- Writing to the display in Java uses System.out.println() and System.out.print() to output text. - Reading from keyboard uses the BufferedReader class along with InputStreamReader to read input as strings. The strings can then be split and converted to int or double as needed. - Key methods for strings include charAt() to get individual characters, and substring() to extract parts of a string.

1. IO Java
CST200 – Week 1: Basic Reading and Writing in
Java
Instructor: Andreea Molnar

2. Outline
• Aims
• Writing to the display
• Reading from the keyboard

3. Introduction
• Input/Ouput (IO) operations in Java are
more complex than described here.
• You will need the information presented
here to complete your assignments and
for the exams

4. Writing
• System.out.println() –prints a new line
• System.out.println(“Hello World”); – prints
Hello World and then moves to the new
line

5. Writing
You can have an expression that will be
printed. Check the definition of an
expression!!!
System.out.println(3+7+8);
//prints 18 and then moves to a new line
System.out.println(“My student ID is “ + 3456E);
//prints My student ID is 3456E

6. Writing
int ID = 3456;
System.out.println(“My student ID is “ + ID);
//prints My student ID is 3456

7. Writing
String firstName = “Andreea”;
String lastName = “Molnar”;
System.out.println(firstName + lastName);
//prints AndreeaMolnar

8. Writing
String firstName = “Andreea”;
String lastName = “Molnar”;
System.out.println(firstName + “ “ + lastName);
//prints Andreea Molnar

9. Writing
String firstName = “Andreea”;
String lastName = “Molnar”;
System.out.println(firstName.charAt(0) + “ “ +
lastName.substring(1, 3));
//prints A ol

10. Writing
String firstName = “Andreea”;
String lastName = “Molnar”;
System.out.println(firstName.charAt(0) + “ “ +
lastName.substring(1, 3));
//prints A ol
If you do not understand how the above output is generated check the
String API, Introduction to Java.ppt or Annex 1 at the end of this
presentation. If any of these do not work for you, please ask.

11. Writing
System.out.println(firstName + “n" +
lastName);
//will print:
//Andreea
//Molnar
Escape
sequence
Meaning
b backspace
t tab
n newline
r return
’’ double quote
’ single quote
backslash
You can use escape sequences.

12. Writing
• System.out.println(“Hello World”); – prints
Hello World and then moves to the new
line
• System.out.print(“Hello World”); - does the
same thing as System.out.println but it
does not move to a new line

13. Writing
System.out.println(“Hello World”);
System.out.println(“I am here”);
System.out.print(“Hello World”);
System.out.print(“I am here”);

14. Reading
This section will provide you with an
example on how to read from the
keyboard using BufferedReader.
You should find the code used attached
under this presentation.

15. Reading
You will need to import the following
classes, in order to make use of them.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
I have added a few more explanations on Annex 2.

16. Reading
Change the main method to throw an
exception:
public static void main(String[] args) throws
IOException {
}
Exception handling is also not covered by this course. You can
find more details at: http://math.hws.edu/javanotes/c3/s7.html

17. Reading
Change the main method to throw an
exception:
public static void main(String[] args) throws
IOException {
}
Exception handling are also not covered by this course.

18. Reading
Use BufferedReader for reading lines of
text from standard input (i.e. keyboard)
BufferedReader reader = new BufferedReader(new
InputStreamReader(System.in));
reader.readLine();
//reads a line of text as a String
IO in Java is not covered in details in this course, and this
example is provided just to help you write algorithms

19. Reading
Use the Integer and Double static
functions to convert a number from a
String to an Integer (int) or Double
(double).

20. Reading
To convert a String to an Integer you can
use: Integer.parseInt(String s);
int number = Integer.parseInt(“23”);
http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html

21. Reading
To convert a String to a Double you can
use: Integer.parseInt(String s);
int number = Integer.parseInt(“23”);
http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html

22. Reading
To convert a String to a Double you can use:
Double.parseDouble(String s);
double number = Double.parseDouble (“23”);
http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html

23. Reading
If you want to read multiple numbers or strings
on the same line, you can use regular
expressions to extract the numbers.

