A First Look at Java Chapter 2 1/29 & 2/2 Imagine! Java: Programming Concepts in Context by Frank M....

transcript

A First Look at Java

Chapter 2

1/29 & 2/2

Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010

Assignment• See my website for the new assignment.

Chapter Topics

A Simple Java Program• Identifiers• Displaying Text

CommentsData Types

• Primitive Types• Reference Types

Chapter Topics

Variables• Declarations• Assignments

Simple Input From the KeyboardConstants

• Unnamed Constants• Named Constants

Objectives

After studying this chapter, you should be able to:• Create valid Java identifiers• Write Java statements that display text• Describe the primitive data types int,

double, char, and boolean• Declare variables and assign them values• Read numbers and characters from the

keyboard

Displaying Text

Two commands• System.out.println

outputs newline character after the output• System.out.print

leaves the cursor on the same line.

Displaying Text

System.out.print(“Black”);

System.out.println(“bird”);

System.out.println(“sings.”);

Output:

Blackbird

sings.

Comments

Documents the programComments

• Begin line with double slash // Ends with the end of the line.

• Span multiple lines between slash-star combination./* . . . . . . */

A Simple Java Program

An online friend lives in Calgary. She always tells me the temperature up there in degrees Celsius. I wrote a program to convert the temperature to Fahrenheit.

public class CtoF {

/** This program converts Celsius to Fahrenheit. */

public static void main(String[] args) { double cel = 20; double fahr;

fahr = 1.8 * cel + 32.0; System.out.println(fahr +

" degrees Fahrenheit"); }//end main method

}//end CtoF class

A Simple Java Program

Note definition of a class• Begins and ends with brace{ … }

Note declaration of main methodpublic static void main(String[] args)

• Also begins and ends with brace

Identifiers

Examples: public, main, CtoF, celNames of things within the program

Made up of letters, digits, underscore, and $ Must not start with a digit Case sensitive

Categories Names you invent Names chosen by another programmer Identifiers part of the Java language

Valid Identifiers?

taxes2013

_gold_mines

99bottles

money$

ConventionsClass names begin with upper caseMethod names and begin with lower caseBraces

Each on its own line (optional)eclipse puts it on the line with the heading

Always in pairsEnd brace commented(optional)

Lines within braces indentedSkip lines for readability (white space)

Primitive TypesData that is a primitive type has a single

simple value.Examples: individual characters,

numbers, boolean values– 'Q', 55.78, true

Are not objects

Primitive Types

Typical Primitive Types int• integer, positive/negative counting

numbers. • 1, -10, 0, 20000

double• Numbers with a decimal part• 1.5, -9.0, 3.14159, 0.0

Typical Primitive Types char• Holds just one character• 'z', '*', '2'

boolean• true or false

Reference TypesCan contain multiple pieces of dataMay also have methods that operate on

those valuesExample – String

“Hello”Contains 5 characters

Name of the class is the data typeCalled a “class type” or “reference type”

Variables

Used to store a piece of dataStores it at a certain memory location Identifier used to name variable• Should begin with lower case letter• Subsequent words use upper case• Should be meaningful

Example: salesTaxRate

Variables

Must be declared –Data type– Identifier

Assignment Statement

Assignment gives a variable a valueSyntax

variable = expression ; • Expression evaluated• Value stored in memory location specified by

variable

Assignment StatementFigure 2-3 Note values before, after

Variables in a Program

Figure 2-4 Effects of sequence of assignments

Reference Variable

String str = “Hello”;

Notes on Output

+ operator in print statements Add numbers• (5+6)

Concatenate Strings.• "to" + "day" --> "today"• 15 + " inches" --> 15 inches

• Strings that span multiple lines

Questions

• What are the rules for writing an identifier (name) in Java?

• What primitive type would you use to store the weight of a car in tons?

• On which side of the = does the variable in an assignment statement belong?

Try a Program

Write a program that sets a variable to a person's name and another variable to their former age. It will output their name and their new age on their birthday. Sample output:

John is 21 today.

Another Example

Write a program to find the sales tax on an item. Assign the cost of the item to a variable. Set a price variable to cost of the item. Output the item's name and the tax.

Scanner and Constants2/5/15 & 2/9/15

Keyboard InputUse Scanner class from Java Class

Library

Must use import statement: import java.util.Scanner;

Next, create Scanner object Scanner keyboard = new Scanner(System.in);

Package name Class name

Keyboard Input

Keyboard is an object which can perform methods.

Use method to receive data from keyboard, store in variable• int x = keyboard.nextInt();• String word = keyboard.next();

•Let’s change the Age program so that it asks the user for the name and age.

Other Scanner Methods

double rate = keyboard.nextDouble();String words = keyboard.nextLine();

//program to give new age on a birthday.import java.util.Scanner;

public class Birthday {public static void main(String[] args) {

String name;int age;//Create a ScannerScanner kbd = new Scanner(System.in);

//Input nameSystem.out.print("Name?");name = kbd.nextLine();

//Input age.System.out.print("Age?");age = kbd.nextInt();age = age + 1;

System.out.println(name + " is " + age + " today.");

kbd.close();}

Constants

ConstantsUnnamed constants

• Literal values – integers, floating-point, boolean

E format may be used for doubles• 1.5e2 --> 150• 8e4 - decimal maybe left off, still a double• 5.1e-4 = ?

ConstantsNamed constants

• Similar to declaring variables, but values can't be changed.

• Use reserved word final• Gives descriptive name, avoids typing

mistakes• All caps by convention

final double TAX = .06;final int DOZEN = 12;final boolean NOT_DONE = false;

Participation Input and Constants

• Write a program to input the cost of a car in dollars. Output the sales tax on the car.

• Tax rate is 6% Use a constant.

1. Input cost – use a Scanner

2. Find tax

3. Output tax

I'll submit the program.

Use of Math class

For the programming assignment you will need to use Math.sqrt( ) and Math.PI.

Questions

• What type of object is used to handle input?

• What package does the Scanner belong to?

• What how is a constant different from other variables?