Computer programs are instructions to the computer
You tell a computer what to do: Exactly what to do. Computers do not understand human languages,
so you need to use a clear set of instructions to communicate your intent.
It’s not unlike other specific instructions we need to follow.
Why Java?
It’s been around for a while - developed by Sun Microsystems, now owned by Oracle
A general-purpose object-oriented language Based
on C/C++
Designed for easy web/internet application development
Widespread use/acceptance
Simple, object oriented, distributed, interpreted, robust, secure, architecture neutral,
portable, high performance, multithreaded, and dynamic.
Java Runs on Servers, PCs and laptops, Mobile and other devices
By convention, class names start with an uppercase letter. In this example, the class name is
Welcome.
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Main Method
Line 3 defines the main method. In order to run a class, the class must contain a method named
main.
The program is executed from the main method.
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Statement
A statement represents an action or a sequence of actions.
The statement System.out.println("Welcome to Java!") in the program below is a statement to
display the greeting "Welcome to Java!“.
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Statement Terminator
Every statement in Java ends with a semicolon (;).
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Reserved words
Reserved words or keywords are words that have a specific meaning to the compiler and cannot be
used for other purposes in the program.
For example, when the compiler sees the word class, it understands that the word after class is
the name for the class.
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Blocks
A pair of braces in a program forms a block that groups components of a program.
// This program prints Welcome to Java!
public class Welcome { //Class block
public static void main(String[] args) { // Method block
System.out.println("Welcome to Java!");
}
}
Special Symbols
Character
Name
Description
{}
Opening and closing braces
Denotes a block to enclose statements.
()
Opening and closing parenthesis
Used with methods.
[]
Opening and closing brackets
Denotes an array.
//
Double slashes
Precedes a comment line
""
Opening and closing quotation marks
Enclosing a string
(i.e., sequence of characters)
;
Semicolon
Marks the end of a statement
/* ... */
Multiline comment
Comment multiple lines
{ Opening and closing braces }
// This program prints Welcome to Java!
public class Welcome { //opening curly
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}//closing curly
(Opening and closing parenthesis)
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) // parenthesis{
System.out.println("Welcome to Java!");
}
}
Semicolon ;
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); //semicolon
}
}
Double slashes //
// This program prints Welcome to Java! //This is single line comment
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
"Opening and closing quotation marks"
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); //String values wrapped in a comment
}
}
"/* Multi line comment */"
/* This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
*/ //Multi line comment
Programming Style and Documentation
Naming Conventions
Class names:
Capitalize the first letter of each word in the name. For example, the class name
ComputeExpression
Indentation :tab( or two spaces)
Spacing: Use blank line to separate segments of the code.
public class Welcome //classname start with uppercase {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
public class Welcome //classname start with uppercase
{
public static void main(String[] args)
{
System.out.println("Welcome to Java!");
}
}
Interpreting/Compiling Source Code
A program written in a high-level language is called a source program or source code
Because a computer cannot understand a source program, a source program must be translated
into machine code for execution
The translation can be done using another programming tool called an interpreter or a
compiler.
Interpreting Source Code
An interpreter reads one statement from the source code, translates it to the machine code
or virtual machine code, and then executes it right away
Note that a statement from the source code may be translated into several machine
instructions.
Compiling Source Code
A compiler translates the entire source code into a machine-code file, and the machine-code
file is then executed, as shown in the following figure.
Java source code is converted to Byte code
Compile Java Code
Javac filename e.g javac myfile.java
Java myfile
javac FirstJavaApp.java
Java FirstJavaApp anything something other words
Operating Systems
The operating system (OS) is a program that manages and controls a computer’s activities.
Popular OS Windows,MAC OS and Linux
such as a Web browser or a word processor, cannot run unless an operating system is
installed and running on the computer
Platform Independence
Write code once
Compiled code can run on any platform that has JRE
JRE is platform dependent
Programming Errors
Syntax Errors => Detected by the compiler
Runtime Errors => Causes the program to abort
Logic Errors => Produces incorrect result
Syntax Errors
Runtime Errors
Logical Errors
TAKE-AWAYS
With computers, precise instructions are extremely important
If you have missing, incorrect, or misspelled words/punctuation, it will reject your
instructions
