The following six areas are the essential for any java programmer to work
1.Oops
2.Exception handling
3.Collection Framework
4.Serialization
5.Multi Threading
6.Strings(stringBuffer,StringBuilder,StringTokenizer)
There are four main features of OOPS.
1) Encapsulation
2) Inheritance
3) Polymorphism
4) Abstraction
Lets we discuss about the about features in details.
Encapsulation
Encapsulation means putting together all the variables (Objects) and the methods into a single unit called Class. So define a user defined data type called the keyword Class followed by the class name and inside the class we need the specify the variables that is also called as attributes and methods.
Example :
Class EncaptulationExample {
int obj;
char cObj;
public void javaMethod(){
}
}
Inheritance
Inheritance is mainly used for code reusability. You already have defined an Object or rather you have already defined set of attributes and characteristics which you like to make you it again and expand up on it. So you are making use of already written class and further extending on that. That why we discussed about the code reusability the concept. In general one line definition we can tell that deriving a new class from existing class, it’s called as Inheritance. You can look into the following example for inheritance concept.
Example 1:
class StudentInfo {
String name;
int rollno;
int get(String n, int r){
name=n; rollno=r; return(0);
}
void showDetails(){
System.out.println(“Name : “+name);
}
}
class InheritanceExampleDemo extends StudentInfo{
public static void main(String args[]){
StudentInfo studObj = new StudentInfo();
studObj.get(“Eswar”,92);
studObj.showDetails();
}
void displayDetails(){
System.out.println(“Sample Info Display”);
}
}
Output:
Name : Eswar
Polymorphism:
In Core Java Polymorphism is one of easy concept to understand. Polymorphism definition is that Poly means Many morphos means froms . Its refer to the objects ability to active Polymorphism depends on its type.
There are two types of Polymorphism available in Java.
1)Static Polymorphism
2) Dynamic Polymorphism
Let’s we discuss about Static Polymorphism, Its Compile time Polymorphism. Method overloading is the concept of two or more methods in a Java Class can have same name and it their arguments lists are different. We also have another two important concepts in Polymorphism, Method Overloading and Method Overridding. The following example program will make you understand the Method Overloading.
Example for Method overloading
class Subjects {
void add(int tamil, int english){
System.out.println(“The total of tamil and english is “+(tamil+english));
}
void add(int tamil,int english,int maths){
System.out.println(“The total of tamil english and maths is “+(tamil+english+maths));
}
}
public class MethodOverloadingDemo{
public static void main(String arg[]){
//create Subjects class object
Subjects sb=new Subjects();
// we have to call add() method by passing 2 values
sb.add(90, 80);
//here also we are calling add() method by passing 3 values, So the 3 arguments (parameters) method will get execute.
sb.add(95,85,100);
}
}
Output for the above program is :
The total of tamil and english is 170
The total of tamil english and maths is 280
Now we will discuss about what is dynamic polymorphism, Its run time polymorphism. We can also call it as Method overridding. Method overridding is the concept of two or more method, constructor have a same name in super and sub class with same signature. This feature is called method overridding.
Method Overloading Example Program:
class MathsSquareDemo1 {
void calculate(double price){
System.out.println(“Sqare value “+(price*price));
}
}
class MathsSquareDemo2 extends MathsSquareDemo1 {
void calculate(double price){
System.out.println(“Sqare value “+(Math.sqrt(price)));
}
}
public class MethodOverriddingDemo{
public static void main(String arg[]){
MathsSquareDemo2 msd=new MathsSquareDemo2();
msd.calculate(25);
}
}
Output :
Sqare value 5.0
Abstraction
Process of exploring relevant details and hiding irrelevant details this feature is known as Abstraction. In other way making simplicity to use complex system. One does not want to understand how to engine works. Similarly one does not have to understand the internal implementation of the software objects.
Abstraction Example : Engine, Driving.
Abstraction main advantage is that every user will get data according to their exact requirement. User will not get confused with unnecessary data. The following example program will make you to understand Abstraction.
Java Abstraction Example Program:
public class AbstractionDemo {
private int accountNo;
private String customerName;
private float accountBalance;
private float profit;
private float loan;
public void dislayClerkInfo(){
System.out.println(“Accout number “+accountNo);
System.out.println(“Customer name “+customerName);
System.out.println(“Account Balance “+accountBalance);
}
}
In the above program clerk can access only the limited details like account number, customer name and account balance. User cannot access other details like profit and loan.
