Stream IO

Streams are used to exchange data between end user, file systems, networks other programs etc.

• data can be binary: Stream classes.
• data can be characters: Reader/Writer classes

Extra functionality can be wrapped around stream classes for example

• Buffer, data handing, etc.

Input Stream

Hierarchy

abstract class InputStream implements AutoCloseable {
    public abstract int read() throws IOException {}

    public void close() throws IOException {}
}

Example

String text = "";
boolean b = true;
double salary = 0.0;
DataInputStream datIn = null;

try {
    InputStream in = new FileInputStream("data.dat");
    InputStream bufIn = new BufferedInputStream(in);
    text = datIn.readUTF();
    b = datIn.readBoolean();
    salary = datIn.readDouble();
} catch(IOException e) {
    e.printStackTrace();
} finally {
    try {
        datIn.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Compact version:

String text = "";
boolean b = true;
double salary = 0.0;

try (DataInputStream datIn = new DataInputStream(new BufferedInputStream(new FileInputStream ("data.dat")))) {
    text = datIn.readUTF();
    b = datIn.readBoolean();
    salary = datIn.readDouble();
} catch (IOException e) {
    e.printStackTrace();
}

Reader

Hierarchy

Example

abstract class Reader implements AutoCloseable {
// Reads a single character, as an integer
// in the range 0 to 65535,
// or -1 if the end of the stream has been reached
public int read() throws IOException { ... }
...
// Closes the stream
public abstract void close() throws IOException { ... }
}

Example

String text = "";
boolean b = true;
double salary = 0.0;

try (BufferedReader bufIn = new BufferedReader(new FileReader("data.txt"))) {
    text = bufIn.readLine();
    b = Boolean.parseBoolean(bufIn.readLine());
    salary = Double.parseDouble(bufIn.readLine());
} catch (IOException e) {
    e.preintStackTrace();
}

Output Stream

Hirearchy

Example

DataOutputStream dos;

try {
    dos = new DataOutputStream(new BufferedOutputStream(new FileOutputStream("data.dat")));
} catch (FileNotFoundException e) {
    System.out.println("data.dat file not created")
    return;
}

try {
    dos.writeUTF("Hello world");
    dos.writeBoolean(true);
    dos.writeDouble(3.21);
    dos.close();
} catch (IOException e) {
    System.out.println("Error writing to data.dat");
    return;
}

Writer

Hierarchy

Example

PrintWriter pw;
BufferedReader br;

try {
    pw = new PrintWriter (
        new BufferedWriter (
            new FileWriter("data.txt")));
} catch (IOException e) {
    System.out.println("data.txt not created");
}

pw.println("Hello World");
pw.println(false);
pw.println(42);
pw.close();

Object IO

reading and writing entire objects.

Objects should be made serializable by implementing the interface Serializable.

Java can read and write all values of object's fields with methods:

Object readObject()void writeObject(Object)

Example

public class Student implements java.io.Serializable {
    private String name;
    private String number;

    public static main(String[] args) {
        Student s1 = null;

        try (ObjectOutputStream oos = new ObjectOutputStream("student.obj")) {
            oos.writeObject(new student("John Doe", "123432"));
        } catch (IOException) {
            e.printStackTrace();
        }

        try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("student.obj"))) {
            s1 = (Student) ois.readObject();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Text Input / Output

Standard Output:

• Static constant System.out of type PrintStream.
• Methods: print, println, printf

Standard Input

• Static constant System.in of type InputStream
• Input should not only be read, but also interpreted.

Scanner Example

Scanner input = new Scanner(System.in);
while (input.hasNextLine()) {
    String line = input.nextLine();
    doSomething(line);
}
input.close();