COMP 14 Introduction to Programming

1
COMP 14Introduction to Programming

• Adrian Ilie
• June 28, 2005

2
Today

• Variables and expressions
• Input/Output
• Writing a whole program

3
Variables

• Associated with data
• Input data
• Output data
• Intermediate data
• We need to define
• Data type
• Identifier
• Values will be assigned in expressions

4
Variables - Steps

• Identify all data from the algorithm
• Determine data types (based on the range and
  nature of the values)
• Find meaningful names
• Example Ch. 1, Exercise 10. Compute average
  score.

5
Declaration of Variables

• dataType identifier
• Must be declared before it can be used
• Can be (but doesn't have to be) initialized when
  declared
• Identifier should start in lowercase, indicate
  separate words with uppercase (good style)
• Example
• number of students in class
• int numStudents
• Multiple variables (of the same data type) can be
  declared on a single line
• int numStudents, numGrades, total

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Assignment

• variable expresssion
• Assignment Operator ()
• expression can be a value (3) or a mathematical
  expression (2 1)
• The expression must evaluate to the same data
  type as the variable was declared

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Assignment

• The assignment operator has a lower precedence
  than the arithmetic operators

First the expression on the right hand side of
the operator is evaluated
answer sum / 4 MAX lowest
2
4
3
1
Then the result is stored in the variable on the
left hand side
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Assignment

• The right and left hand sides of an assignment
  statement can contain the same variable

First, one is added to the original value of count
count count 1
Then the result is stored back into
count (overwriting the original value)
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Example (average score)

• Write assignments and expressions
• Declaration of variables (good style)
• At the beginning
• Before being used

10
Named Constant

• static final dataType IDENTIFIER value
• Declared by using the reserved word final
• Always initialized when it is declared
• Identifier should be in ALL CAPS, separate words
  with underscore (_) (good style)
• Example
• 1 inch is always 2.54 centimeters
• final double CM_PER_INCH 2.54

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Questions

• What is stored in the memory location referred
  to by the identifier num after each of the
  following statements is executed in order?
• int num ______
• num 3 ______
• num 5 4 - 2 ______
• num num 2 ______
• num 3.4 5 ______

unknown
3
7
14
error
would give an error since 3.4 5 would not
result in an int
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Questions

• Which of the following are valid Java assignment
  statements? Assume that i, x, and percent have
  been declared as double variables and properly
  initialized.
• i i 5 ______
• x 2 x ______
• x 2.5 x ______
• percent 10 ______

valid
invalid
valid
invalid
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Input

• Standard input
• Dialog windows (tomorrow)
• File (tomorrow)

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Reading Keyboard Input

• We can let the user assign values to variables
  through keyboard input
• In Java, input is accomplished using objects that
  represent streams of data
• A stream is an ordered sequence of bytes
• System.in is the standard input stream object (by
  default, the keyboard)

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Reading Keyboard Input

• The idea is that, with System.in, we have access
  to a stream of bytes coming from the keyboard.
• In other words, we have access to the keys
  pressed on the keyboard.
• But how do we actually get the values of the keys
  that are pressed?

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Reading Keyboard Input

• The input stream is made up of multiple objects
• InputStreamReader charReader new
  InputStreamReader (System.in)
• BufferedReader keyboard new BufferedReader
  (charReader)
• OR, all in one statement
• BufferedReader keyboard new BufferedReader (new
  InputStreamReader (System.in))

1 character at a time
Whole lineat once
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Reading Keyboard Input

• The readLine method of the BufferedReader class
  reads an entire line of input as a String
• String line keyboard.readLine()

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Example (average score)

• Input scores
• Input weights

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String to Numbers

• String to int
• Integer.parseInt(strExpression)
• Integer.parseInt("6723") 6723
• String to float
• Float.parseFloat(strExpression)
• Float.parseFloat("345.76") 345.76
• String to double
• Double.parseDouble(strExpression)
• Double.parseDouble("1234.56") 1234.56

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Example (average score)

• Convert scores to int
• Convert weights to double

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Output

• Standard output device is usually the monitor
• Access the monitor using the standard output
  object
• System.out
• Two methods to output a string
• print
• println

Puts the cursor on the next line at the end
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Examples
System.out.println ("Hi") System.out.println
("There") Hi There
System.out.print ("Hi") System.out.println
("There") HiThere
int num 53 System.out.println (num) 8
int num 5 System.out.println ("The sum is "
(num 3)) The sum is 8
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String Concatenation

• A string cannot be split between two lines
• String greeting "How are you doing
• today"
• Concatenation () - produces one string where the
  second string has been appended to the first
  string
• String greeting How are you doing
  today?

