Transcript System.out.println(“Hello World!”)

JAVA PROGRAMMING WITH ECLIPSE PART 1
PLEASE READ ALL THE CONTENT ON THE SLIDES. WATCH THE RECOMMENDED VIDEOS LISTED.
DOWNLOAD AND INSTALL SOFTWARE
 Install Java JDK 8 (JDK: Java Development Kit)
 Install Eclipse IDE for Java Developers
 Eclipse is a IDE: Integrated Development Environment to write Java code in.
 Both Java and Eclipse are supported on Windows and OSX
 Download links are provided on the MSE101 ACCESS Website
POSSIBLE ASSIGNMENTS
 Possible Assignments
 http://www.java-made-easy.com/fun-java-game.html
 Create a Java Class that takes in a user input
THE STEPS FOR CREATING & RUNNING A JAVA
PROGRAM
A program is a sequence of instructions that perform a task.
4
STEPS FOR CREATING & RUNNING A PROGRAM
1. Write the source code in a
“anyFileName.java” file. Note the
.java extension.
2. The javac program will Compile
the source code into byte code
creating a .class file
3. JVM (Java Virtual Machine) will
interprets the byte code into
machine language 1s and 0s and
the computer then executes the
instructions
5
SOURCE CODE IS WRITTEN IN A .JAVA FILE
Step1. When a programmer writes Java code, it is stored in a source code file that has a .java
extension like SampleCode.java.
The source code file is what the programmer compiles and runs to see the output of a program.
We will write our Java progams in the IDE software called Eclipse. IDE stands for Integrated
Development Environment. An IDE lets you write, compile, and execute programs.
Source code can be typed in any simple text editor, but we will use Eclipse, which is both a text
editor and a compiler.
Eclipse is a nice Java editor and compiler because it will point out syntax errors when java code
is typed incorrectly. A program will not run until all syntax errors are removed.
6
SOURCE CODE IS COMPILED TO BYTE CODE
Step 2. An IDE (Integrated Development Environment) like Eclipse, has a built in compiler
that translates the Java source code into Java byte code.
When source code is compiled into byte code, the byte code is stored in a file that has the
same name as the source code file but with a .class extension. It is not a text file and
you cannot open it! For example, the source code file RobotMan.java will be compiled
into a byte code file named RobotMan.class.
The file is created during the compiling process, however, Eclipse sometimes translates .java files
into .class files behind the scenes before you ever compile them. You can see these files in
your package folders if you open your workspace folder on your hard drive.
7
BYTE CODE IS INTERPRETED TO MACHINE CODE
Step 3. The JVM (Java Virtual Machine) must interpret the byte code into machine language.
Interpret basically means to translate. The JVM is software that acts like hardware. That is
why it is called a virtual machine. Java bytecodes are translated on the fly to 1s and 0s
instead of being stored in an executable file, which some languages do. The main advantage
of an interpreter is that it will run on any computer and there are JVMs for Apple and
Windows machines. This makes the code portable from one platform to another.
Every kind of computer, whether it is an Apple, Windows, or Unix machine, has an operating
system that contains a JVM software component. The JVM was automatically installed when
your computer’s system software was installed and updates to the JVM may be contained in
any system software updates you install now or in the future.
8
WHAT ARE APPLETS?
Applets are small Java programs that are embedded in web pages. When you load a web page
that has a game in it, the game is an applet.
More specifically, the applet is usually a jar file that contains the byte code for all the .java files
that are needed for the game. A jar file is like a zip file, where a number of files are
compressed into one file.
An HTML web page can contain a special kind of tag called an Applet tag. That tag tells the
web page that the byte code is stored in a jar file with a specific name.
The browser software, like FireFox, Chrome, Camino, or Safari will then use its built-in JVM
interpreter to translate the byte code into machine language 1s and 0s.
9
JUST-IN-TIME COMPILATION BY THE JVM
Some JVMs support just-in-time compilation (JIT). JITs will translate
byte code instructions the first time they are encountered into machine
language code and that code is saved so that the next time that code is
encountered it can be executed without being re-interpreted. This can
speed up the interpreting of the byte code of a program so it will run
faster.
