Tuesday, December 18, 2012

Java Tutorial : Crash Course


Java Tutorial 

Java is:
platform independent programming language
similar to C++ in syntax


Interesting features:
  • automatic type checking,
  • automatic garbage collection,
  • simplifies pointers; 
  • no directly accessible pointer to memory, 
  • simplified network access, 
  • multi-threading!

How it works .......
Platform Independent Why ?
Only depends on the Java Virtual Machine (JVM),
code is compiled to bytecode, which is interpreted by the resident JVM,
JIT (just in time) compilers attempt to increase speed.
Security:
Pointer denial - reduces chances of virulent programs corrupting host,
Applets even more restricted -
May not run local executables,
Read or write to local file system,
Communicate with any server other than the originating server.
Object-Oriented
Java supports OOD
  • Polymorphism
  • Inheritance
  • Encapsulation

Java programs contain nothing but definitions and instantiations of classes
Everything is encapsulated in a class!
Java Advantages
  • Portable - Write Once, Run Anywhere
  • Security has been well thought through
  • Robust memory management
  • Designed for network programming
  • Multi-threaded (multiple simultaneous tasks)
  • Dynamic & extensible (loads of libraries)


Basic Java Syntax :

Primitive Types and Variables :
boolean, char, byte, short, int, long, float, double etc.
These basic (or primitive) types are the only types that are not objects (due to performance issues).
This means that you don’t use the new operator to create a primitive variable.
Declaring primitive variables:
float initVal;
int retVal, index = 2;
double gamma = 1.2, brightness
boolean valueOk = false;

Initialisation :
If no value is assigned prior to use, then the compiler will give an error
Java sets primitive variables to zero or false in the case of a boolean variable
All object references are initially set to null
An array of anything is an object
Set to null on declaration
Elements to zero false or null on creation

Declarations :
int index = 1.2;   // compiler error
boolean retOk = 1;  // compiler error
double fiveFourths = 5 / 4;   // no error!
float ratio = 5.8f;  // correct
double fiveFourths = 5.0 / 4.0;  // correct
1.2f is a float value accurate to 7 decimal places.
1.2 is a double value accurate to 15 decimal places.
Assignment
All Java assignments are right associative
  int a = 1, b = 2, c = 5
  a = b = c
   System.out.print(
  “a= “ + a + “b= “ + b + “c= “ + c)
What is the value of a, b & c
Done right to left: a = (b = c);
Basic Mathematical Operators
* / % + - are the mathematical operators
* / % have a higher precedence than + or -
double myVal = a + b % d – c * d / b;
Is the same as:
double myVal = (a + (b % d)) –
    ((c * d) / b);

Statements & Blocks :
A simple statement is a command terminated by a semi-colon:
  name = “Fred”;
A block is a compound statement enclosed in curly brackets:
  {
  name1 = “Fred”; name2 = “Bill”;
  }
Blocks may contain other blocks

Flow of Control :
Java executes one statement after the other in the order they are written
Many Java statements are flow control statements:
Alternation:   if, if else, switch
Looping:  for, while, do while
Escapes:  break, continue, return
If – The Conditional Statement
The if statement evaluates an expression and if that evaluation is true then the specified action is taken
if ( x < 10 ) x = 10;
If the value of x is less than 10, make x equal to 10
It could have been written:
if ( x < 10 )
x = 10;
Or, alternatively:
if ( x < 10 ) { x = 10; }

Relational Operators :
  • ==  Equal 
  • !=  Not equal
  • >=  Greater than or equal
  • <=  Less than or equal
  • >  Greater than
  • <  Less than

If… else
The if … else statement evaluates an expression and performs one action if that evaluation is true or a different action if it is false.
   if (x != oldx) {
  System.out.print(“x was changed”);
}
else {
  System.out.print(“x is unchanged”);
}
Nested if … else
if ( myVal > 100 ) {
  if ( remainderOn == true) {
    myVal = mVal % 100;
  }
  else {
  myVal = myVal / 100.0;
  }
}
else
{
  System.out.print(“myVal is in range”);
}
else if
Useful for choosing between alternatives:
if ( n == 1 ) {
  // execute code block #1
}
else if ( j == 2 ) {
  // execute code block #2
}
else {
  // if all previous tests have failed, execute code block #3
}

CORRECT!
if( i == j ) {
  if ( j == k )
  System.out.print(
      “i equals k”);
}
else
  System.out.print(“i is not equal to j”);  // Correct!
The switch Statement
switch ( n ) {
  case 1:
  // execute code block #1
  break;
  case 2:
  // execute code block #2
  break;
  default:
  // if all previous tests fail then          //execute code block #4
  break;
}
The for loop
Loop n times
for ( i = 0; i < n; n++ ) {
  // this code body will execute n times
  // ifrom  0 to n-1
}
Nested for:
for ( j = 0; j < 10; j++ ) {
  for ( i = 0; i < 20; i++ ){
  // this code body will execute 200 times
  }
}
 
while loops
while(response == 1) {
  System.out.print( “ID =” + userID[n]);
  n++;
  response = readInt( “Enter “);
}
do {… } while loops
do {
  System.out.print( “ID =” + userID[n] );
  n++;
  response = readInt( “Enter ” );
}while (response == 1);
Break
A break statement causes an  exit from the innermost containing while, do, for or switch statement.
for ( int i = 0; i < maxID, i++ ) {
  if ( userID[i] == targetID ) {
  index = i;
  break;
  }
}  // program jumps here after break
Continue
Can only be used with while, do or for.
The continue statement causes the innermost loop to start the next iteration immediately
for ( int i = 0; i < maxID; i++ ) {
  if ( userID[i] != -1 ) continue;
  System.out.print( “UserID ” + i + “ :” +     userID);
}

