput and DataOutput interfaces

The DataOutput interface defines the following methods for writing a number, a character, a boolean value, or a string in binary format:

writeChars          writeFloat

writeByte           writeDouble

writeInt            writeChar

writeShort          writeBoolean

writeLong           writeUTF

For example, writeInt always writes an integer as a 4-byte binary quantity re­gardless of the number of digits, and writeDouble always writes a double as an 8-byte binary quantity. The resulting output is not human-readable, but it will use the same space for each value of a given type and reading it back in will be faster than parsing text.

The writeUTF method writes string data using a modified version of the 8-bit Unicode Transformation Format. Instead of simply using the standard UTF-8 encoding, sequences of Unicode code units are first represented in UTF-16, and then the result is encoded using the UTF-8 rules. This modified encoding is different for characters with codes higher than OxFFFF. It is used for backward compatibility with virtual machines that were built when Unicode had not yet grown beyond 16 bits.

Since nobody else uses this modification of UTF-8, you should only use the writeUTF method to write strings intended for a Java virtual machine—for exam­ple, in a program that generates bytecodes. Use the writeChars method for other purposes.

To read the data back in, use the following methods defined in the DataInput interface:

readInt             readDoubte

readShort           readChar

readLong            readBootean

readFtoat           readUTF

The DataInputStream class implements the DataInput interface. To read binary data from a file, combine a DataInputStream with a source of bytes such as a FiteInputStream:

var in = new DataInputStream(new FiteInputStream(“emptoyee.dat”));

Similarly, to write binary data, use the DataOutputStream class that implements the DataOutput interface:

var out = new DataOutputStream(new FiteOutputStream(“emptoyee.dat”));

2. Random-Access Files

The RandomAccessFile class lets you read or write data anywhere in a file. Disk files are random-access, but input/output streams that communicate with a network socket are not. You can open a random-access file either for reading only or for both reading and writing; specify the option by using the string “r” (for read access) or “rw” (for read/write access) as the second argument in the constructor.

var in = new RandomAccessFile(“employee.dat”, “r”);

var inOut = new RandomAccessFile(“employee.dat”, “rw”);

When you open an existing file as a RandomAccessFile, it does not get deleted.

A random-access file has a file pointer that indicates the position of the next byte to be read or written. The seek method can be used to set the file pointer to an arbitrary byte position within the file. The argument to seek is a long integer between zero and the length of the file in bytes.

The getFilePointer method returns the current position of the file pointer.

The RandomAccessFile class implements both the DataInput and DataOutput interfaces. To read and write from a random-access file, use methods such as readInt/ writeInt and readChar/writeChar that we discussed in the preceding section.

Let’s walk through an example program that stores employee records in a random-access file. Each record will have the same size. This makes it easy to read an arbitrary record. Suppose you want to position the file pointer to the third record. Simply set the file pointer to the appropriate byte position and start reading.

long n = 3; – 1) * RECORD_SIZE);

var e = new Employee();

e. readData(in);

If you want to modify the record and save it back into the same location, remember to set the file pointer back to the beginning of the record: – 1) * RECORD_SIZE);


To determine the total number of bytes in a file, use the length method. The total number of records is the length divided by the size of each record.

long nbytes = in.length(); // length in bytes

int nrecords = (int) (nbytes / RECORD_SIZE);

Integers and floating-point values have a fixed size in binary format, but we have to work harder for strings. We provide two helper methods to write and read strings of a fixed size.

The writeFixedString writes the specified number of code units, starting at the beginning of the string. If there are too few code units, the method pads the string, using zero values.

public static void writeFixedString(String s, int size, DataOutput out) throws IOException


for (int i = 0; i < size; i++)


char ch = 0;

if (i < s.length()) ch = s.charAt(i);




The readFixedString method reads characters from the input stream until it has consumed size code units or until it encounters a character with a zero value.

Then, it skips past the remaining zero values in the input field. For added efficiency, this method uses the StringBuitder class to read in a string.

public static String readFixedString(int size, DataInput in) throws IOException


var b = new StringBuilder(size);

int i = 0;

var done = false;

while (!done && i < size)


char ch = in.readChar();


if (ch == 0) done = true;

else b.append(ch);


in.skipBytes(2 * (size – i));

return b.toString();


We placed the writeFixedString and readFixedString methods inside the DataIO helper class.

To write a fixed-size record, we simply write all fields in binary.

DataIO.writeFixedString(e.getName(), Employee.NAME_SIZE, out);


LocalDate hireDay = e.getHireDay();




Reading the data back is just as simple.

String name = DataIO.readFixedString(Employee.NAME_SIZE, in);

double salary = in.readDouble();

int y = in.readInt();

int m = in.readInt();

int d = in.readInt();

Let us compute the size of each record. We will use 40 characters for the name strings. Therefore, each record will contain 100 bytes:

  • 40 characters = 80 bytes for the name
  • 1 double = 8 bytes for the salary
  • 3 int = 12 bytes for the date

The program shown in Listing 2.2 writes three records into a data file and then reads them from the file in reverse order. To do this efficiently requires random access—we need to get to the last record first.

3. ZIP Archives

ZIP archives store one or more files in a (usually) compressed format. Each ZIP archive has a header with information such as the name of each file and the compression method that was used. In Java, you can use a ZipInputStream to read a ZIP archive. You need to look at the individual entries in the archive. The getNextEntry method returns an object of type ZipEntry that describes the entry. Read from the stream until the end, which is actually the end of the current entry. Then call closeEntry to read the next entry. Do not close zin until you read the last entry. Here is a typical code sequence to read through a ZIP file:

var zin = new ZipInputStream(new FileInputStream(zipname));

ZipEntry entry;

while ((entry = zin.getNextEntry()) != null)


read the contents of zin




To write a ZIP file, use a ZipOutputStream. For each entry that you want to place into the ZIP file, create a ZipEntry object. Pass the file name to the ZipEntry constructor; it sets the other parameters such as file date and decompression method. You can override these settings if you like. Then, call the putNextEntry method of the ZipOutputStream to begin writing a new file. Send the file data to the ZIP output stream. When done, call closeEntry. Repeat for all the files you want to store. Here is a code skeleton:

var fout = new FileOutputStream(“”);

var zout = new ZipOutputStream(fout);

for all files


var ze = new ZipEntry(filename);


send data to zout




ZIP input streams are a good example of the power of the stream abstraction. When you read data stored in compressed form, you don’t need to worry that the data are being decompressed as they are being requested. Moreover, the source of the bytes in a ZIP stream need not be a file—the ZIP data can come from a network connection.

Source: Horstmann Cay S. (2019), Core Java. Volume II – Advanced Features, Pearson; 11th edition.

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