Transcript TCU CoSc 10403 Programming with Java

TCU CoSc 10403
Introduction to Programming
(with Java)
Getting to Know Java
Problem Solving
• The purpose of writing a program is to solve a
problem. Problem solving consists of multiple
steps:
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Understanding the problem.
Breaking the problem into manageable pieces.
Designing a solution.
Implementing the solution.
Testing the solution and fixing any problems that exist.
The Java Programming Language
• A programming language defines a set of rules:
– That determine exactly how a programmer can combine
words and symbols of the language into
programming statements.
• Programming statements are the instructions that are carried
out when the program is executed.
– Java is an object-oriented programming language
(more about that later).
• Java is one of the fastest growing programming technologies of
all time.
Reasons Why Java is Popular
• Java is the first programming language to deliberately
embrace the concept of writing programs that can be
executed on the Web.
• Moreso, Java is a useful general purpose programming
language.
• The Java language is accompanied by a library of extra
software that we can use when developing programs.
– The library provides the ability to create graphics,
communicate over networks, and interact with databases.
– The set of supporting libraries is huge and versatile.
I’ve been telling you that Java is an
Object Oriented Programming Language
- what exactly does that mean?
To understand - we need to discuss and
understand what an Object is and how it
relates to a Class!!
Anatomy of a Java class
A class is a description of a template (or pattern) that
specifies the types of data values that an instance of the
class can hold and the operations that it can perform.
In OOP, rather than describe each and every object, the
programmer describes the general type of an object (a class)
and then uses the class to create (or instantiate) as many of
the objects as are needed.
Example: a class describing”generic” students at TCU.
Instance variables - the data that is maintained for students
(ex., name, address, age, …)
Constructors - methods that are involved in the construction
of an instance of a class.
Methods - subprograms that permit instance variables to be
accessed and modified (ex., setName(), setMajor(),
getAge(), setAge(), …)
Example: consider a banking problem
A class
(the concept)
Bank
Account
Multiple objects
from the same class
An object
(the realization)
John’s Bank Account
Balance: $5,257
Bill’s Bank Account
Balance: $1,245,069
Mary’s Bank Account
Balance: $16,833
Let’s discuss these concepts further!!
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A class -- the basic building block of an object-oriented language.
•
It is a template/pattern that describes the data and behavior associated with
instances of that class.
•
When you instantiate a class you create an object that looks and feels like other
instances of the same class.
– The data associated with a class or object is stored in instance variables;
– the behavior associated with a class or object is implemented with methods. Methods
are similar to the functions or procedures in other procedural languages.
** Some author wrote: Julia Child's recipe for rack of lamb is a real-world example of a class. Her
rendition of the rack of lamb is one instance of the recipe, and mine is quite another. (While both racks
of lamb may "look and feel" the same, I imagine that they "smell and taste" different.)
Constructors
•
A constructor:
– is a special method that is used to set up a newly created object
– often sets the initial values of variables (i.e., it can be passed
information that can be used to initialize some variables in the object)
– has the same name as the class
– does not return a value
– has no return type, not even void
• The programmer does not have to define a constructor for a class
(there is a default constructor for every class – its syntax is:
theNameOfTheclass()
• For example, a class named Student could have a constructor that
receives a parameter specifying the Student’s name:
Student tommy = new Student(“Tom Jones”);
Inheritance
•
Classes are defined in a hierarchy
•
A class can extend (inherit from) some other class
Object => Component => Container => Panel => Applet
•
Thus, an Applet can do everything that Panel does and some other things. A
Panel can do everything a Container can do, etc…
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Advantage
– Easy to extend or specialize something that already exists. Note: we take
advantage of this when we write:
public class HelloWorld extends Applet
{ … }
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Disadvantage
– Sometimes difficult to find everything that a class does; you may have to
look up the tree.
Inheritance
• One class can be used to derive another via inheritance.
• A class can extend (inherit from) some other class
Person class (called a superclass)
Subclasses or
“derived” classes
- may have more
than just 2 levels.
Student class (extends Person)
Faculty class(extends Person)
Administrator class(extends Person)
Java Components
• Package (what the import statement brings in!!)
– collection of related classes
• Class
– description of an object (template)
– variables + methods
• Variable – name for memory location for a data item
• Method – specifies an action on an object
– retrieve/return info or change info
• Object
– instance of a class
– created by the “new” operator
• Java program = collection of cooperating objects
Creating Objects
jackSmith
• The new operator creates a new object from a class:
• Example:
Person jackSmith = new Person ();
Person sallyJones = new Person();
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This declaration asserts that jackSmith and
sallyJones are variables that refer to objects
created from the Person class.
Instance of
the Person
class
containing a
copy of all
instance
variables and
all methods
sallyJones
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They are initialized (set to point) to the objects
created by the new operator
• The newly created object is actually created and
allocated to memory by a call to a very special method
contained within the class - a constructor of the class.
Instance of
the Person
class
containing a
copy of all
instance
variables and
all methods
Choosing Identifier Names
Identifier names should be descriptive.
• Avoid meaningless names such as a or x.
• Unless the name is actually descriptive, such as using x and y to
represent (x, y) coordinates.
– Avoid using unnecessarily long names (unless they
promote better readability - example: yearToDateSalary).
• A “name” in Java is a series of identifiers separate by
the dot (period) character.
– The name System.out is the ‘name’ of an object through
which we invoke the println method. (println is a method
inside the object.)