Understanding Java I/O Streams: Guide & Examples

Some basic I/O

Example of Output Stream • Our first program involved the output stream • // First Program HelloWorld • public class HelloWorld { • public static void main(String args []){ • System.out.println(“Hello world...”): }}

System.out.println(“Hello world...”): • System.out is called the standard output object • This will display a line of text in the command window • In Java , any source and destination for I/O is considered a stream of bytes or characters. • To perform output we insert bytes or characters into a stream. To perform input we extract bytes or characters from a stream. • java.lang.System class contains three predefined streams • System.out • System.err for errors • System.in

Consider • System.in

System.in • Standard input is this stream • Some books refer to the input method • System.in.readln();

Problem • System.in.readln() doesn’t seem to work with system here • Solution import the scanner class

Consider the following program • // Second Program HelloWorld • import java.util.Scanner; • public class HelloWorld3 { • public static void main(String args []){ • Scanner input = new Scanner( System.in ); • String theName = input.nextLine(); • System.out.println(“input was“ + theNAme);}}

It contains the lines • // Second Program HelloWorld • import java.util.Scanner; • This imports an input Utility class called Scanner • Scanner input = new Scanner( System.in ); • This creates an instance called input of the scanner utility

String theName = input.nextLine(); • This calls a method of the class called nextLine() • This gets input from the screen • It binds this to a variable called theName which is declared to be a string type by the String Keyword

So now we have a basic input method which seems to work • Next we need to consider the basic components of programs in order to start building more complex examples. • We will start with basic syntax and operators

Java Syntax Primitive data types Operators Control statements

Primitive data types • Identical across all computer platforms  portable

Primitive data types • char (16 bits) a Unicode character • byte (8 bits) • int (32 bits) a signed integer • short (16 bits) a short integer • long (64 bits) a long integer

Primitive data types • float (32 bits) a real number • double (64 bits) a large real number • boolean (8 bits) • values are true or false(keywords) • cannot be converted to and from other data typese.g.while (i!=0) not while(i)

Primitive Data Types

Operators • Addition Subtraction • + - • Increment operators (postfix and prefix)++ -- • Multiplication Division Modulus* / % • Equality== != • Assignment operators= += -= *= /= %=

Arithmetic Operators

Back to our simple input program • Remember the last night we had a program to input an integer, manipulate it and output it.

We used • input.nextLine(); Returns a String • It would need to be converted if we are to use this string as an integer using the following parseInt method

Parsing Strings for integers • Integer.parseInt(String s); • The parseInt() function parses a string and returns an integer.

Using our Scanner input method • // Second Program HelloWorld • import java.util.Scanner; • public class HelloWorld4 { • public static void main(String args []){ • Scanner input = new Scanner( System.in ); • String theName = input.nextLine(); • int num1 = Integer.parseInt(theName); • num1 = num1 + 1; • System.out.println("result" + num1);}}

Some anomalies • Note the String declaration starts with an Uppercase Letter • And the int declaration starts with a lowercase letter

String Contatenation anomaly • Note we use the arithmetic operator + to increment num1 by 1 • We use the same notation + to concatenate the result num1 to the string result an example of operator overloading which in turn is part of polymorphism

Use of parseInt • int num1 = Integer.parseInt(theName); • We use the method parseInt on the string theName to produce our integer num1

Exercise • Write code to input 2 integers , add them together and output the result.

Towards more complicated programming • Next Conditionals and Loops • Now all Conditionals and Loops are characterised by Conditions (called guards in the case of Loops) • Conditions are boolean expressions largely based on relational operators • Next we look at these operators

Operators • Relational operators< <= > >= • Conditional operator?:

Logical Operators • Not ! • Logical AND && • Logical OR || • Boolean logical AND & • Boolean logical inclusive OR | • Boolean logical exclusive OR ^(true if only one operand is true)

Relational Operators

If Statements • Similarto C/C++ syntax: • if statement if (x != n){ …}else if { … }else { … }

Example of Classic Conditional Program • Consider our Leap Year program

// Second Program HelloWorld • import java.util.Scanner; • public class Leapyear { • public static void main(String args []){ • Scanner input = new Scanner( System.in ); • String theName = input.nextLine(); • int num1 = Integer.parseInt(theName); • if(num1 % 4 ==0 && (num1 % 100 != 0 ||num1 % 400 == 0)) • System.out.println("leap"); • else • System.out.println("not leap"); • }}

Exercises • Write javaclasses which will • 1: Input two Integers and output the largest • 2 :Input 3 sides of a triangle and determine whether the triangle is scalene, isoceles or equilateral. • 3 Input a month and a year and calculate how many days are in that month

Exercises continued • Remember • 30 days hath September , April , June and November, • All the Rest have 31, • Except February which has 28, • save for one year in four • When it has one day more

More Exercises • An insurance company offers different classes of insurance based on age • Over 20 and less than 40 is Category A • Between 40 and 60 is Category B • Over 60 is Category C • Write a program to accept in someones age and tell them the class of insurance they are entitled to.

Control Statements • switch statement switch (n){ case 1: … break; case 2: case 3: … break; default: break;};

Loop Statements • for statement for (int i=0; i<max; i++){ … }; • while statement while (x==n ){ …}; • do statement do {…} while( x<=y && x!=0);

General Points to Note • Case sensitive • Use lower case • objects have capitalised first letter

Reserved Words

Understanding Java I/O Streams: Guide & Examples

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