Transcript Introduction to Java

Introduction to Java
Java program example
/*
Introduction to OOP with Java 4th Ed, McGraw-Hill
Wu/Otani
Chapter 2 Sample Program: Displays a Window
File: Ch2Sample1.java
*/
import javax.swing.*;
class Ch2Sample1 {
public static void main(String[] args) {
JFrame myWindow;
myWindow = new JFrame();
myWindow.setSize(300, 200);
myWindow.setTitle("My First Java Program");
myWindow.setVisible(true);
}
}
Running the program
produces this result:
How it works
• Declares a class named Chapter2Sample1,
including a single method called main which
contains all of the instructions in the program
• Declares a JFrame object named myWindow
• Sends messages to the object requesting it to set
its size and title, then display the window
Line-by-line analysis
/*
Introduction to OOP with Java 4th Ed, McGraw-Hill
Wu/Otani
Chapter 2 Sample Program: Displays a Window
File: Ch2Sample1.java
*/
• The program begins with a comment – all of the
text between the /* and */ symbols is not, strictly
speaking, code – the markers indicate that the
computer can ignore this section, which contains
information strictly for the human reader – later,
we’ll look at comments in more depth
Line-by-line analysis
import javax.swing.*;
• import statement: allows the program to
use code from a package in the Java API –
in this case, we’re importing code from the
swing package, which contains class
definitions for graphical objects, like
windows
Import statements
o import statements always appear at the beginning of a
program file
o syntax: import packagename.classname;
o example of this syntax: import javax.swing.JFrame;
o this syntax indicates that we want the JFrame class, which is part
of the swing package, which is a subpackage of a larger package
called javax
o a package is simply a grouping of related Java classes – later on,
we’ll see how to create our own packages
o note the semicolon – in Java, this indicates the end of an
instruction
o the word import is a Java reserved word – that is, this is a symbol
that belongs to the Java language, and it can only be used in an
import statement
Import statements
o We can choose to import an entire package rather than a
single class – then the syntax would be import
packagename.*;
o Our program example uses this syntax – the line
import javax.swing.*;
indicates we wish to import the entire swing package
o this is more common than importing a single class
o import statements are often used when we want to incorporate code
from the Java standard classes
o it would have been possible to write this program without the
import statement, but that would have made other statements more
complicated – we’ll see how in a moment
o there are usually multiple import statements in a program
Analysis continued
• Class declaration: every Java program
consists of one or more classes
• There must be at least one main class (that
is, a class containing a main method) –
program execution starts with the first
instruction in this main method
Class declaration
• Class declaration syntax:
class classname {
class member declarations
}
• In our example program, the class declaration
looks like this:
class Ch2Sample1 {
/* main method declaration here */
}
Class declaration
class classname {
class member declarations
}
• Note the curly brackets – these indicate the
beginning (left bracket) and ending (right bracket) of a
block of code
• The contents of a class are always contained within a code
block
• The same is true for the body of a method – in fact, a
method is always a subblock within the block containing
the class
• Most programs contain additional subblocks, which are
enclosed within methods
Analysis continued
• Method declaration: a method declaration consists of
a heading and a body containing a set of instructions
to be executed to perform a task
• The general syntax for method declaration is:
modifier(s) returntype methodname (parameter(s)) {
instructions
}
Method declaration
• The main method header always looks exactly like
this:
public static void main (String [ ] args) {
– For the record, public and static are modifiers, void is
the return type, main is the name of the method, and
String[] and args are the data type and name of the
parameter, respectively
– The purpose and meaning of all of these things will
become clear during the semester – for now, just
memorize this sequence of words and symbols, you’ll
be using them a lot
Analysis continued
• Instructions: the body of main consists of a
sequence of program instructions – when the
program executes, each instruction will be
performed exactly once, in the order given
• Every instruction ends with a semicolon (;)
• Although it isn’t necessary from the computer’s
point of view that every instruction be on its own
line of code, it’s important in terms of human
readability, which is an important factor in writing
code that is easy to maintain
Instructions
• The instructions in this program include:
– an object declaration
– an object creation
– three messages
Object declaration
• Object declaration: designates the name of
an object and the class to which it belongs
– syntax: classname objectname;
– example: JFrame myWindow;
– you can declare more than one object in a
single line of code – for example, the following
code declares two JFrame objects:
JFrame window1, window2;
Object declaration
• In order to declare an object of a particular class, the class
must already be defined – in this instance, we are using
one of the standard classes from the Java API
• JFrame is a class defined in the swing package, which is
part of the javax package – the import statement we
looked at earlier made JFrame and all other swing classes
available, as you may recall
• Without the import statement, we still could have declared
a JFrame object, but instead of just the class name we
would have had to use JFrame’s fully qualified name,
which lists the package(s) it belongs to – in that case, our
declaration would have looked like this:
javax.swing.JFrame myWindow;
Object declaration
• The name of an object is an identifier chosen by
the programmer – a valid Java identifier has the
following characteristics:
• A sequence of one or more letters, digits,
underscores (_) and/or dollar signs ($) – no spaces
are allowed
• The first character must be a letter
• Java reserved words can’t be used for identifiers –
for example, you can’t name an object “import” or
“public” or “class”
Java identifiers
• Java is case-sensitive: this means the language
distinguishes between upper and lowercase
characters (therefore, although it’s not advisable,
you could create an object named “Class”)
• The Java standard naming convention adds
these additional guidelines:
– Class names start with a capital letter
– Object names start with a lowercase letter
– If an identifier contains multiple words, the first letter
of every word (except the first if the thing being named
isn’t a class) is capitalized
Analysis continued
• object creation: a declaration simply reserves a name;
to actually create an object, memory must be allocated
for storage – this operation is accomplished using the
new operator
• syntax: objectname = new
classname(argument(s));
• example: myWindow = new JFrame();
Object creation
• The object creation statement is a message to a
special method of the class called the class
constructor
• In this example, there are no arguments passed to
the constructor – we will look at arguments when
we examine messages, below
• It is possible to declare and create an object using
a single line of code, as in the example below:
JFrame myWindow = new JFrame();
Analysis continued
• Messages: a message is a request to an object (or
class) for some task to be performed – the message
specifies the name of the method requested (which
must be defined within the specified object’s class)
and any data that must be passed to the object in
order for it to accomplish the task
• Syntax:
objectname.methodname(argument(s));
Messages
• There are messages in the sample program – they
are:
myWindow.setSize(300, 200);
myWindow.setTitle("My First Java Program");
myWindow.setVisible(true);
Messages
• Each of the messages is addressed to an object named
myWindow
– the first one calls the setSize method and sends it 2 arguments
which specify the width and length of the window to be created
– the second one calls the setTitle method and passes it a single
argument in the form of the string literal “My First Java Program”
– this argument specifies the title of the window
– the third one calls the setVisible method, with the argument true –
true is a Java reserved word – this method displays the window – if
we passed the opposite argument to the method (false), the
window would disappear
Comments
• Comments: provide internal documentation for your code - we have
already seen an example of a comment – there are 3 types:
• multiline comments: this is the kind we have seen – any text enclosed
between the 2-character symbol /* and the symbol */ is considered a
comment
• single line comments: a comment that begins with the 2-character symbol
// and ends at the end of the line of text containing it – this type of
comment is often used to provide narrative explanation for code on the
same line – for example:
myWindow.setVisible(true);
// displays window
• javadoc comments: specialized type of comment that can appear before
class declarations, method declarations, and other program elements – this
type of comment is used to create external documentation in the form of
a web page– we will look at this type of comment in depth later in the
semester