10
MACHINE LANGUAGE CODE IS EXECUTED
Step 4. Finally, the machine language
code (patterns of 1s and 0s), those
instructions that the computer
understands at the lowest level, are
executed. This allows the program
to appear in its runnable form, either
in a console window, an applet
window, or a GUI (Graphical User
Interface) window. Again the overall
process is ….
11
SECTION 2
FRAMEWORK OF A SIMPLE JAVA PROGRAM
1. PACKAGE DECLARATION
2. IMPORT STATEMENTS
3. CLASS DECLARATION LINE
4. MAIN METHOD
12
FRAMEWORK OF A SIMPLE JAVA PROGRAM
The Framework (in order) of a simple console or standalone program:
1.
package declaration for the file listed first (where it is stored)
2.
followed by import statements (if any)
3.
class declaration line
Sample
4.
main method
import java.util.Scanner;
public class DistanceCalculator
{ // opening curly brace for the DistanceCalculator class
public static void main(String args[ ])
{ // opening curly brace for main method
Skeleton
Program
Note:
• Curly braces always appear in pairs.
• All comments start with two slashes //
// code for program goes inside these curly braces
} // ending curly brace for main method
} // ending curly brace for the DistanceCalculator class
All Java console and JFrame (standalone) programs have this framework. The framework for applets is different and you will
become familiar with it later.
13
CLASSES IN JAVA
A Java program can use more than one class file (a .java file). In fact, it may be made up of many .java files.
And most of the time, every .java source code file in a Java program contains only one class.
However, it is possible to have more than one class in a .java file, but this is usually done in more
advanced programming projects where the code is more complicated and there is a reason for doing
it.
So assume that every .java file contains only one class, but a java program can use one or more .java files.
A class is a module of code that can stand alone by itself and if made “public” other files can “see it” and
“access parts of it”. Eclipse is a nice IDE for organizing Java files so that classes are accessible to each
other if they are in the same package folder or if an appropriate import statement is used to direct
Eclipse to where a package is located. Eclipse manages this behind the scenes automatically and lets
you know if something is “unorganized”, so you can fix it.
14
DECLARING CLASSES IN A FILE
A class is declared with a line like:
public class RobotMan
and has a set of curly braces that go with it { and }.
In Java, a class like RobotMan must be stored in a file named RobotMan.java. If they don’t
match, then Eclipse or any other IDE will flag it with an error and it must be corrected
before the file is used in a program. Eclipse has short cuts to renaming the file (compilation
unit) or the class if necessary. You’ll learn about that soon.
Simple Java programs can be designed so that they contain just one class. We will now look at
some examples.
15
SECTION 3
THREE TYPES OF
JAVA PROGRAMS
16
THREE TYPES OF JAVA PROGRAMS
There are basically three types of Java programs that a programmer can
make:
1.
A console text output program.
2.
A standalone graphics or GUI program that can be displayed in a
standard window frame.
3.
An applet graphics or GUI program that is embedded in a web page.
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CONSOLE TEXT OUTPUT PROGRAMS
Some Java programs, like RobotMan, are displayed only in a console window, because
they show only text output. You will learn how to do more than draw things with
text characters in a console window. Some programs may make mathematical
calculations and display them.
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GRAPHICS & GUI PROGRAMS
Besides console text programs, you can have applet or standalone programs that contain graphics or GUI components:
1.
GUI components are elements in a program like text fields, buttons, labels, text areas, or menus.
2.
Graphics are lines, and curves, and pretty much anything that can be drawn.
So you can have:
1.
A web applet program with GUI components.
2.
A web applet program with graphics.
3.
A standalone program with GUI components.
4.
A standalone program with graphics.
5.
A web applet or standalone program that has both GUI components and graphics.
