Download 3 - ELTE

Basic Java Syntax and Coding
Conventions
Copyright © 2004, Oracle. All rights reserved.
Objectives
After completing this lesson, you should be able to do
the following:
• Identify the key components of the Java language
• Identify the three top-level constructs in a Java
program
• Identify and describe Java packages
• Describe basic language syntax and identify Java
keywords
• Identify the basic constructs of a Java program
• Compile and run a Java application
• Examine the JavaBean architecture as an example
of standard coding practices
• Use the CLASSPATH variable and understand its
importance during compile and run time
3-2
Copyright © 2004, Oracle. All rights reserved.
Examining Toolkit Components
The J2SE/J2EE from Sun provides:
• Compiler
• Bytecode interpreter
• Documentation generator
J2SE
3-4
Copyright © 2004, Oracle. All rights reserved.
Exploring Packages in J2SE/J2EE
The J2SE/J2EE from Sun provides standard packages
for:
• Language
• Windowing
• Input/output
• Network communication
J2SE
3-5
Copyright © 2004, Oracle. All rights reserved.
Documenting Using the J2SE
The J2SE/J2EE from Sun provides documentation
support for:
• Comments
– Implementation
– Documentation
•
Documentation generator
J2SE
3-6
Copyright © 2004, Oracle. All rights reserved.
Contents of a Java Source
•
A Java source file can contain three top-level
constructs:
– Only one package keyword followed by the package
name, per file
– Zero or more import statements followed by fully
qualified class names or “*” qualified by a package
name
– One or more class or interface definitions
followed by a name and block
•
3-7
File name must have the same name as the public
class or public interface.
Copyright © 2004, Oracle. All rights reserved.
Establishing Naming Conventions
Naming conventions include:
• Class names
– Customer, RentalItem, InventoryItem
•
File names
– Customer.java, RentalItem.java
•
Method names
– getCustomerName(), setRentalItemPrice()
•
Package names
– oracle.xml.xsql, java.awt, java.io
3-8
Copyright © 2004, Oracle. All rights reserved.
More About Naming Conventions
•
Variables:
– customerName, customerCreditLimit
•
Constants:
– MIN_WIDTH, MAX_NUMBER_OF_ITEMS
•
•
3-10
Uppercase and lowercase characters
Numerics and special characters
Copyright © 2004, Oracle. All rights reserved.
Defining a Class
Class definitions typically include:
• Access modifier
• Class keyword
• Instance fields
• Constructors
• Instance methods
• Class fields
• Class methods
3-12
Copyright © 2004, Oracle. All rights reserved.
Rental Class: Example
Access modifier
public class Rental {
//Class variable
static int lateFee;
// Instance variables
int rentalId;
String rentalDate;
float rentalAmountDue;
…
// Instance methods
float getAmountDue (int rentId) {
…
}
…
}
3-13
Copyright © 2004, Oracle. All rights reserved.
Declaration
Instance
variable
Instance
method
Creating Code Blocks
•
•
•
Enclose all class declarations.
Enclose all method declarations.
Group other related code segments.
public class SayHello {
public static void main(String[] args) {
System.out.println("Hello world");
}
}
3-15
Copyright © 2004, Oracle. All rights reserved.
Defining Java Methods
•
•
Always define within a class.
Specify:
–
–
–
–
Access modifier
Static keyword
Arguments
Return type
[access-modifiers] [static] "return-type"
"method-name" ([arguments]) {
"java code block“ … }
return
3-16
Copyright © 2004, Oracle. All rights reserved.
Examples of a Method
public float getAmountDue (String cust){
// method variables
int numberOfDays;
float due;
float lateFee = 1.50F;
String customerName;
// method body
numberOfDays = getOverDueDays();
due = numberOfDays * lateFee;
customerName = getCustomerName(cust);
return due;
}
3-17
Copyright © 2004, Oracle. All rights reserved.
Declaration
Method
variables
Method
statements
Return
Declaring Variables
•
•
•
•
•
•
3-18
You can declare variables anywhere in a class
block, and outside any method.
You must declare variables before they are used
inside a method.
It is typical to declare variables at the beginning of
a class block.
The scope or visibility of variables is determined
in the code block.
You must initialize method variables before using
them.
Class and instance variables are automatically
initialized.
Copyright © 2004, Oracle. All rights reserved.
Examples of Variables
in the Context of a Method
public float getAmountDue (String cust) {
float due = 0;
int numberOfDays = 0;
float lateFee = 1.50F;
{int tempCount = 1; // new code block
due = numberOfDays * lateFee;
tempCount++;
…
}
return due;
// end code block
}
3-19
Copyright © 2004, Oracle. All rights reserved.
Method
variables
Temporary
variables
Rules for Creating Statements
•
•
•
3-20
Use a semicolon to terminate statements.
Define multiple statements within braces.
Use braces for control statements.
Copyright © 2004, Oracle. All rights reserved.
What Are JavaBeans?
A JavaBean is a platform-neutral reusable software
component that:
• Can be manipulated visually in a builder tool
• Communicates with other JavaBeans via events
• Comprises visible components that must inherit
from other visible components
• Provides an architecture for constructing the
building blocks of an application
3-21
Copyright © 2004, Oracle. All rights reserved.
Managing Bean Properties
•
•
Properties are the bean class member variables.
(Variables can be primitive types or objects.)
A property can be:
– Unbound, which is a simple property
– Bound, which triggers an event when the field is
altered
– Constrained, in which changes are accepted or
vetoed by interested listener objects
3-22
Copyright © 2004, Oracle. All rights reserved.
Exposing Properties and Methods
Getter
methods
(public)
T
3-23
private
T var;
T[] arr;
getVar()
Setter
methods
(public void)
setVar(T val)
T[] getArr()
setArr(T[] val)
boolean isVar()
setVar(boolean val)
Copyright © 2004, Oracle. All rights reserved.
JavaBean Standards at Design Time
The benefits at design time include:
• A facilitated interaction between designer, tool,
and bean
• Instantiated and functioning beans in a visual tool
• Highly iterative development environment
• Building applications in small bits that plug in and
out
• Storage and recovery of instantiated
objects
3-24
Copyright © 2004, Oracle. All rights reserved.
Compiling and Running
a Java Application
•
To compile a .java file:
prompt> javac SayHello.java
… compiler output …
•
To execute a .class file:
prompt> java SayHello
Hello world
prompt>
•
3-25
Remember that case matters.
Copyright © 2004, Oracle. All rights reserved.
The CLASSPATH Variable
•
•
Is defined in the operating system
Directs the JVM and Java applications where to
find .class files
•
References built-in libraries or user-defined
libraries
Enables interpreter to search paths, and loads
built-in classes before user-defined classes
Can be used with “javac” and “java” commands
•
•
3-26
Copyright © 2004, Oracle. All rights reserved.
CLASSPATH: Example
Location of .class files in the oe package
Setting CLASSPATH
C:\>set CLASSPATH=D:labs\les03\classes\oe
3-27
Copyright © 2004, Oracle. All rights reserved.
Summary
In this lesson, you should have learned the following:
• J2SE provides basic Java tools.
• J2SE provides a rich set of predefined classes and
methods.
• Java programs are made up of classes, objects,
and methods.
• Adhering to programming standards makes code
easier to read and reuse.
3-28
Copyright © 2004, Oracle. All rights reserved.
Practice 3: Overview
This practice covers:
• Examining the Java environment
• Writing and running a simple Java application
• Examining the course solution application
• Inspecting classes, methods, and variables
• Creating class files and an application class with a
main( ) method
•
3-29
Compiling and running an application
Copyright © 2004, Oracle. All rights reserved.