Consists of classes Classes include methods Programmers can create classes they need to form Java programs
Rich collection of classes in the Java libraries
Known as Java APIs (Application Programming Interface)
You can
Create your own classes Use existing Java classes
Introduction to Java
Introduction to Java
Learning Java
Typical Java Development Environment
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Two aspects 1. 2.
Java Language – to program your own classes Classes in the extensive Java class libraries
Edit Compile Load Verify Execute
Introduction to Java
Java programs normally go through five phases
Sun provides a Java standard Edition implementation called Java Development Kit (JDK) that includes the tools you need to write s/w in Java Introduction to Java
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Phase 1- Creating a program 7
8
Editing a file with an editor
Type the program – source code Make necessary corrections use any text editor Integrated development environment (IDEs)
Eclipse, NetBeans, JBuilder
With .java extension
Example: javac Welcome.java
If the program compiles, the compiler produces a .class file
Save the program
Use the javac (the Java compiler) to compile the program
Can
Phase 2 – Compiling a Java program into Bytecodes
Welcome.class The compiled version of the program
Welcome.java
Introduction to Java
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Phase 2 – Compiling a Java program into Bytecodes (contd.)
Introduction to Java
Virtual Machine - VM 10
Java compiler translates Java source code into bytecodes. Bytecodes are executed by the Java Virtual Machine (JVM)
VM is a software application that simulates a computer
But, hides the underlying operating system and hardware from the programs that interact with the VM.
If the same VM is implemented on many computer platforms, applications that it executes can be used on all those platforms. JVM is one of the most widely used VM.
Introduction to Java
Introduction to Java
Java Bytecodes
Java Virtual Machine - JVM
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Platform independent instructions
Not dependent on a particular hardware platform
Portable
The same bytecodes can execute on any platform containing a JVM.
Introduction to Java
A part of the JDK. The foundation of the Java platform. The JVM is invoked by the java command . Example: java Welcome
This will then initiate the steps necessary to execute the application. This begins Phase 3.
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Phase 3 – Loading a program into memory
The program must be placed in memory before it can execute.
Phase 4 – Bytecode Verification 14
Known as loading
The class loader takes the .class files containing the program’s bytecodes and transfers them to primary memory. The class loader also loads any of the .class files provided by Java that your program uses.
The bytecode verifier examines their bytecodes to ensure that they are valid, and do not violate Java’s security restrictions. Java enforces strong security
Computer viruses and worms
Introduction to Java
Introduction to Java
Phase 5 - Execution 15
Two compilations phases 16
The JVM executes the program’s bytecodes. JVMs execute bytecodes using a combination of interpretation and so called just-in-time (JIT) compilation.
During compilation
Source code bytecode For
Bytecodes Machine language for the actual computer on which the program executes
Introduction to Java
Introduction to Java
First program in Java 17
portability
During Execution
Line 1 18
Comments Insert comments to document programs and improve their readability In Java // : end-of-line (or single line) comment /* …. */ : Traditional (or multiple-line comments) /** … */ : Javadoc comments
Introduction to Java
Introduction to Java
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Javadoc comments 19
Line 3 20
All text between Javadoc comment delimiters is ignored. Enable programmers to embed program documentation directly in their programs The javadoc utility program (part of the Java SE Development Kit) reads Javadoc comments and uses them to prepare your program’s documentation in HTML format.
Blank line
Insert blank lines and spaces to make it more readable
Introduction to Java
Introduction to Java
Line 4
Line 4 (contd.)
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public class Welcome1 Begins a class declaration Every program in Java consists of at least one class declaration
Defined by the programmer
You will learn about public later
A public class must be saved in a file that has the same name as the class with the .java extension
The class name is Welcome1 By convention, all class names in Java begin with a capital letter
Starts with the keyword public
In terms of both spelling and capitalization
and capitalize the first letter of each word they include
e.g. SampleClassName Introduction to Java
Introduction to Java
Lines 5 and 13
Line 7
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public static void main(String args[])
Braces Left brace begins the body of every class declaration A corresponding right brace must end the class declaration
Lines 6- 11 are indented
Introduction to Java
The starting point of the program The parentheses after the identifier main, indicate that it is a method Java class declarations normally contain one or more methods For a Java application, exactly one of the methods must be called main and must be defined as above. Keyword void indicates that this method will perform a task but will not return any information when it completes its task String args[] in parentheses is a required part of the method main's declaration.
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Line 9
Method print
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System.out.println("Welcome to Java Programming!");
A statement – ends with a ; System.out is the standard output object
System.out.println
Does not position cursor at the beginning of the next line System.out.print("Welcome!");
Allows to display characters in the command window Display the argument in command window A
line of text
Introduction to Java
Introduction to Java
Newline character 27
Escape sequence 28
Indicates to System.out.print and println methods to start on a newline.
// Addition program that displays the sum of two numbers. import java.util.Scanner; // program uses class Scanner public class Addition { public static void main( String args[] ) { // create Scanner to obtain input from command // window Scanner input = new Scanner( System.in ); Introduction to Java
int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and number2
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Another program (contd.) 31
Another program (contd.) 32
System.out.print( "Enter first integer: " ); number1 = input.nextInt(); // read first number
sum = number1 + number2; // add numbers System.out.printf( "Sum is %d\n", sum );
System.out.print( "Enter second integer: " ); number2 = input.nextInt(); // read second number
} // end method main } // end class Addition
Introduction to Java
Introduction to Java
Import declaration 33
Import declaration (contd.) 34
Helps the compiler locate a class that is used in this program Rich set of predefined classes These classes are grouped into packages
import java.util.Scanner;
Named collection of related classes
Indicates that this program uses predefined Scanner class from package java.util
All import declarations must appear before the first class declaration in the file
Collectively referred to as Java class library or Java Application Programming Interface (Java API) Use import declaration to identify the predefined classes used in Java program Introduction to Java
Introduction to Java
Input data
Input data (contd.)
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A Scanner enables a program to read data Data can come from many sources
File on disk Keyboard
number1 = input.nextInt(); // read first number Scanner objects input's nextInt method to obtain an integer from the user at the keyboard
Before using a Scanner, the program must create it and specify the source of the data Scanner input = new Scanner( System.in ); The standard input object, System.in, enables to read data from keyboard
Introduction to Java
Introduction to Java
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Variable declaration statements 37
Arithmetic operators 38
int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and number2
System.out.print( "Enter first integer: " ); System is a class. It is a part of package java.lang Class System is not imported with an import declaration
Print-formatted
By default, package java.lang is imported in every Java program Classes
in java.lang are the only ones in the Java API that do not require an import declaration Introduction to Java
Introduction to Java
Output of variables with printf() 41
42
The printf function has general form: System.out.printf("string…..", variables, numbers);
Output of variables with printf() (contd.) System.out.printf ("Some number = %d", number); Some number = 42
Introduction to Java
Introduction to Java
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printf conversions
Formatting with printf
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d u x o s c f e g
signed decimal integer unsigned decimal integer hexadecimal integer octal integer string single character fixed decimal floating point scientific notation floating point use f or e, whichever is shorter
The conversion specifiers can be extended % [-] [fwidth] [.p] X
%-10.3f
Introduction to Java
Introduction to Java
Formatting symbols
Examples
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[fwidth] [-] [.p]
Object to
field width left justified. For a floating point type number of decimal places For a string number of characters