Arrays
Am array is a list of similar things
An array has a fixed:
name
type
length
These must be declared when the array is created.
Arrays sizes cannot be changed during the execution of the code
Declaring Arrays
int myArray[];
declares myArray to be an array of integers
myArray = new int[8];
sets up 8 integer-sized spaces in memory, labelled myArray[0] to myArray[7]
int myArray[] = new int[8];
combines the two statements in one line
Assigning Values
refer to the array elements by index to store values in them.
myArray[0] = 3;
myArray[1] = 6;
myArray[2] = 3;   ...
can create and initialise in one step:
int myArray[] = {3, 6, 3, 1, 6, 3, 4, 1};
Iterating Through Arrays
for loops are useful when dealing with arrays:
for (int i = 0; i < myArray.length; i++) {
  myArray[i] = getsomevalue();
}
Arrays of Objects
So far we have looked at an array of primitive types.
integers
could also use doubles, floats, characters…
Often want to have an array of objects
Students, Books, Loans ……
Need to follow 3 steps.
Declaring the Array
1. Declare the array
  private Student studentList[];
this declares studentList
2 .Create the array
  studentList = new Student[10];
this sets up 10 spaces in memory that can hold references to Student objects
3. Create Student objects and add them to the array: studentList[0] = new Student("george", "Computing");

Java Methods & Classes
Classes ARE Object Definitions
OOP - object oriented programming
code built from objects
Java these are called classes
Each class definition is coded in a separate .java file
Name of the object must match the class/object name    

The three principles of OOP
Encapsulation
Objects hide their functions (methods) and data (instance variables)
Inheritance
Each subclass inherits all variables of its superclass
Polymorphism
Interface same despite different data types

Methods :
A method is a named sequence of code that can be invoked by other Java code.
A method takes some parameters, performs some computations and then optionally returns a value (or object).
Methods can be used as part of an expression statement.
 
public float convertCelsius(float tempC) {
  return( ((tempC * 9.0f) / 5.0f) + 32.0 );
  }
Method Signatures
A method signature specifies:
The name of the method.
The type and name of each parameter.
The type of the value (or object) returned by the method.
The checked exceptions thrown by the method.
Various method modifiers.
modifiers type name ( parameter list ) [throws exceptions ]
public float convertCelsius (float tCelsius ) {}
public boolean setUserInfo ( int i, int j, String name ) throws IndexOutOfBoundsException {}
Public/private
Methods/data may be declared public or private meaning they may or may not be accessed by code in other classes …
Good practice:
keep data private
keep most methods private
well-defined interface between classes - helps to eliminate errors
Using objects
Here, code in one class creates an instance of another class and does something with it …
Fruit plum=new Fruit();
int cals;
cals = plum.total_calories();
Dot operator allows you to access (public) data/methods inside Fruit class
Constructors
The line
plum = new Fruit();
invokes a constructor method with which you can set the initial data of an object
You may choose several different type of constructor with different argument lists
 eg Fruit(), Fruit(a) ...
Overloading
Can have several versions of a method in class with different types/numbers of arguments
  Fruit() {grams=50;}
    Fruit(a,b) { grams=a; cals_per_gram=b;}
By looking at arguments Java decides which version to use
Java Development Kit
javac - The Java Compiler
java -   The Java Interpreter
jdb -     The Java Debugger
appletviewer -Tool to run the applets
javap - to print the Java bytecodes
javaprof - Java profiler
javadoc - documentation generator
javah - creates C header files
Stream Manipulation
Streams and I/O
basic classes for file IO
FileInputStream, for reading from a file
FileOutputStream, for writing to a file
Example:
Open a file "myfile.txt" for reading
FileInputStream fis = new FileInputStream("myfile.txt");
Open a file "outfile.txt" for writing
FileOutputStream fos = new FileOutputStream ("myfile.txt");

General IO stream manipulation utilities.

 This class provides static utility methods for input/output operations.
 closeQuietly - these methods close a stream ignoring nulls and exceptions
 read - these methods read data from a stream
 write - these methods write data to a stream
 copy - these methods copy all the data from one stream to another
 contentEquals - these methods compare the content of two streams

Object Serialization :

Serialization is the conversion of an object to a series of bytes, so that the object can be easily saved to persistent storage or streamed across a communication link. The byte stream can then be deserialised - converted into a replica of the original object.
import java.io.*; 
public class SerializationDemo { 
public static void main(String args[]) { 
// Object serialization 
try { 
MyClass object1 = new MyClass("Hello", -7, 2.7e10); 
System.out.println("object1: " + object1); 
FileOutputStream fos = new FileOutputStream("serial"); 
ObjectOutputStream oos = new ObjectOutputStream(fos); 
oos.writeObject(object1); 
oos.flush(); 
oos.close(); 
} 
catch(Exception e) { 
System.out.println("Exception during serialization: " + e); 
System.exit(0); 
} 
// Object deserialization 
try { 
MyClass object2; 
FileInputStream fis = new FileInputStream("serial"); 
ObjectInputStream ois = new ObjectInputStream(fis); 
object2 = (MyClass)ois.readObject(); 
ois.close(); 
System.out.println("object2: " + object2); 
} 
catch(Exception e) { 
System.out.println("Exception during deserialization: " + 
e); 
System.exit(0); 
} 
} 
} 
class MyClass implements Serializable { 
String s; 
int i; 
double d; 
public MyClass(String s, int i, double d) { 
this.s = s; 
this.i = i; 
this.d = d; 
} 
public String toString() { 
return "s=" + s + "; i=" + i + "; d=" + d; 
} 
}
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