COMP 14Introduction to Programming

Miguel A. Otaduy

May 14, 2004

Today

• Variables and expressions

• Input/Output

• Writing a whole program

Variables

• Associated with data– Input data– Output data– Intermediate data

• We need to define:– Data type– Identifier

• Values will be assigned in expressions

Variables. Steps

1. Identify all data from the algorithm2. Determine data types (based on the

range and nature of the values)3. Find meaningful names

• Example: Ch. 1, Exercise 11. Compute average miles per gallon.

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 classint numStudents;

• Multiple variables (of the same data type) can be declared on a single lineint numStudents, numGrades, total;

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

Assignment

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

First the expression on the right handside of the = operator is evaluated

Then the result is stored in thevariable on the left hand side

answer = sum / 4 + MAX * lowest;

14 3 2

Assignment• The right and left hand sides of an assignment

statement can contain the same variable

First, one is added to theoriginal value of count

Then the result is stored back into count(overwriting the original value)

count = count + 1;

Example (miles per gallon)• Write assignments and expressions

• Declaration of variables (good style)a) At the beginningb) Before being used

Named Constantstatic 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 centimetersfinal double CM_PER_INCH = 2.54;

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 wouldnot result in an int

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.

1. i = i + 5;

2. x + 2 = x;

3. x = 2.5 * x;

4. percent = 10%

validinvalid

valid

invalid

Input

1. Standard input

2. Dialog windows (on Monday)

3. File (on Monday)

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)

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?

Reading Keyboard InputThe 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

Reading Keyboard Input

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

String line = keyboard.readLine();

Example (miles per gallon)

• Input odometer values

• Input gallons put in the car

String to Numbers

• String to intInteger.parseInt(strExpression)

Integer.parseInt("6723") 6723

• String to floatFloat.parseFloat(strExpression)

Float.parseFloat("345.76") 345.76

• String to doubleDouble.parseDouble(strExpression)

Double.parseDouble("1234.56") 1234.56

Example (miles per gallon)

• Convert miles to int

• Convert gallons to double

Output

• Standard output device is usually the monitor

• Access the monitor using the standard output object– System.out

• Two methods to output a string:1.print

2.println Puts the cursor at the next line

Examples

System.out.println ("Hi");System.out.println ("There");HiThere

int num = 5;System.out.println ("The sum is " +

(num + 3));The sum is 8

int num = 5+3;System.out.println (num);8

System.out.print ("Hi");System.out.println ("There");HiThere

String Concatenation

• A string cannot be split between two linesString 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?”;

How are you doing today?greeting

X

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 = 1226str

Example (miles per gallon)

• Output result

• Formatting numeric strings

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

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

Package

java.langjava.appletjava.awtjavax.swingjava.netjava.utiljavax.xml.parsers

Purpose

General supportCreating applets for the webGraphics and graphical user interfacesAdditional graphics capabilities and componentsNetwork communicationUtilitiesXML document processing

Packages

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

• Some of the packages in the standard class library are:

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

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

Using Predefined Classes and Methods• Example method call:int num = (int) Math.round (4.6);

– why don't we have to import the Math class?

• (Dot) . Operator: used to access the method in the class

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

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

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

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

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

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

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

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

Skeletonimport statements if any

public class ClassName{

declare named constants and/or stream objects

public static void main (String[] args) throws IOException

{variable declarationsexecutable statements

}}

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

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

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

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

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

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 */

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 Sunday night.– Hint***

• Read Ch. 3