Copyright © 2014 by John Wiley & Sons. All rights reserved.1 Introduction.

transcript

Introduction

What is java? Developed by Sun Microsystems General-purpose and Object-oriented language Widespread acceptance

Java Features Simple

• no pointers• automatic garbage collection• rich pre-defined class library

Object oriented• all functions are associated with objects• potentially better code organization and reuse

Platform independent• Same application runs on all platforms• Java bytecode can run on any JVM, on any platform

• …including mobile phones and other hand-held devices

Java Virtual Machine (JVM)

Steps:1. Write all source code is in plain text files ending with

the .java extension. 2. Compile those source files into .class files by

the javac compiler. • A .class file contains bytecodes — the machine language of

the Java Virtual Machine(Java VM).

3. Run the bytecode/.class file using JVM

Java Virtual Machine (JVM) Because the Java VM is available on many

different operating systems, the same .class files are capable of running on any platform.

Java Versions

Writing first application You need:

• The Java SE Development Kit 7 (JDK 7)

• A text editor:

• Simple text editors: – Notepad, Notepad++ , …..

• Integrated Development Environment (IDE):– Eclipse, Netbeans, DrJava , …..

QUESTION

What is the difference between JDK and JRE?• Java Download page has both!

• http://www.oracle.com/technetwork/java/javase/downloads/jre7-downloads-1880261.html

ANSWER:• You need to answer the question: Do you want to run Java

programs or do you want to develop Java programs?• JRE:

• If you want to run Java programs, but not develop them, download the JRE.

• JDK:• If you want to develop Java applications, download the Java

Development Kit, or JDK.• The JDK includes the JRE, so you do not have to download both

separately.

Installing Java JDK

On Windows:• http://docs.oracle.com/javase/7/docs/webnotes/install/windo

ws/jdk-installation-windows.html On Linux:

• http://docs.oracle.com/javase/7/docs/webnotes/install/linux/linux-jdk.html

On Mac OS X:• http://docs.oracle.com/javase/7/docs/webnotes/install/mac/

mac-jdk.html

Your first Program: HelloPrinter.java

Choice 1: Using simple text editor• STEP1: Write and save your program in text editor as

HelloPrinter.java

• STEP2: Compile the source file into a .class file

• STEP3: Run the program

Programming Question

In your home directory create a folder cs160. Inside it create a folder ch01.

Write HelloPrinter program using gedit editor Save program in cs160/ch01 as HelloPrinter.java Open a terminal and navigate (cd) to cs160/ch01 Compile program (if no errors, verify .class file is generated) Run program

Your first Program: HelloPrinter.java

Choice 2: Using IDE1. Start the Java development environment (We use

DrJava).2. Write a simple program.3. Run the program.4. Organize your work.

Becoming Familiar with Your Programming Environment

1. Write and save program (File->save)

2. Compile3. Run

Comments Java supports three types of comments:

1. single-line comment:

2. multiline comment

3. document comment

Identifiers

Names chosen by the programmer

E.g. variable names, method names, class names etc.

Identifiers are subject to the following rules:• Identifiers may contain letters (both uppercase and

lowercase), digits, and underscores (_).• Identifiers begin with a letter or underscore.• There’s no limit on the length of an identifier.• Lowercase letters are not equivalent to uppercase letters. (A

language in which the case of letters matters is said to be case-sensitive.)

• Cannot be reserved words (keywords)

Reserved Words

Here is a list of keywords in the Java programming language. You cannot use any of the following as identifiers in your programs.

Multiword Identifiers

When an identifier consists of multiple words, it’s important to mark the boundaries between words.

One way to break up long identifiers is to use underscores between words:

last_index_of

Another technique is to capitalize the first letter of each word after the first:

lastIndexOf

This technique is the one commonly used in Java.