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APPLET GUI OUTPUT PROGRAMS
Java applets are small Java programs downloaded from web pages that run in a Web
browser.A JVM is incorporated into every browser, so the applet can be
interpreted, executed and displayed inside the browser. Both applet and standalone
programs can have GUI components.
A GUI program with fields, buttons, labels, and a text area.
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SECTION 4
DATA TYPES AND THE BASICS
In this section, you will learn
very AND
important
information about:
OFsome
INPUT
OUTPUT
1.
print and println Statements
2.
Variables
Simple data types
The assignment operator =
How to declare and construct a Scanner object so you can read input from the
keyboard
The new operator
3.
4.
5.
6.
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JAVA IS HOT SOURCE CODE
public class JavaIsHot
{
public static void main(String args[ ])
{
System.out.println( "
d
");
System.out.println( "
o l
");
System.out.println( "
l r
");
System.out.println( "
l o
");
System.out.println( "
e w
");
System.out.println( "
H
");
System.out.println( "
xxxxxxxxxxxxxxxxx
System.out.println( "
x
System.out.println( "
x
System.out.println( "
x
System.out.println( "
x is Hot! x
System.out.println( "
x
System.out.println( "
xxxxxxxxxxxxxx
x x
Java
");
x x
xxxx
x
");
");
");
");
");
");
} // end of main method
} // end of JavaIsHot class
System.out.println statements are used to print text to the console output window.
22
PRINT AND PRINTLN STATEMENTS
Output to a console text window in Java is accomplished through the use of print
and println (pronounced print-line) statements.
print and println are two operations (or as we say in Java) … methods that can
be called. These operations output text information to the console window in
Eclipse.
We call methods when we want to accomplish some operation or task.
23
PRINT AND PRINTLN STATEMENTS
Here are lines of code that call the two methods print and println:
1.
System.out.print(“Hello World”);
2.
System.out.println(“Hello World”);
Think of methods as operations, but we always refer to them as methods in Java.
Some other languages refer to them as functions or procedures.
When we say we are “calling a method”, what we mean is that we are executing
an operation!
24
PRINT AND PRINTLN STATEMENTS
You may be wondering what the difference between
the two methods print and println are. print will
display everything on one line but not start a new line
for anything else that will be printed by the next print
or println statement. println will display everything on
one line and then start a new line of output for the
output of any other print or println statement. Here
is an example:
(Note the extra spaces at the end of the first two
print statements or the sentences would jam up
together in output)
The output in the console window is all on one line:
public class HelloWorld
{
public static void main(String args[ ])
{
//Consider these three lines of code:
System.out.print(“Hello World! ”);
System.out.print(“How are you doing? ”);
System.out.print(“I am doing fine.”);
} // end of main method
} // end of HelloWorld Class
Hello World! How are you doing? I am doing fine.
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PRINT AND PRINTLN STATEMENTS
Again … print will display everything on one line but not start a new line of output. println will display everything on one line and then start a new line of output.
public class HelloWorldNew
{ //Filename: HelloWorldNew.java
public static void main(String args[ ])
{
//Consider these three lines of code:
System.out. print(“Hello World! ”);
System.out. println(“How are you doing?”);
System.out. println(“I am doing fine.”);
} // end of main method
} // end of HelloWorldNew Class
The output is on two lines in the console window:
Hello World! How are you doing?
I am doing fine.
26
PRINT STATEMENTS AND SYNTAX ERRORS
So when we use the line of code:
System.out.print(“Hello World”);
We are calling the print method. Printing to a console window requires that we place System.out. prior to
the word print.
Please notice the periods “.” (method selector operators) that separate the words System, out, and print.
Also, notice that the first S of System is capitalized (upper case). The reason is System is the name of a
class.
Also, notice that after the word print there are parentheses that include in double quotes the information
that we want to display to the screen and the entire line of code ends in a semicolon.
All of this is required and must be typed correctly or you will get a syntax error. Programs with syntax
errors won’t run!
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SEMICOLONS END MOST JAVA STATEMENTS
•
A Semicolon (;) marks the end of most Java statements. A statement is a “sentence” in a program.
