Introduction To Object-Oriented Programming Flashcards
What is an object in Java?
An entity that has state and behavior is known as an object e.g., chair, bike, marker, pen, table, car, etc. It can be physical or logical (tangible and intangible). The example of an intangible object is the banking system.
An object has three characteristics:
State:represents the data (value) of an object.
Behavior:represents the behavior (functionality) of an object such as deposit, withdraw, etc.
Identity:An object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. However, it is used internally by the JVM to identify each object uniquely
For Example, Pen is an object. Its name is Reynolds; color is white, known as its state. It is used to write, so writing is its behavior.
An object is an instance of a class.A class is a template or blueprint from which objects are created. So, an object is the instance(result) of a class.
Object Definitions:
An object isa real-world entity.
An object isa runtime entity.
The object isan entity which has state and behavior.
The object isan instance of a class.
What is a class in Java
class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can’t be physical.
A class in Java can contains:
Fields
Methods
Constructors
Blocks
Nested class and interface
Instance variable in Java
A variable which is created inside the class but outside the method is known as an instance variable. Instance variable doesn’t get memory at compile time. It gets memory at runtime when an object or instance is created. That is why it is known as an instance variable.
new keyword in Java
The new keyword is used to allocate memory at runtime. All objects get memory in Heap memory area.
3 Ways to initialize object
There are 3 ways to initialize object in Java.
By reference variable
By method
By constructor
Initialization through reference
Initializing an object means storing data into the object. Let’s see a simple example where we are going to initialize the object through a reference variable.
classStudent{
intid;
Stringname;
}
classTestStudent2{
publicstaticvoidmain(Stringargs[]){
Students1=newStudent();
s1.id=101;
s1.name=”Sonoo”;
System.out.println(s1.id+”“+s1.name);//printingmemberswithawhitespace
}
}
Initialization through method
In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.
classStudent{
introllno;
Stringname;
voidinsertRecord(intr,Stringn){
rollno=r;
name=n;
}
voiddisplayInformation(){System.out.println(rollno+”“+name);}
}
classTestStudent4{
publicstaticvoidmain(Stringargs[]){
Students1=newStudent();
Students2=newStudent();
s1.insertRecord(111,”Karan”);
s2.insertRecord(222,”Aryan”);
s1.displayInformation();
s2.displayInformation();
}
}
Initialization through a constructor
What are the different ways to create an object in Java?
There are many ways to create an object in java. They are:
By new keyword
By newInstance() method
By clone() method
By deserialization
By factory method etc.
Anonymous object
Anonymous simply means nameless. An object which has no reference is known as an anonymous object. It can be used at the time of object creation only.
If you have to use an object only once, an anonymous object is a good approach. For example:
newCalculation();//anonymousobject
Calling method through a reference:
Calculationc=newCalculation();
c.fact(5);
Calling method through an anonymous object
newCalculation().fact(5);
example of an anonymous object in Java.
classCalculation{
voidfact(intn){
intfact=1;
for(inti=1;i<=n;i++){
fact=fact*i;
}
System.out.println(“factorialis”+fact);
}
publicstaticvoidmain(Stringargs[]){
newCalculation().fact(5);//callingmethodwithanonymousobject
}
}