Java Conventions A rule that we agree to follow, even though it’s not required

by the language

1. Begin a class name with an uppercase letter:

Student

EmployeeString

2. Names of variables and methods, never start with an uppercase letter.

age variable nameprint() method name

3. Identifier names follow camel casing (For subsequent words in name, first word capitalizd)

totalSalary variable name

getName() method nameSalariedEmployee class name

Variables

Stores values/state

Must be declared before they can be used • static typing

Primitive Data Types

Java supports eight basic data types known as primitive types.

integer

floating point

Declaring Variables

Syntax: <variable type> <variable name> ;

Example:int i; // Declares i to be an int

variable

Several variables can be declared at a time:int i, j, k;

It’s often better to declare variables individually.

Initializing Variables

Assign a value to the variable for the first time.

us =, the assignment operator:i = 0;byte z = 22;double h = 3.1l3;char x = ‘x’;

Variables always need to be initialized before the first time their value is used.

You can initialize variables at the time of declaration too:boolean found=true, contains;int a, b=20, c;

Changing the Value of a Variable

The assignment operator can be used both to initialize a variable and to change the value of the variable later in the program:

i = 1; // Value of i is now 1…i = 2; // Value of i is now 2

Literals A token that represents a particular number or

other value.

Examples of int literals:0 437 30343

Examples of double literals:37.0 37. 3.7e1 3.7e+1 .37e2 370e-1

The only boolean literals are true and false.

char literals are enclosed within single quotes:'a' 'z' 'A' 'Z' '0' '9' '%' '.' ' '

Constants: final

Represents values that do not change.

Use keyword final when declaring a variable as a constant

Convention: • use all-uppercase names for constants• Words separated by underscore(“_”)

final double PENNY_VALUE = 0.01;final double NICKEL_VALUE = 0.05;

Class Declaration

Classes:• are the fundamental building blocks of Java programs:• The name of the public class MUST match the name of the file

containing the class: • Class HelloPrinter must be contained in a file named

HelloPrinter.java

1 public class HelloPrinter 2 { 3 public static void main(String[] args) 4 { 5 // Display a greeting in the console window 6 7 System.out.println("Hello, World!"); 8 } 9 }

Class declaration

Class body

Methods

Each class contains declarations of methods.

Each method contains a sequence of instructions describing how to carry out a particular task.

A method is called by specifying the method and its arguments.

Every Java application contains a class with a main method • When the application starts, the instructions in the main

method are executed

Method declaration

Method body

Statements

The body of the main method contains statements.

Every statement ends with a semicolon (“;”)

Our method has a single statement:System.out.println("Hello, World!");

It prints a line of text:Hello, World!

Statement 1

Print statement System.out.print

• Prints in the same line System.out.println

• Prints in a new line• System.out.println();//Prints in an empty line

Printing Multiple Items• The + operator can be used to combine multiple items into a

single string for printing purposes:

int I = 10;

System.out.println(“The value of i is: " + i);

• At least one of the two operands for the + operator must be a string.

Syntax 1.1 Java Program

Indentation

Programmers use indentation to indicate nesting.

An increase in the amount of indentation indicates an additional level of nesting.

The HelloPrinter program consists of a statement nested inside a method nested inside a class:

public class HelloPrinter{ public static void main(String[] args) { // Display a greeting in the console window System.out.println("Hello, World!"); }}

public class HelloPrinter{

public static void main(String[] args) {

// Display a greeting in the console window System.out.println("Hello, World!"); }}

Brace Placement

Brace placement is another important issue.

Option1:• Put each left curly brace at the end of a line.

• The matching right curly brace is lined up with the first character on that line:

Brace Placement

Option2:• Some programmers prefer to put left curly braces on

separate lines:

• This makes it easier to verify that left and right braces match up properly. However, program files become longer because of the additional lines.

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

// Display a greeting in the console window System.out.println("Hello, World!"); }}

Write a tester class IdentifierDemo that does the following: • Create seven variables, one for each of the primitive

number types in Java• Initialize each variable with any appropriate value. • Print out the name of each variable and its value. • Modify the value of each variable with an assignment

statement • Print out the names of the variables and their new values.• Next, create a double constants pi with value

3.141592653589793.• Print the name of the constant and its value.

