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

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

12. Writing
•
System.out.println(“Hello World”);
•
System.out.print(“Hello World”);
– prints
Hello World and then moves to the new
line
- 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 attached under this
presentation the code used.

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;
You may want to look again over JavaClassLibrary video from
pluralsight. 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://pluralsight.com/training/Courses/TableOfContents/java2

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. You
can find more details at:
http://pluralsight.com/training/Courses/TableOfContents/java2

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 (check
the definition of an algorithm)
http://pluralsight.com/training/Courses/TableOfContents/java2

19. Reading
Use the Integer and Double static
functions to convert a number from a
String to an Integer (int) or Double
(double).
You may want to check again the video about boxing and
unboxing:
http://pluralsight.com/training/Courses/TableOfContents/java1

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 variable if you do not
know it !!!
My first program
lin
e

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

28. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String firstString = input[0];
inp
0
ut
ind
ex
firstStri
my
first
program
1
2
my
my first program
lin
e

29. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String secondString= input[1];
inp
0
ut
ind
ex
secondStr
my
first
program
1
2
first
my first program
lin
e

30. Reading
String line = reader.readLine();
String [] input = line.split(“ “);
String thirdString= input[2];
inp
0
ut
ind
ex
thirdStri
my
first
program
1
2
program
my first program
lin
e

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

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.
inp
ut
ind
45
90
78
0
1
2
45*90*78
lin

33. Reading
String line = reader.readLine();
String [] input = line.split(“*“);
int no1 = Integer.parseInt(input[0]);
inp
ut
ind
ex
45
90
78
0
1
2
no1
45*90*78
45
lin
e

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

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

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

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

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

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

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

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

42. Annex 2
•
•
•
When you use an already implemented class from 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 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