Examples: Filename NumSum.java
public class NumSum
{
public static void main(String args[ ])
{
//Consider these three lines of code:
System.out.println(“Hello World!”);
num1 = reader.nextInt();
sum = num1 + num2;
} // end of main method
} // end of NumSumClass
Some Java lines do not end in a semicolon. You will learn which ones don’t as you are introduced to other coding
structures.
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USING VARIABLES TO STORE VALUES
A variable names a memory address (location) in RAM memory where a value can be stored.
The variable is considered to be a nickname for the memory address.
The memory address of a variable may be some unusual combination of 1s or 0s or a hexadecimal number,
so its great that we can decide on the name of variables. We do this when we declare them.
We can think of a variable as a small box that holds a value.
X
an int variable
Y
a double variable
name
a string variable
29
SIMPLE JAVA DATA TYPES
In Java, we have different kinds of data types. Here you will learn three of them. We can store each of the three
types of data values in different kinds of variables.
We can have …
1.
int variables that can hold integers (type int)
2.
double variables that can hold real numbers or what we refer to also as floating-point numbers (type
double)
3.
String variables that refer to an object that can hold a string of characters, in other words, a bunch of
characters that make up a word (type String)
The data types int and double are simple numeric data types and we store those kind of values in simple
variables that are not considered object variables.
However, Strings are objects and a String value must be stored in an object variable. Let’s look at how we do
that with the assignment operator.
30
THE ASSIGNMENT OPERATOR
The assignment operator is the = character and you can initialize variables to literal numeric or string values in one line of
code.
So to store a value in a variable, we use the assignment operator.
Here is how to declare the different kinds of variables and store values in them or as we like to say initialize them:
int x = 129;
// x is the int variable and 129 is being stored.
double y = 3.14159; // y is the double variable and π is stored.
String name = “Java”;
In the last line of code, name is the String variable and the word “Java” is being stored. Notice the double quotes around
Java since we are storing a literal String value Don’t place double quotes when you store numbers.
31
NUMERIC VARIABLES
A numeric variable names a memory address (location) in RAM memory where a number
can be stored. It is considered to be a nickname for the memory address. Numeric
variables are of type int or double.
When we use the line of code:
double celsius;
then enough RAM memory is allocated so that a floating-point value can be stored in
celsius. Java does this automatically for you! A double variable gets twice as much
memory as an int variable. Thus, the data type was named double.
32
IMPORTANT FACTS ABOUT VARIABLES
Important points to remember about variables:
• A variable’s value may change during a program, but its name remains
constant.
• A variable’s type defines what kind of values can be stored in RAM. The
type of the variable cannot be changed while a program is running.
Remember when we say type we are referring to int, double, boolean,
String, or other data types.
33
JAVA’S MATHEMATICAL OPERATORS
Java basically has 5 mathematical operators.
Addition is represented by the + sign.
Subtraction is represented by the - sign.
Multiplication is represented by the * sign.
Division is represented by the / sign.
Mod is represented by the % sign. Mod gives the remainder of int division. (You’ll be amazed at how much you will use mod)
In the Convert.java program, the line of code:
celsius = ( fahrenheit - 32.0 ) * 5.0 / 9.0;
uses three of the above mathematical operators.
You’ll see the code for the program in a few of slides.
34
DECLARING AND USING SCANNER OBJECTS
We also use the assignment operator when creating objects (constructing objects) so an object variable can
refer to them.
If we want to receive input from the keyboard during a program, then we need to construct a Scanner object
first. This allows the user to enter numbers or string values into a program.
To do this we need to import Java’s Scanner class with the line:
import java.util.Scanner; // goes above the class declaration line
Next, inside the main method, we need to construct the Scanner object with the line:
Scanner reader = new Scanner(System.in);
We need to use System.in as the parameter because this indicates the keyboard, which is the default input
device for Java. Note: Scanner is a class so the S is capitalized.