What happens if you try to a assign a value to a constant?

Answerpublic class IdentifierDemo{ public static void main(String[] args) { boolean bln = true; // booleans can only be 'true' or 'false' byte b = 20; short s = 500; char c = '%'; // must use single quotes to denote characters int i = 1000000; // decimal notation float f = 1.5f; // trailing 'f' distinguishes from double long l = 2000000L; // trailing 'L' distinguishes from int System.out.println("bln="+bln); System.out.println("b="+b); System.out.println("c="+c); System.out.println("i="+i); System.out.println("f="+f); System.out.println("bln="+bln); System.out.println("l="+l);

bln = false; // booleans can only be 'true' or 'false' b = 70; s = 800; c = 't'; // must use single quotes to denote characters i = 23; // decimal notation f = 3.0f; // trailing 'f' distinguishes from double l = 5600000L; // trailing 'L' distinguishes from int System.out.println(); System.out.println("bln="+bln); System.out.println("b="+b); System.out.println("c="+c); System.out.println("i="+i); System.out.println("f="+f); System.out.println("bln="+bln); System.out.println("l="+l); final double pi = 3.141592653589793; // doubles are higher precision

System.out.println("pi="+pi); }}

IdentifierDemo.java

Java Language Basics: Operators

Java provides a rich set of operators to manipulate variables.• Arithmetic Operators• Relational Operators• Bitwise Operators• Logical Operators• Assignment Operators• Misc Operators

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

Java Language Basics: Operators

Arithmetic Operators:

• Assume variables A=10 , B=20

QUESTION

What is the output of following java expressions?

1) 1 / 22) 5 / 33) 6.1 + 24) 7 % 35) 1.0/0.06) -1.0/0.07) 0.0/0.0

Answer

Operator Precedence

What’s the value of 6 + 2 * 3?• (6 + 2) * 3, which yields 24?• 6 + (2 * 3), which yields 12?

Operator precedence resolves issues such as this.

*, /, and % take precedence over + and -.

Examples:5 + 2 / 2 5 + (2 / 2) 68 * 3 - 5 (8 * 3) - 5 196 - 1 * 7 6 - (1 * 7) –19 / 4 + 6 (9 / 4) + 6 86 + 2 % 3 6 + (2 % 3) 8

Associativity

Precedence rules are of no help when it comes to determining the value of 1 - 2 - 3.

Associatively rules come into play when precedence rules alone aren’t enough.

The binary +, -, *, /, and % operators are all left associative:2 + 3 - 4 (2 + 3) - 4 12 * 3 / 4 (2 * 3) / 4 1

Parentheses in Expressions

Parentheses can be used to override normal precedence and associativity rules.

Parentheses in the expression (6 + 2) * 3 force the addition to occur before the multiplication.

Question

What is the default parenthesis used in java for following expression:

2+3*4+1+4/2

Answer

(((2+(3*4))+1)+(4/2))

Compound Assignment Operators

The compound assignment operators make it easier to modify the value of a variable.

A partial list of compound assignment operators:+= Combines addition and assignment-= Combines subtraction and assignment*= Combines multiplication and assignment/= Combines division and assignment%= Combines remainder and assignment

Compound Assignment Operators

Examples:i += 2; // Same as i = i + 2;i -= 2; // Same as i = i - 2;i *= 2; // Same as i = i * 2;i /= 2; // Same as i = i / 2;i %= 2; // Same as i = i % 2;

Write a tester class FtoC that convert a Fahrenheit temperature to Celsius.

• In the main method declare two variables fahrenheit and celsius.

• Initialize fahrenheit to 98.6.

• Calculate value of celcius using following equation:

• Expected output:

Answer

// Converts a Fahrenheit temperature to Celsius

public class FtoC { public static void main(String[] args) { double fahrenheit = 98.6; double celsius = (fahrenheit - 32.0) * (5.0 / 9.0); System.out.println("Celsius equivalent: “ + celsius);

FtoC.java

Note that the ratio 5.0/9.0 and value 32.0 is a constant in FtoC program. Modify your program so that you define a constant DEGREE_RATIO (5.0/9.0) and FREEZING_POINT(32.0) and adjust your equation accordingly.

