Lesson 3 Introducing the Java Language
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Objectives • After completing this lesson, you should be able to: – – – – – –
– – –
–
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Define a class Identify the components of a class Explain the term object Describe the purpose of a variable Discuss methods and describe how to use a main method Describe the elements that comprise a Java class, such as declarations, return values, and the proper use of brackets and braces Identify keywords and describe their purpose Test and execute a simple program Describe some common causes of syntax errors Describe the purpose and features of an IDE debugger
Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Relevance • How do you test something that you have built, such as a house, a piece of furniture, or a program?
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Identifying the Components of a Class Shirt Order Date
Window
OrderEntry Customer
Button
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Structuring Classes – Class declaration – Field de larations lass attri utes are alled fields • Fields may also be initialized at declaration time.
– Methods (optional) – Comments (optional)
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Structuring Classes public class Shirt {
Field declarations
Method
public int shirtID = 0; // Default ID for the shirt public String description = "-description required-"; // default // The color codes are R=Red, B=Blue, G=Green, U=Unset public char colorCode = 'U'; public double price = 0.0; // Default price for all shirts public int quantityInStock = 0; // This method displays the values for an item public void displayInformation() { System.out.println("Shirt ID: " + shirtID); System.out.println("Shirt description:" + description); System.out.println("Color Code: " + colorCode); System.out.println("Shirt price: " + price); System.out.println("Quantity in stock: " + quantityInStock); } // end of display method } // end of class
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Class declaration
Symbols Used in Defining a Java Source
– – – – – – –
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Braces Parentheses Semicolons Commas Single quotation marks Double quotation marks Single-line comment
Putting It All Together – Syntax for declaring a class: [modifiers] class class_identifier
– Class example:
public class Shirt{ public double price; public void setPrice(double priceArg){ price = priceArg; }
} Open and closing braces for the Shirt class
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Quiz • Select the class declaration that adheres to the class-naming guidelines. a. b. c. d.
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class Shirt public Class 501Pants public Shirt public Class Pants
Field Declarations and Assignments • • • • •
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public public public public public
int shirtID = 0; String description = "-description required-"; char colorCode = 'U'; double price = 0.0; int quantityInStock = 0;
Comments – Single-line:
• •
public int shirtID = 0; // Default ID for the shirt public double price = 0.0; // Default price for all shirts
•
// The color codes are R=Red, B=Blue, G=Green
– Traditional: • • •
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/******************************************* * Attribute Variable Declaration Section * *******************************************/
Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Methods • • •
• • • • • • • •
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– Syntax: [modifiers] return_type method_identifier ([arguments]){ method_code_block }
– Example: public void displayInformation() { System.out.println("Shirt ID: " + shirtID); System.out.println("Shirt description:" + description); System.out.println("Color Code: " + colorCode); System.out.println("Shirt price: " + price); System.out.println("Quantity in stock: " + quantityInStock); } // end of display method
Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Keywords abstract
default
for
package
synchronized
assert
do
if
private
this
boolean
double
implements
protected
throw
break
else
import
public
throws
byte
enum
instanceof
return
transient
case
extends
int
short
true
catch
false
interface
static
try
char
final
long
strictfp
void
class
finally
native
super
volatile
continue
float
new
switch
while
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Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Creating and Using a Test Class • Example: class ShirtTest { public static void main (String[] args) { Shirt myShirt; myShirt= new Shirt(); myShirt.displayInformation();
} }
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main Method – A special method that the JVM recognizes as the starting point for every Java technology program that runs from a command line – Syntax: public static void main (String[] args)
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Compiling a Program 1. Go to the directory where the source code files are stored. 2. Enter the following command for each .java file you want to compile. – Syntax:
•
javac filename
•
javac Shirt.java
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– Example:
Executing (Testing) a Program 1. Go to the directory where the class files are stored. 2. Enter the following for the class file that contains the main method: • Syntax: •
java classname
• Example: •
• • • • •
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java ShirtTest
• Output: Shirt ID: 0 Shirt description:-description requiredColor Code: U Shirt price: 0.0 Quantity in stock: 0
Compiling and Running a Program by Using an IDE
Save is equivalent to javac. Run is equivalent to java.
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Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Avoiding Syntax Problems
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Topics – Define a class, identify class components, and use variables – Discuss methods and the use of a main method – Identify keywords – Test and execute a simple Java program – Describe some common causes of syntax errors – Describe the purpose and features of an IDE debugger
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Working with an IDE Debugger
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Summary • In this lesson, you should have learned how to: – – – – – – – – – –
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Define a class Identify the components of a class Explain the term object Describe the purpose of a variable Discuss methods and describe how to use a main method Describe the elements that comprise a Java class, such as declarations, return values, and the proper use of brackets and braces Identify keywords and describe their purpose Test and execute a simple program Describe some common causes of syntax errors Describe the purpose and features of an IDE debugger
Practice for Lesson 3 Overview: • In this practice, you are given a completed Java program. During the practice, you will: – – – – –
Open the Java program Examine the lines of code Modify the program Compile the program Test it by executing the program
• In this practice, you create a Java class and compile it. You also create another Java class to test the previous class. • In this practice, you debug the ShirtTest program by using the NetBeans debugger. During the practice, you will: – Set a breakpoint – Examine and modify field values – Use a step through
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