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;
}
}
Step 3 :