35
GENERAL FORM FOR CONSTRUCTING OBJECTS
• In programming, the process of constructing an object is called instantiation.
• In general, constructing or instantiating and object takes the general form:
<Name of Class> <variable> = new <Name of Class> (<parameters>);
You can see that the line of code below follows this form:
Scanner reader = new Scanner (System.in);
You can think of reader as something that is “scanning the keyboard waiting for
input”.
36
CONSTRUCTING A SCANNER OBJECT WITH NEW
In the line of code :
Scanner reader = new Scanner(System.in);
the name of the class Scanner is used twice.
The first part of the line:
Scanner reader
declares reader to be an object variable of type Scanner. We need to do this or reader can’t
refer to the Scanner object we construct.
The second part of the line:
new Scanner(System.in);
constructs the Scanner object and “attaches it” to the keyboard. The word new is the new
operator that we use to construct things. The assignment operator makes reader refer to
the Scanner object.
37
MORE ABOUT READING INPUT FROM THE
KEYBOARD
Assume the following lines of code appear in the main method of a program.
Filename: ScanIn.java
public class ScanIn
{
public static void main(String args[ ])
{
//Consider these three lines of code:
Scanner reader = new Scanner (System.in);
Notice when we prompt the user, we use a print statement
NOT a println statement. We can receive different kinds of
input using reader. We use the object variable reader to
“call the method nextLine() to receive a String value from
the keyboard. We use reader to “call the method nextInt()
to receive an integer and we use reader to “call the method
nextDouble() to receive a floating-point value from the
keyboard.
System.out.print(“Enter your name and press return: ”);
String name = reader.nextLine();
System.out.print(“Enter your age and press return: ”);
int age = reader.nextInt();
Note: the print statements “prompt” the user to enter data,
otherwise he or she wouldn’t know the computer is waiting
for input!
System.out.print(“Enter your gpa and press return: ”);
double gpa = reader.nextDouble();
} // end of main method
} // end of ScanIn Class
38
ECHOING THE INPUT FROM THE KEYBOARD
We can now “echo the input” (print the information back to the screen that was entered) by using
some println statements. In each line, we will print a literal string (something in double quotes) and
the value contained in a variable. We use a plus sign to concatenate the literal string value and the
value stored in the variable together to make a larger string that is then printed.
System.out.println(“Your name is: ” + name);
System.out.println(“Your age is: ” + age);
System.out.println(“Your gpa is: ” + gpa);
Notice that there are no double quotes around the variables name, age, and gpa.
39
SAMPLE CODE : NOTE TWO COLUMNS
public class ScanIn
System.out.print(“Enter your gpa and press
return: ”);
{
public static void main(String args[ ])
double gpa = reader.nextDouble();
{
System.out.println(“Your name is: ” + name);
//Consider these three lines of code:
Scanner reader = new Scanner (System.in);
System.out.print(“Enter your name and press
return: ”);
String name = reader.nextLine();
System.out.print(“Enter your age and press
return: ”);
int age = reader.nextInt();
System.out.println(“Your age is: ” + age);
System.out.println(“Your gpa is: ” + gpa);
} // end of main method
} // end of ScanIn Class
ALGORITHM FOR CONVERTING TEMPERATURES
If we wanted to write the code for a program that would convert Fahrenheit temperatures to
Celsius, it would be good to stop and develop an algorithm. An algorithm is a step by step
procedure for solving a problem. This helps our code to be more efficient and we save a lot of
time, because we consider all that needs to be done … NOT just the mathematical formula
needed to convert the temperature.
41
ALGORITHM FOR CONVERTING TEMPERATURES
Consider this algorithm(break down to solve the problem):
1.
Prompt the user to enter a Fahrenheit temperature.
2.
Read the value from the keyboard and store it in a variable.
3.
Use the value stored in the variable in an assignment statement that applies the correct
mathematical formula in code form.
4.
Store the value calculated in a second variable.
5.
Output the converted value to the screen.
