Java Exceptions:
1.) ArithmeticException
2.) CollectionException
3.) ArrayIndexOutOfBoundsException
4.) ClassNotFoundException
5.) IOException
6.) FileNotFoundException
7.) ArrayListException
8.) NumberFormatException
9.) FontFormatException
10.) NullPointerException
☺ Benefits of Inheritance in OOP : Reusability - Once a behavior (method) is defined in a superclass, that behavior is automatically inherited by all subclasses. – Thus, you can encode a method only once and they can be used by all subclasses. – A subclass only needs to implement the differences between itself and the parent.
☺Overriding Methods - If for some reason a derived class needs to have a different implementation of a certain method from that of the superclass, overriding methods could prove to be very useful. A subclass can override a method defined in its superclass by providing a new implementation for that method.
To override, the getName method of the superclass Person, we write in the subclass Student,
public class Student extends Person{
:
:
public String getName(){
System.out.println("Student: getName");
return name;
}
:
}
Now, when we invoke the getName method of an object of the subclass Student, the overridden getName method would be called, and the output would be,
Student: getName
☺Final Classes
– Classes that cannot be extended
– To declare final classes, we write,
public final ClassName{
. . .
}
Example:
public final class Person {
. . .
}
Other examples of final classes are your wrapper classes and Strings.
☺Final Methods
– Methods that cannot be overridden
– To declare final methods, we write,
public final [returnType] [methodName]([parameters]){
. . .
}
Static methods are automatically final.
●Polymorphism
– The ability of a reference variable to change behavior according to what object it is holding.
– This allows multiple objects of different subclasses to be treated as objects of a single superclass, while automatically selecting the proper methods to apply to a particular object based on the subclassit belongs to. To illustrate polymorphism, let us discuss an example.
☺Given the parent class Person and the subclass Student of the previous examples, we add another subclass of Person which is Employee.
☺Abstract class
– a class that cannot be instantiated.
– often appears at the top of an object-oriented programming class hierarchy, defining the broad types of actions possible with objects of all subclasses of the class.
☺An interface
– is a special kind of block containing method signatures (and possibly constants) only.
– defines the signatures of a set of methods, without the body.
– defines a standard and public way of specifying the behavior of classes.
– allows classes, regardless of their locations in the class hierarchy, to implement common behaviors.
– NOTE: interfaces exhibit polymorphism as well, since program may call an interface method, and the proper version of that method will be executed depending on the type of object passed to the interface method call.
●Object - is composed of a set of data (properties) which are variables describing the essential characteristics of the object, and consists of a set of methods (behavior) that describes how an object behaves.
– An object is an instance of a class.
•Classes provide the benefit of reusability.
• Software programmers can use a class over and over again
to create many objects.
•Class Variable - a class variable is a variable defined in a class (i.e. a member variable) of which a single copy exists, regardless of how many objects of the class exist.
Classes consist of
– Instance variables
– Instance methods
– Class Variables (static member variables)
•variables that belong to the whole class.
•This means that they have the same value for all the objects in the same class.
•Class Instantiation - the process of creating objects from a class is called instantiation. So an object is always an instance of a class which represents the blueprint.
The object is constructed using the class and must be created before being used in a program.
Objects are manipulated through object references (also called reference values or simply references)
•The new operator
– allocates a memory for that object and returns a reference of that
memory location to you.
– When you create an object, you actually invoke the class'
constructor.
•The constructor
– is a method where you place all the initializations, it has the same
name as the class.
•Method - A Java method is a set of Java statements which can be included inside a Java class.
Java methods are similar to functions or procedures in other programming languages.
•The following are characteristics of methods:
– It can return one or no values
– It may accept as many parameters it needs or no parameter at all.
Parameters are also called arguments.
– After the method has finished execution, it goes back to the method
that called it.
Every Java program must have one main() method.
Here is the main() method from a Java program which prints "Hello World":
public static void main (String[] args) {
// This Java program prints "Hello World!"
System.out.println{"Hello World!");
}
☻Method Declaration - a method declaration is the heading of a method containing the name of the method, its parameters, and its access level. The method heading in Java is organized as such:
[access keywords] [return type] [method name] ( [parameters separated by commas] )
for instance:
public String toString(); is public (accessible by any class), returns a String, is called toString, and takes no parameters.
Other features could be added to the method declaration for a more specialized method such as static (method could be called without an object of that class), native (implemented using the native code, usually what C has already done, i.e. square root, power etc.).
☻Static Methods - methods that can be invoked without instantiating a class (means without invoking the new keyword).
– Static methods belong to the class as a whole and not to a certain instance (or object) of a class.
– Static methods are distinguished from instance methods in a class definition by the keyword static.
• To call a static method, just type,
Classname.staticMethodName(params);
parameters during calculations.
– all primitive data types when passed to a method are pass-by-value.
● Pass-by-Reference – When a pass-by-reference occurs, the reference to an object is passed to the calling method. This means that, the method makes a copy of the reference of the variable passed to the method.
– However, unlike in pass-by-value, the method can modify the actual object that the reference is pointing to, since, although different references are used in the methods, the location of the data they are pointing to is the same.
To read input data, by default we use the computer keyboard. However, some of the data we need maybe stored in some data file. For example, if you have a class record stored in a file and you want to retrieve it, or simply want to read a text file using your java program.
Reading from a file:
The FileInputStream class allows you to read data from a file.
The class has the following constructors:
FileInputStream(File file)
Creates a FileInputStream by opening a connection to an actual file, the file named by the File object file in the file system.
o FileInputStream(FileDescriptor fdObj)
Creates a FileInputStream by using the file descriptor fdObj, which represents an existing connection to an actual file in the file system.
o FileInputStream(String name)
Creates a FileInputStream by opening a connection to an actual file, the file named by the path name name in the file system.
c.) JAVA Features
e.) Difference between JAVA application and JAVA appletsIn simple terms, an applet runs under the control of a browser, whereas an application runs stand-alone, with the support of a virtual machine. As such, an applet is subjected to more stringent security restrictions in terms of file and network access, whereas an application can have free reign over these resources.
Applets are great for creating dynamic and interactive web applications, but the true power of Java lies in writing full blown applications. With the limitation of disk and network access, it would be difficult to write commercial applications (though through the user of server based file systems, not impossible). However, a Java application has full network and local file system access, and its potential is limited only by the creativity of its developers.
f.) What makes JAVA an object oriented programming language?
Java is called an Object Oriented Programming Language because many of the object oriented programming concepts are implemented in Java.
Some concepts are:
1. Class
2. Object
3. Inheritance
4. Encapsulation
5. Polymorphism
6. Abstraction