X
How are you doing today?
greeting
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String Concatenation

• Operator can be used to concatenate two strings
  or a string and a numeric value or character
• Precedence rules still apply
• Example
• String str
• int num1 12, num2 26
• str "The sum " num1 num2

The sum 1226
str
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Example (average score)

• Output result
• Formatting numeric strings

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Writing a Whole Program

• Class - used to group a set of related operations
  (methods), allows users to create their own data
  types
• Method - set of instructions designed to
  accomplish a specific task
• Package - collection of related classes
• Library - collection of packages

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Class Libraries

• A collection of classes that we can use when
  developing programs
• The Java standard class library is part of any
  Java development environment
• The System class and the String class are part of
  the Java standard class library

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Packages

• The classes of the Java standard class library
  are organized into packages.
• Some of the packages in the standard class
  library are

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Using Packages

• We need to import some of the packages we want
  to use
• java.io for BufferedReader
• import packageName
• import java.io.
• imports all of the classes in the java.io package
• import java.io.BufferedReader
• imports only the BufferedReader class from the
  java.io package

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Using Predefined Classes and Methods

• To use a method you must know
• Name of class containing method (Math)
• Name of package containing class (java.lang)
• Name of method (round), its parameters (double
  a), what it returns (long), and function (rounds
  a to the nearest integer)
• See Appendix E for more Java predefined classes

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Using Predefined Classes and Methods

• Example method call
• int num (int) Math.round (4.6)
• (Dot) . Operator used to access the method in
  the class

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Creating a Java Program

• Java application program - collection of one or
  more classes
• every application must have at least one class
• Class
• basic unit of a Java program
• collection of methods and data members
• Method - set of instructions designed to
  accomplish a specific task
• print, readLine

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Programming in Java

• Java programming language
• object-oriented approach to problem solving
• In the Java programming language
• a program is made up of one or more classes
• a class contains one or more methods
• a method contains program statements

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Creating a Java Program

• All Java application programs must have a method
  called main
• there can be only one main method in any Java
  application program
• Most of the time, our programs will have only one
  class
• Name of source file must be ClassNameWithMain.java

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Anatomy of a Java Program

• Syntax of class
• Syntax of main method

public class ClassName classMembers
public static void main (String
args) statement1 ... statementn
throws clause
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Throws Clause

• throws clause - exceptions thrown by the main
  method
• exception - occurrence of an undesirable
  situation that can be detected during program
  execution
• can either be handled or thrown
• readLine throws the exception IOException
• We won't handle the exception, we'll just throw it

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Throws

• If we're allowing user input to the program, the
  heading of the main method should look like

public static void main (String args) throws
IOException
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Import Statements

• Tell the compiler which packages are used in the
  program
• Import statements and program statements
  constitute the source code
• Source code saved in a file with the extension
  .java
• Source code file must have the same name as the
  class with the main method

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The main method

• Heading
• Body
• statements enclosed by
• declaration statements
• used to declare things such as variables
• executable statements
• perform calculations, manipulate data, create
  output, accept input, etc.

public static void main (String args) throws
IOException
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Skeleton

• import statements if any
• public class ClassName
• declare named constants and/or stream objects
• public static void main (String args)
  throws IOException
• variable declarations
• executable statements

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static

• Heading of the main method has the reserved word
  static
• Statements to declare named constants and input
  stream objects are outside the main method
• These must also be declared with the static
  reserved word

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Style

• Syntax
• beware! a syntax error in one place might lead to
  syntax errors in several other places
• Use of semicolons, braces, commas
• all Java statements end with semicolon
• braces enclose the body of a method and set it
  off from other parts of the program (also have
  other uses)
• commas separate list items

43
Style

• Semantics
• set of rules that gives meaning to a language
• beware! the compiler will not be able to tell you
  about semantic errors (example missing
  parentheses in mathematical expression)
• Documentation
• comments
• naming rules
• use meaningful identifiers
• prompt lines
• let the user know what type of input is expected

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Style and White Space

• White space
• blanks, tabs, blank lines
• used to separate words and symbols
• extra space is ignored by computer
• blank line between variable declaration and rest
  of code
• Programs should be formatted to enhance
  readability, using consistent indentation

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Comments

• Not used by the computer
• only for human consumption
• Used to help others understand code
• explain and show steps in algorithm
• comments are essential!
• Should be well-written and clear
• Comment while coding
• Also called inline documentation

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Java Comments

• // this is a one-line comment
• comments out the rest of the line after marker
  //
• / this is a multi-line
• comment /
• comments out everything between markers / and
  /

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To do

• Practice input. Ch. 2 (pp.45-54)
• Practice output. Ch. 2 (pp.55-65)
• Ch. 2 examples
• Convert Length
• Make Change
• Homework 2 due Thursday night.
• Read Ch. 3