42
import java.util.Scanner;
public class Convert
{
THE CONVERT.JAVA PROGRAM
public static void main(String args[])
{
Scanner reader = new Scanner(System.in);
double fahrenheit;
double celsius;
// prompt the user to enter a value from the keyboard when the program runs.
System.out.print("Enter degrees Fahrenheit and press return: ");
// read the value from the keybaord and store it in the variable fahrenheit.
fahrenheit = reader.nextDouble();
// calculate the equivalent celsius value and store it in the variable celsius
celsius = (fahrenheit - 32.0) * 5.0 / 9.0;
// print out the following string of characters to the terminal window.
System.out.print("The equivalent in Celsius is ");
// print out the value stored in the variable celsius to the terminal window.
System.out.println(celsius);
}
}
Note the key Scanner lines identified by the red arrows.
43
MORE ABOUT IMPORT STATEMENTS
In the Convert.java code you saw that the first line after the package declaration was an import
statement:
import java.util.Scanner;
This tells the compiler where to find a class that will be used during the program. The class may be
either in a Java library file or another file you have in a folder. The import statement contains the path
name of where to find the class.
The import statement tells us that the Scanner class is found in a sub-package folder named util that is in
the java package folder.
Now you know enough that you can finish the second half of the Convert.java program!
44
SECTION 5
CALLING METHODS
IN THIS SECTION,YOU WILL LEARN SOME VERY IMPORTANT INFORMATION
ABOUT:
1.
2.
3.
METHOD CALLS AND PARAMETERS
CONCATENATION
READABILITY OF CODE
45
METHOD CALLS WITH PARAMETERS
Consider the following line of code:
System.out.println(“Hello World!”);
•
System is a class and out is an object of that class that knows how to display or
print characters in a console or terminal window.

println is the name of the method (operation) being executed. (Another way to
say it is we are “sending the message println to the object represented by
System.out”.)

The item inside the parentheses “Hello World” is the parameter of what needs
to be printed. Here the parameter is a string (string of characters) that make up the
words “Hello World”. Notice they appear in quotation marks. This tells Java to print
to the screen the literal string value “Hello World”. The parameter could be a variable
that contains a value instead of a literal string value in double quotes.
46
GENERAL RULE OF METHOD CALLS
The general form for calling methods is:
<name of object> . <name of method> (<parameters>)
A message may require zero, one, or multiple parameters. Here are some examples:
•
•
•
To print a blank line, we can use System.out.println(); without any parameters in the ( ).
To print “Hello World”, we need only one parameter the literal string value “Hello
World” in the parenthesis … System.out.println(“Hello World!”);
To print “Hello World. Java Rules”, we still need only one parameter but the parameter
may be the concatenation of two literal string values as in …
System.out.println(“Hello World!” + “Java Rules!”); Notice that we concatenate two literal strings together using a + symbol.
•
To call a method named calculateArea with two parameters length and width, we use the
code ...
calculateArea(length, width);
Notice the two parameters are separated by a comma.
47
THE METHOD SELECTOR IS THE PERIOD
The Method selector operator is the period . and is always placed between the object’s name and the method’s
name. It is also placed between the name of a class and the name of an object when needed as in System.out.
Scanner reader = new Scanner(System.in);
System.out.println(“Enter an integer: ”);
int num1 = reader.nextInt();
System.out.println(“Enter an integer: ”);
int num2 = reader.nextInt();
System.out.println(“Enter a floating-point number: ”);
double num3 = reader.nextDouble();
Above, reader has been declared to be a Scanner class variable that represents a new Scanner object. That Scanner object
can read data from the input of the keyboard. Reader can call either of the methods nextInt() or nextDouble() to
read integers or floating-point numbers from the keyboard. The variable reader is also referred to specifically as an
object variable.
48
DECLARING INT AND DOUBLE VARIABLES
TIME OUT!!! What are we doing with the lines?
int num1 = reader.nextInt();
double num3 = reader.nextDouble();
In the first line, we declare a variable named num1, but we have to tell what type of data we intend for it to
hold. Since we want it to hold an integer (whole number), we use Java’s int data type. The type of data
must be listed before the name of the variable!
