Transcript Java: Getting Started

Java
Getting Started
"Hello, World" Application
public class Hello {
public static void main (String args[ ]) {
System.out.println ("Hello, World!");
}
}
• Memorize this:
public static void main (String args[ ])
• Use System.out.println for output
Compiling a Java program
• The program defined public class Hello
• Therefore, the file must be named Hello.java
• Compile the program with javac Hello.java
– This creates a file named Hello.class
– Hello.class contains Java byte code
• Run the program with java Hello
– This runs the Java virtual machine (interpreter)
"Hi, World" Applet
import java.applet.*;
import java.awt.*;
public class HiWorld extends Applet {
public void paint (Graphics g) {
g.drawString ("Hi, world!", 50, 100);
}
}
No main method
• Yes, applets have a main method...
• ...but it's in the browser, not in your code!
import Statements
• import java.applet.*; makes available all the
classes and objects in the java.applet package.
• import is not #include -- it doesn't make the
program bigger, it just makes things easier to
reference.
• Without import java.applet.*; you could still
say java.applet.paint (...)
instead of paint (...)
The paint method
• public void paint (Graphics g) {
g.drawString ("Hi, world!", 50, 100);
}
• This is a method you supply in class HiWorld
• It overrides the predefined paint (Graphics)
method in Applet
• If you don't define this method, you get the
inherited method (which does nothing!)
"Hi, World" Applet (repeat)
import java.applet.*;
import java.awt.*;
public class HiWorld extends Applet {
public void paint (Graphics g) {
g.drawString ("Hi, world!", 50, 100);
}
}
The Person class
class Person {
String name;
int age = 20;
}
void birthday ( ) {
age++;
System.out.println (name + " is now "+ age);
}
The People program
public class People {
public static void main (String args [ ]) {
Person john;
// "People" uses "Person"
john = new Person ( );
Person mary = new Person ( );
}
}
john.name = "John Doe";
john.birthday ( );
Sending a message
• You don't "call a function," you send a message
to an object
• The object may execute one of its own methods
• The object may execute an inherited method
• You generally need a pretty clear idea of what
methods are being inherited
toString
• toString( ) is defined at Object, and is
therefore inherited by every object
• System.out.println, when given any
object, automatically calls that object's
toString method
• The resultant output is better than nothing
• It's a good idea to define toString( ) for
every class you write
An example toString( ) method
class Person {
String name;
int age;
}
public String toString ( ) {
return name + ", age " + age;
• Returns something like "John, age 34"
Parameter transmission
• In Java,
– primitives (int, double, char, boolean, etc.) are
passed by value
– objects (Person, String, arrays, etc.) are passed
by reference
• This rule is simple and easy to work with
• You never accidentally get a copy of an object
• If you need to copy an object, do it "by hand"
Procedural thinking
• Procedural programs use functional
decomposition
– Decide what program is to do
– Write a top level procedure, inventing new
lower-level procedures as you go
– Write lower-level procedures
– Continue until program is fully coded
• Goal: small, relatively independent parts
Object thinking
• Decide what program is to do
• Decide what objects you need
• For each object, decide
– what are its responsibilities
– with which other objects it must collaborate
• Goal: small, relatively independent parts
The End