Download Java Programming Basics

Java Programming
Basics
SE-1011
Dr. Mark L. Hornick
1
Computer of the future?
SE-1011
Dr. Mark L. Hornick
2
Algorithms are implemented in
Java methods

A method is a named collection
of Java instructions that
implement an algorithm

The method for actually computing
the number of days between two
dates might be named
computeDays

The method names - or method
identifiers - are made up by you
(…but you have to follow some
rules…)
SE-1011
Dr. Mark L. Hornick
3
Good and bad method
identifiers
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computeDays
printResult
getStartDate
days
getIt
method1
PRINTit
x109
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Good method names are
 meaningful
 use verbs that imply an
action
 use camel-case format

Bad method names are
 Vague or meaningless
 don’t use verbs
 use an unconventional
format
SE-1011
Dr. Mark L. Hornick
4
Java is also case-sensitive…

This means that the following method
identifiers are considered separate and
distinct:
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getStartDate
getStartdate
Avoid creating identifiers that differ only in capitalization
SE-1011
Dr. Mark L. Hornick
5
Java’s reserved words cannot
be used as identifiers
abstract default if private this
boolean do implements protected throw
break double import public throws byte
else instanceof return transient case
extends int short try catch final
interface static void char finally
long strictfp volatile class float
native super while const for new
switch continue goto package
synchronized
Don’t create identifiers that differ from reserved words
only in capitalization (e.g. Continue)
SE Focus
Dr. Mark L. Hornick
6
Algorithms range from trivially
simple to incredibly complex

An extremely simple program may
consist of a single algorithm that just
prints something to the screen

Realistic programs typically implement
multiple algorithms of varying complexity
SE-1011
Dr. Mark L. Hornick
7
If an algorithm is complex, it is
usually broken into multiple methods

Normally, each method handles a
single task

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this keeps things organized and
simpler
Sub-tasks are delegated to
subordinate methods

Structuring a program into tasks and
sub-tasks is a skill you’ll develop as
you gain experience creating Java
programs
SE-1011
Dr. Mark L. Hornick
8
Related methods are grouped
together in named classes

Normally, the methods with a
class share information, but hide
information from other classes

This keeps information from being
changed or corrupted accidentally

However, there are ways for
methods of one class to send or
receive information from methods of
another class
SE-1011
Dr. Mark L. Hornick
9
Good and bad class names
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DayCalculator
Lab2Program
TVController
days
Calculate
Class1
x109

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Good class names are
 meaningful
 use nouns that imply the
purpose
 Start with an uppercase
letter
Bad class names are
 Vague or meaningless
 don’t use nouns
 use an unconventional
format
SE-1011
Dr. Mark L. Hornick
10
Every Java program must have
a class that contains a primary
method, named main

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This name main (not a verb) exists for
historical reasons, and cannot be
changed
The class containing the main method
can be named anything (that makes
sense), like DayCalculator

The class containing the main method,
regardless of it’s actual name, is referred to
as the main class
SE-1011
Dr. Mark L. Hornick
11
How a Java program gets
executed
sd Sequence Diagram
MainClass
DayCalculator
user
Windows OS
Java VM
start program
load main class of program
main(optional_arguments)
<done>
<done>
<done>
SE-1011
Dr. Mark L. Hornick
12
Summary: The basic Java
Program
To create a runnable program, we must first define a main
class that represents our program

We can give the main class any reasonable name, like
“MyMainClass”
class Basic Jav a Program
MyMainClass
+
main(String[]) : void
When we “run” the program, the Java Virtual Machine (VM)
accesses the main class we defined
Then the VM sends a message to our program’s main
class that tells it to run.
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The “run” message is sent by the VM as a call to a method
named main that our program’s main class must have
defined.
SE-1011
Dr. Mark L. Hornick
13
A Java program is composed of
one or more classes

One of the classes in the program must be the
main class
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That is, it must have a method called main()
The main class itself can have any (valid) name
All of the Java instructions for a class and a
class’s methods must reside in a file having
the same name as the class

DayCalculator.java must be the name of the file
containing all the Java instructions for a class
named DayCalculator.
SE-1011
Dr. Mark L. Hornick
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Java Edit-Compile-Run Cycle
Step One: Create a program with an editor.
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This is where you type in the Java instructions,
using a text editor (or something like Eclipse),
and save the program to a file.
The name of the class has to match the name of
the file containing the class and have the .java
file extension.
This is called a source file.
SE-1011
Dr. Mark L. Hornick
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Java Edit-Compile-Run Cycle

Step 2: Compile the source file.
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The process of compiling the source file creates a
bytecode file, which is not human-readable.
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The name of the compiler-generated bytecode file
will have the suffix .class while its prefix is the
same as the source file’s.

DayCalculator.java is compiled, creating
DayCalculator.class, the bytecode (or class) file
SE-1011
Dr. Mark L. Hornick
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Java Edit-Compile-Run Cycle

Source file vs. its bytecode file:
SE-1011
Dr. Mark L. Hornick
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Java Edit-Compile-Run Cycle
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Step 3: Execute the bytecode file.
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The java interpreter (VM) will go through the
bytecode file and execute the instructions in it.
If an error occurs while running the program, the
interpreter will catch it and stop its execution.
The VM starts execution at the bytecode
instructions that correspond to the Java statement
public static void main()
SE-1011
Dr. Mark L. Hornick
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Java Edit-Compile-Run Cycle

The result after the interpreter executes the
instructions in the bytecode file.
SE-1011
Dr. Mark L. Hornick
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Besides Java instructions, programs also
contain comments which state the purpose
of the program, explain the meaning of
code, and provide other descriptions to help
others to understand your code.
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Comments are not compiled or executed
Comments are just text you make up
Comments augment the program by providing more
understandable, human-friendly information that
help readers of your program understand what you
have written.
SE-1011
Dr. Mark L. Hornick
20
A program template to use as the
starting point for all Java
applications
SE-1011
Dr. Mark L. Hornick
21
In Java, classes are further
grouped into logical packages

Packages provide a further means of
organizing related classes
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You create package names yourself
Package names start with lowercase
Classes in the same package reside in the same
file folder, where the folder is the package name
SE-1011
Dr. Mark L. Hornick
22
Quiz 1 tomorrow at start of Lab
Learning outcomes since the start of the course:
 Define the term program.
 Define the term algorithm.
 Why do we use pseudocode?
 Why do we use flowcharts?
 How does a Java pseudocode variable differ from a variable you
use in Math?
 List some possible pseudocode variables.
 Given psuedocode, draw the corresponding flowchart.
 Given a flowchart, write the corresponding pseudocode.
 What two types of control-flow does a diamond represent in a
flowchart?
SE-1011
Dr. Mark L. Hornick
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