24. Reading
String line = reader.readLine();
//assuming that the read line provides strings
or numbers or characters separated by space
String [] input = line.split(“ “);
http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#split(ja
va.lang.String)

25. Reading
//+ is used as a separator
String [] line = line.split(“+“);
//* is used as a separator
String [] line = line.split(“*“);

26. Reading
String line = reader.readLine();
//assuming that the user introduces the
following string: “my first program”, the line
variable will contain the value my first program
Check the definition of a variable if you do not
know it !!! My first program
line

27. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
First value in an array starts at index 0!!!
my first program
line
my first program
0 1 2input
index

28. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String firstString = input[0];
my first program
line
my first program
0 1 2input
index
my
firstString

29. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String secondString= input[1];
my first program
line
my first program
0 1 2input
index
first
secondString

30. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String thirdString= input[2];
my first program
line
my first program
0 1 2input
index
program
thirdString

31. Reading
String line = reader.readLine();
//assuming now that the user introduces the
following string: “45*90*78”
String [] input = line.split(“*“);
45*90*78
line
45 90 78
0 1 2input
index

32. Reading
String line = reader.readLine();
String [] input = line.split(“*“);
int no1 = Integer.parseInt(input[0]);
Although input looks like it contains numbers, the numbers are in fact
represented as strings, therefore they need to be converted to int.
45*90*78
line
45 90 78
0 1 2input
index

33. Reading
String line = reader.readLine();
String [] input = line.split(“*“);
int no1 = Integer.parseInt(input[0]);
45*90*78
line
45 90 78
0 1 2input
index
45
no1

34. Reading
String line = reader.readLine();
String [] input = line.split(“*“);
int no1 = Integer.parseInt(input[1]);
45*90*78
line
45 90 78
0 1 2input
index
90
no2

35. Reading
String line = reader.readLine();
String [] input = line.split(“*“);
int no1 = Integer.parseInt(input[2]);
45*90*78
line
45 90 78
0 1 2input
index
78
no3

36. Summary
• Writing: use System.out.println() and
System.out.print();
• Reading: use BufferedReader class

37. Annex 1
String firstName = “Andreea”;
char firstCharacter= firstName.charAt(0);
A n d r e e a
0 1 2 3 4 5 6
index

38. Annex 1
String firstName = “Andreea”;
char firstCharacter= firstName.charAt(0);
char secondCharacter = firstName.charAt(1);
char thirdCharacter = firstName.charAt(2);
char fourthCharacter = firstName.charAt(3);
//…
A n d r e e a
0 1 2 3 4 5 6
index

39. Annex 1
System.out.println(firstCharacter);
//will print A
System.out.println(secondCharacter);
//will print n
A n d r e e a
0 1 2 3 4 5 6
index

40. Annex 1
String lastName = “Molnar”;
String subString= lastName.substring(1, 3));
M o l n a r
0 1 2 3 4 5
index

41. Annex 1
String lastName = “Molnar”;
String subString= lastName.substring(1, 3));
System.out.println(lastName);
//will print ol
M o l n a r
0 1 2 3 4 5

42. Annex 2
• When you use an already implemented class from the Java
framework, you are basically using a class from a package.
• java.lang package is automatically imported this is why for the
HelloWorld program you didn’t need to import anything.
• To import an entire package you can write import package.*. For
example: import java.util.*;

43. Annex 2
• Importing a package allows you to use the class from a library
without fully using its fully qualified name. For example in the
case of BufferedReader, without importing
java.io.BufferedReader you would have to write:
java.io.BufferedReader reader = new java.io.BufferedReader (…);
http://docs.oracle.com/javase/7/docs/api/java/io/BufferedReader.html