The assignment operator = means we want to store something in num1. What we want to store is what is on
the right side of the = sign. It could be an arithmetic expression or a reader line.
When a program runs and it encounters a line like reader.nextInt(), the program will pause and wait for
something to be entered from the keyboard. Once the user enters an integer and presses return, then the
number is read by reader’s nextInt() method, and then the value is stored in num1.
With the second line of code, we declare a variable num3 that can hold a floating-point number (one that
contains a decimal point like 4.237). The method nextDouble() is used to read the value from the
keyboard and store it in the double (floating-point) variable num3.
49
CONCATENATING OUTPUT ITEMS
Consider this segment of code:
System.out.println(“Enter an integer: ”);
int num1 = reader.nextInt();
// a whole number is stored in num1
System.out.println(“Enter an integer: ”);
int num2 = reader.nextInt();
// a whole number is stored in num2
int sum = num1 + num2;
// a whole number is stored in sum
System.out.println(“The sum of ” + num1 + “ and ” + num2 + “ is ” + sum);
In the last output line, we concatenate 6 things together to be written to output. Three of the items are literal strings
and three are variables of type int that hold integers. Notice there are NO double quotes around the variables.
You never place double quotes around any variable … only literal string values. Also, notice the blank spaces at
the beginning and end of some literal string values. This keeps the numbers from being jammed up against the
words.
50
REVIEW OF SIMPLE AND OBJECT VARIABLES
In the Convert.java program, simple variables like fahrenheit and celsius each hold a single
floating-point number.
Object variables like reader and System.out hold references to objects.
Object variables are used to send messages to objects.
reader.nextDouble() sends the message “get me the next floating point number” from the keyboard.
In summary, to write effective Java programs, a programmer does not need to have detailed
knowledge of the inner workings of any object, he or she just needs to know how to construct
objects and how to send messages to the object by calling methods.
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THE READABILITY OF CODE
It is important for your code to be readable by others. In the real world, programmers are on software
teams as they develop and maintain software. So everyone must be able to read your code!
Programmers have developed a standard for how code should be formatted.
•
The main factors that determine whether code is readable or not are
 Spacing (referring to extra blank lines that space things out)
 Indentation (referring to tabs or indentions on a specific line)
•
Spacing and Indentation are just for programmer readability. The compiler ignores any kind of spacing and
indentation. It just checks to make sure that everything is syntactically correct (spelled correctly)!
•
Eclipse assists you with indenting segments of code as you type by automatically properly indenting your
next line depending on the kind of Java code you are writing. But if you mess up the indenting then all you
have to do is select all code by typing Control “a” on a Windows machine or Apple “a” on a Mac, then
type either Control “i” for Windows or Apple “i” for a Mac and everything will get indented properly.
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COMMENTS ARE USED TO …
Comments are used to:
• begin a program with a statement of its purpose
• explain the purpose of a variable that needs it
• explain the purpose of a major segment of code, like a method
• explain the workings of complex or tricky sections of code
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THE THREE TYPES OF PROGRAM COMMENTS
Program comments come in three varieties:
 End of line or Single comments: All text
following a double forward slash (//) on a
single line may explain a line of code and are
ignored by the compiler.
public class HelloWorld
{
public static void main(String args[ ])
{
//Consider these three lines of code:
System.out.println(“Hello World!”);
} // end of main method
} // end of HelloWorld
 Multi-line comments: All text occurring
between a /* and a */ is ignored by the
compiler. Multi-line comments are used
where lengthy explanations are needed or
you want to deactivate a large segment of
code temporarily.
Notice the //. Everything after the // are programmer comments about the code.
It is does not perform any task when the code is executed.
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USEFUL READING AND YOUTUBE VIDEO
 How to Create first Application in Java
 http://www.java-made-easy.com/java-hello-world.html
 https://www.youtube.com/watch?v=m9vN0HJoUTI