Answer// Converts a Fahrenheit temperature to Celsius

public class FtoC { public static void main(String[] args) { final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; double fahrenheit = 98.6; double celsius = (fahrenheit - FREEZING_POINT) * DEGREE_RATIO; System.out.println("Celsius equivalent: “+celsius);

FtoC.java

Converting Floating-Point Numbers to Integers - Cast

The compiler disallows the assignment of a double to an int because it is potentially dangerous

This is an error:double balance = total + tax;int dollars = balance; // Error: Cannot assign double to int

Use the cast operator (int) to convert a floating-point value to an integer.

double balance = total + tax;int dollars = (int) balance;

Cast discards fractional part

Syntax 4.2 Cast

Packages Java classes are grouped into packages.

• java.net package classes for networking• java.io package input/output functions• javax.swing package graphical user interface development

• Refere javadoc API

To use a class in a package, you must import the package• E.g. to use Rectangle class in java.awt package:

import java.awt.Rectangle;

Put the line at the top of your program.

You DON’T need to import classes in the java.lang package such as String and System.

Syntax 2.4 Importing a Class from a Package

Read user input

Use a class called Scanner to read keyboard input.

To use the Scanner class, import it by placing the following at the top of your program file (before class definition):

import java.util.Scanner

2. Obtain a Scanner object:Scanner in = new Scanner(System.in);

3. Prompt user a message asking to input data: System.out.println(“Enter the Fahrenheit temperature : ”);

By default, a scanner uses white space to separate tokens.White space characters include blanks, tabs, and line terminators.

2. Read the user input:• Read int: use nextInt() method• Read double: nextDouble() method

• Read string: next() method

Refer Scanner page in Oracle JavaAPI

nextInt() example:

System.out.print("Please enter the number of bottles: ");int bottles = in.nextInt();

• When the nextInt method is called,

• The program waits until the user types a number and presses the Enter key;

• After the user supplies the input, the number is placed into the bottles variable;

• The program continues.

To read different and multiple inputs from user, creating one scanner object is enough.

Complete Exampleimport java.util.Scanner;

public class ScannerTester{ public static void main(String args[]) { //create scanner object Scanner in = new Scanner(System.in);

//read values System.out.println("Enter name,age and isMarried seperated by whitespaces:"); String name = in.next(); int age = in.nextInt(); boolean isMarried = in.nextBoolean();

//print values System.out.println("Name:"+name+"\nage:"+age+"\nisMarried:"+isMarried);

//close scanner in.close(); }}

output

Write a tester class ScannerDemo.java that accepts two numbers from user and prints the sum.

Sample output:

Answerimport java.util.Scanner;

public class ScannerDemo{ public static void main(String args[]) { Scanner in = new Scanner(System.in); System.out.print("Enter number 1: "); int n1 = in.nextInt(); System.out.print("Enter number 2: "); int n2 = in.nextInt(); int total = n1 + n2; System.out.println(n1+ " + "+n2+ " = "+total);

in.close(); }}

ScannerDemo.java

Formatted Output Use the printf method to specify how values should be formatted.

Suppose you have a variable price with value 1.215962441314554

If you use

System.out.println(price)

Will print :1.215962441314554

If you want a formatted output(e.g. only 2 decimal points displayed):

System.out.printf("%.2f", price);

Will print:1.22

Formatted Output You can also specify a field width:

System.out.printf("%10.2f", price);

This prints 10 characters, six spaces followed by the four characters 1.22

This commandSystem.out.printf("Price per liter:%10.2f",

price);

PrintsPrice per liter: 1.22

Formatted Output

Formatted Output You can print multiple values with a single call to the printf method.

ExampleSystem.out.printf("Quantity: %d Total: %10.2f",quantity, total);

Output explained:

Copyright © 2014 by John Wiley & Sons. All rights reserved.1 Introduction.

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