An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following: A user has entered invalid data. A file that needs to be opened cannot be found. A network connection has been lost in the middle of communications, or the JVM has run out of memory. Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner. To understand how exception handling works in Java, you need to understand the three categories of exceptions: Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation. Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation. Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation. Exception Hierarchy: All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class. Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Example : JVM is out of Memory. Normally programs cannot recover from errors. The Exception class has two main subclasses : IOException class and RuntimeException Class. Here is a list of most common checked and unchecked Java's Built-in Exceptions. Exceptions Methods: Following is the list of important medthods available in the Throwable class. SN Methods with Description 1 public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. 2 public Throwable getCause() Returns the cause of the exception as represented by a Throwable object. 3 public String toString() Returns the name of the class concatenated with the result of getMessage() 4 public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. 5 public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack. 6 public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace. Catching Exceptions: A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following: try { //Protected code }catch(ExceptionName e1) { //Catch block } A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter. Example: The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception. // File Name : ExcepTest.java import java.io.*; public class ExcepTest{ public static void main(String args[]){ try{ int a[] = new int[2]; System.out.println("Access element three :" + a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :" + e); } System.out.println("Out of the block"); } } This would produce following result: Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 Out of the block Multiple catch Blocks: A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following: try { //Protected code }catch(ExceptionType1 e1) { //Catch block }catch(ExceptionType2 e2) { //Catch block }catch(ExceptionType3 e3) { //Catch block } The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through all catches, in which case the current method stops execution and the exception is thrown down to the previous method on the call stack. Example: Here is code segment showing how to use multiple try/catch statements. try { file = new FileInputStream(fileName); x = (byte) file.read(); }catch(IOException i) { i.printStackTrace(); return -1; }catch(FileNotFoundException f) //Not valid! { f.printStackTrace(); return -1; } The throws/throw Keywords: If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature. You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords. The following method declares that it throws a RemoteException: import java.io.*; public class className { public void deposit(double amount) throws RemoteException { // Method implementation throw new RemoteException(); } //Remainder of class definition } Amethod can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException: import java.io.*; public class className { public void withdraw(double amount) throws RemoteException, InsufficientFundsException { // Method implementation } //Remainder of class definition } The finally Keyword The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred. Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code. A finally block appears at the end of the catch blocks and has the following syntax: try { //Protected code }catch(ExceptionType1 e1) { //Catch block }catch(ExceptionType2 e2) { //Catch block }catch(ExceptionType3 e3) { //Catch block }finally { //The finally block always executes. } Example: public class ExcepTest{ public static void main(String args[]){ int a[] = new int[2]; try{ System.out.println("Access element three :" + a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :" + e); } finally{ a[0] = 6; System.out.println("First element value: " +a[0]); System.out.println("The finally statement is executed"); } } } This would produce following result: Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 First element value: 6 The finally statement is executed Note the followings: A catch clause cannot exist without a try statement. It is not compulsory to have finally clauses when ever a try/catch block is present. The try block cannot be present without either catch clause or finally clause. Any code cannot be present in between the try, catch, finally blocks. Declaring you own Exception: You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes: All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. If you want to write a runtime exception, you need to extend the RuntimeException class. We can define our own Exception class as below: class MyException extends Exception{ } You just need to extend the Exception class to create your own Exception class. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods. Example: // File Name InsufficientFundsException.java import java.io.*; public class InsufficientFundsException extends Exception { private double amount; public InsufficientFundsException(double amount) { this.amount = amount; } public double getAmount() { return amount; } } To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException. // File Name CheckingAccount.java import java.io.*; public class CheckingAccount { private double balance; private int number; public CheckingAccount(int number) { this.number = number; } public void deposit(double amount) { balance += amount; } public void withdraw(double amount) throws InsufficientFundsException { if(amount <= balance) { balance -= amount; } else { double needs = amount - balance; throw new InsufficientFundsException(needs); } } public double getBalance() { return balance; } public int getNumber() { return number; } } The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount. // File Name BankDemo.java public class BankDemo { public static void main(String [] args) { CheckingAccount c = new CheckingAccount(101); System.out.println("Depositing $500..."); c.deposit(500.00); try { System.out.println("\nWithdrawing $100..."); c.withdraw(100.00); System.out.println("\nWithdrawing $600..."); c.withdraw(600.00); }catch(InsufficientFundsException e) { System.out.println("Sorry, but you are short $" + e.getAmount()); e.printStackTrace(); } } } Compile all the above three files and run BankDemo, this would produce following result: Depositing $500... Withdrawing $100... Withdrawing $600... Sorry, but you are short $200.0 InsufficientFundsException at CheckingAccount.withdraw(CheckingAccount.java:25) at BankDemo.main(BankDemo.java:13) Common Exceptions: In java it is possible to define two catergories of Exceptions and Errors. JVM Exceptions: - These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException, Programmatic exceptions . These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.
