java.io.DataOutputStream and java.io.DataInputStream give us the power to write and read primitive data type to a media such as file. Both of these classes have the corresponding methods to write primitive data type and to read it back.

Using this class make it easier to read int, float, double data and others without needing to interpret if the data should be an int or a float data. Let’s see our code below.

package org.kodejava.io;

import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.DataOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class PrimitiveStreamExample {

    public static void main(String[] args) {
        // Prepares some data to be written to a file.
        int cityIdA = 1;
        String cityNameA = "Green Lake City";
        int cityPopulationA = 500000;
        float cityTempA = 15.50f;

        int cityIdB = 2;
        String cityNameB = "Salt Lake City";
        int cityPopulationB = 250000;
        float cityTempB = 10.45f;

        /*
        Create an instance of FileOutputStream with cities.dat as the file
        name to be created. Then we pass the input stream object in the
        DataOutputStream constructor.
        */
        try (FileOutputStream fos = new FileOutputStream("cities.dat");
             DataOutputStream dos = new DataOutputStream(fos)) {

            /*
             Below we write some data to the cities.dat. DataOutputStream
             class have various method that allow us to write primitive type
             data and string. There are method called writeInt(),
             writeFloat(), writeUTF(), etc.
            */
            dos.writeInt(cityIdA);
            dos.writeUTF(cityNameA);
            dos.writeInt(cityPopulationA);
            dos.writeFloat(cityTempA);

            dos.writeInt(cityIdB);
            dos.writeUTF(cityNameB);
            dos.writeInt(cityPopulationB);
            dos.writeFloat(cityTempB);
        } catch (IOException e) {
            e.printStackTrace();
        }

        /*
         Now we have a cities.dat file with some data in it. Next you'll see
         how easily we can read back this data and display it. Just like the
         DataOutputStream the DataInputStream class have the corresponding
         read methods to read data from the file. Some method names
         are readInt(), readFloat(), readUTF(), etc.
        */
        try (FileInputStream fis = new FileInputStream("cities.dat");
             DataInputStream dis = new DataInputStream(fis)) {

            // Read the first data
            int cityId1 = dis.readInt();
            String cityName1 = dis.readUTF();
            int cityPopulation1 = dis.readInt();
            float cityTemperature1 = dis.readFloat();

            printOut(cityId1, cityName1, cityPopulation1, cityTemperature1);

            // Read the second data
            int cityId2 = dis.readInt();
            String cityName2 = dis.readUTF();
            int cityPopulation2 = dis.readInt();
            float cityTemperature2 = dis.readFloat();

            printOut(cityId2, cityName2, cityPopulation2, cityTemperature2);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void printOut(int cityId1, String cityName1, int cityPopulation1, float cityTemperature1) {
        System.out.println("Id: " + cityId1);
        System.out.println("Name: " + cityName1);
        System.out.println("Population: " + cityPopulation1);
        System.out.println("Temperature: " + cityTemperature1);
    }
}

The generated result of our program are:

Id: 1
Name: Green Lake City
Population: 500000
Temperature: 15.5
Id: 2
Name: Salt Lake City
Population: 250000
Temperature: 10.45