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Introduction to object
oriented programming in
JAVA
Computer Applications in Power Systems – Advance course EH Dr. Arshad Saleem Lecture material contribution from: John Lewis and William Loftus. Java Software Solutions, foundation of program design
contents
- Installation and setup
- OOP architecture revisit
- Programming in JAVA
- Working together
- Hello World – the first program
- Declaring variables and assignments
- Implementing basic structures
- Conditions and continuations
- Implementing inheritance in JAVA
- Working individually/ in groups
- Implementing a class diagram
- Implementing class relationships
- Implementing methods
- Implementing use cases
- HomeWork
Java Program Structure
- A program is made up of one or more classes
- A class contains one or more methods
- A method contains program statements
- A Java application always executes the main method
HelloWorld.java
//HelloWorld, my first program in Java
class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World...")
try{System.in.read();}
catch(Exception e){}
}// method main
}// class HelloWorld
Importing Packages
- Using a class from the Java API can be accomplished by using its fully qualified name: java.lang.System.out.println ();
- Or, the package can be imported using an import statement , which has two forms: import java.applet.*; import java.util.Random;
- The java.lang package is automatically imported into every Java program
Java Applets
- A Java applet is a Java program that is intended to be sent across a network and executed using a Web browser
- A Java application is a stand alone program
- Applications have a main method, but applets do not
- Applets are derived from the java.applet.Applet class
- See Confucius.java and No_Parking.java
- Links to applets can be embedded in HTML documents
Variables
- Multiple variables can be declared on the same line: int total, count, sum;
- Variables can be initialized (given an initial value) in the declaration: int total = 0, count = 20; float unit_price = 57.25;
- See Piano_Keys.java
Assignment Statements
- An assignment statement takes the following form: variable-name = expression ;
- The expression is evaluated and the result is stored in the variable, overwriting the value currently stored in the variable
- See United_States.java
- The expression can be a single value or a more complicated calculation
Constants
- When appropriate, constants are better than variables because: - they prevent inadvertent errors because their value cannot change
- They are better than literal values because:
- they make code more readable by giving meaning to a value
- they facilitate change because the value is only specified in one place
BasicMath.java
Circumference is: 37. Area is: 113. class BasicMath { public static void main (String[] args) { final double PI = 3.14159; int radius = 6; double area, circum; circum = PI * radius * 2; area = PI * radius * radius; System.out.println("Circumference is: " + circum); System.out.println("Area is: " + area); try{System.in.read();} catch(Exception e){} }//method main }//class BasicMath
Input and Output
- The Java API allows you to create many kinds of streams to perform various kinds of I/O
- To read character strings, we will convert the System.in stream to another kind of stream using: BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in), 1);
- This declaration creates a new stream called stdin
Echo.java
import java.io.*; class Echo { public static void main (String[] args) throws IOException { BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in),1); String message; System.out.println ("Enter a line of text: "); message=stdin.readLine(); System.out.println ("You entered: "" + message + """); } }
The if Statement
condition false statement true
The if-else Statement
condition statement true false statement