Collection Framework
Core Collection InterfacesThe core interfaces that define common functionality and allow collections to be manipulated independent of their implementation.
The 6 core Interfaces used in the Collection framework are:
Collection Interface
Map InterfaceConcrete ClassesThe concrete classes that are specific implementations of the core interfaces, providing data structures that a java program can use.
Note: Concrete Classes for the Map is shown in the previous section.
Standard utility methods and algorithms
Standard utility methods and algorithms
that can be used to perform various operations on collections, such as sorting, searching or creating customizedcollections.
How are Collections Used
import java.util.*;
public class CollectionsDemo {
public static void main(String[] args) {
List a1 = new ArrayList();
a1.add("Beginner");
a1.add("Java");
a1.add("tutorial");
System.out.println(" ArrayList Elements");
System.out.print("\t" + a1);
List l1 = new LinkedList();
l1.add("Beginner");
l1.add("Java");
l1.add("tutorial");
System.out.println();
System.out.println(" LinkedList Elements");
System.out.print("\t" + l1);
Set s1 = new HashSet(); // or new TreeSet() will order the elements;
s1.add("Beginner");
s1.add("Java");
s1.add("Java");
s1.add("tutorial");
System.out.println();
System.out.println(" Set Elements");
System.out.print("\t" + s1);
Map m1 = new HashMap(); // or new TreeMap() will order based on keys
m1.put("Windows", "98");
m1.put("Win", "XP");
m1.put("Beginner", "Java");
m1.put("Tutorial", "Site");
System.out.println();
System.out.println(" Map Elements");
System.out.print("\t" + m1);
} }
Output
ArrayList Elements
[Beginner, Java, tutorial]
LinkedList Elements
[Beginner, Java, tutorial]
Set Elements
[tutorial, Beginner, Java]
Map Elements
{Tutorial=Site, Windows=98, Win=XP, Beginner=Java}
The 6 core Interfaces used in the Collection framework are:
- Collection
- Set
- List
- Iterator (Not a part of the Collections Framework)
- SortedSet
- Map
- SortedMap
Collection Interface
Map InterfaceConcrete ClassesThe concrete classes that are specific implementations of the core interfaces, providing data structures that a java program can use.
Note: Concrete Classes for the Map is shown in the previous section.
Standard utility methods and algorithms
Standard utility methods and algorithms
that can be used to perform various operations on collections, such as sorting, searching or creating customizedcollections.
How are Collections Used
- The collections stores object references, rather than objects themselves. Hence primitive values cannot be stored in a collection directly. They need to be encapsulated (using wrapper classes) into an Object prior to storing them into a Collection (such as HashSet, HashMap etc).
- The references are always stored as type Object. Thus, when you retrieve an element from a collection, you get an Object rather then the actual type of the collection stored in the database. Hence we need to downcast it to the Actual Type while retrieving an element from a collection.
- One of the capabilities of the Collection Framework is to create a new Collection object and populate it with the contents of an existing Collection object of a same or different actual type.
import java.util.*;
public class CollectionsDemo {
public static void main(String[] args) {
List a1 = new ArrayList();
a1.add("Beginner");
a1.add("Java");
a1.add("tutorial");
System.out.println(" ArrayList Elements");
System.out.print("\t" + a1);
List l1 = new LinkedList();
l1.add("Beginner");
l1.add("Java");
l1.add("tutorial");
System.out.println();
System.out.println(" LinkedList Elements");
System.out.print("\t" + l1);
Set s1 = new HashSet(); // or new TreeSet() will order the elements;
s1.add("Beginner");
s1.add("Java");
s1.add("Java");
s1.add("tutorial");
System.out.println();
System.out.println(" Set Elements");
System.out.print("\t" + s1);
Map m1 = new HashMap(); // or new TreeMap() will order based on keys
m1.put("Windows", "98");
m1.put("Win", "XP");
m1.put("Beginner", "Java");
m1.put("Tutorial", "Site");
System.out.println();
System.out.println(" Map Elements");
System.out.print("\t" + m1);
} }
Output
ArrayList Elements
[Beginner, Java, tutorial]
LinkedList Elements
[Beginner, Java, tutorial]
Set Elements
[tutorial, Beginner, Java]
Map Elements
{Tutorial=Site, Windows=98, Win=XP, Beginner=Java}