Transcript Chapter 2 Your First Java Application

Chapter 2
Your First Java
Application
2
Program Concepts
Modern object-oriented programs
help us build models to manage the
complexity found in a problem
domain.
The problem domain describes real-world
objects and concepts that a computer
program is trying to solve.
2
Models
A model is a simplification of a
complex system. A good model:
Helps to identify the most important
aspects of a problem.
Helps a programmer to focus on the
problem he or she is trying to solve
instead of on the complexity of the
problem’s data.
2
What is a metaphor?
A metaphor is a word or phrase used
in place of another word or phrase to
denote a likeness.
Computer programs use “metaphors” to
represent real objects.
2
Objects
Objects represent real-world things:
Ship
Rudder
Wheel
Instruments
Objects have:
properties (characteristics)
methods (behaviors)
2
Classes and Objects
A class is a definition of a type:
Like a template, a class defines the
characteristics and behaviors of the type.
An object is an instance of a class:
Can be instantiated and manipulated
An object’s characteristics are defined by
the class that was used to create the
object.
2
Fields
Fields define the properties of a class.
Can be intrinsic types (int, boolean…)
Can be user-defined objects
State is the current value of a field in
an object.
2
Methods
Methods describe the capabilities of
the class.
Every method must be called from
another method. The only exception is
main(), which is called by the OS.
Methods can accept parameters.
2
Object Relationships
Association
A method of one object calls the method
of another object.
Composition
Some objects are composed of other
objects.
2
Specialization
Hierarchies of classes
move from a
generalized class to a
more specific class.
2
Creating Programs
Syntax and Semantics
A language’s exact keywords,
punctuation, and order of terms are called
its syntax.
Semantics refers to the meaning of one’s
instructions; what the program is trying
to accomplish.
2
Writing a Java Program
Writing a Java program requires five
steps:
Analyze
Design
Write
Compile
Test
2
Compiling the Code
Programs must be compiled in order
for the CPU to understand and execute
the instructions.
Java byte code is run through the Java
Virtual Machine (JVM), which translates
the code for the CPU.
.java files are compiled by the javac (java
compiler) program into .class files.
2
Types of Java Programs
Applet
Requires an .HTML file with the <applet> tag to
reference the .class file
Can also be run using the appletviewer.exe
program file
Application
Run by the java.exe program file
Run without a browser interface, often using the
console window
2
HelloWorld.java Analysis
The keyword import allows existing
class libraries to be reused.
The keyword public signifies that a
class is visible to other classes.
Braces {} indicate the beginning and end
of a block of code.
Braces must always be “balanced”
Align opening and closing braces
2
More HelloWorld.java Analysis
The keyword void signifies that a
method does not return a value.
Identifiers are the names given to classes,
fields, and methods.
Identifiers are case sensitive.
Identifiers often use camel case notation.