OOPS concept interview questions


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Q) What are different oops concept in java?

Ans) OOPs stands for Object Oriented Programming. The concepts in oops are similar to any other programming languages. Basically, it is program agnostic.

oops concept in java

The different OOps concepts are :

Q1) What is polymorphism?

Ans) The ability to identify a function to run is called Polymorphism. In java, c++ there are two types of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).

Mehtod overriding: Overriding occurs when a class method has the same name and signature as a method in parent class. When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time.

Overloading: Overloading is determined at the compile time. It occurs when several methods have same names with:

  • Different method signature and different number or type of parameters.
  • Same method signature but different number of parameters.
  • Same method signature and same number of parameters but of different type
Example of Overloading
int add(int a,int b)
 float add(float a,int b)
 float add(int a ,float b)
 void add(float a)
 int add(int a)
 void add(int a) //error conflict with the  method int add(int a)
class BookDetails {
  String title;
  setBook(String title){}
}
class ScienceBook extends BookDetails {
  setBook(String title){} //overriding
  setBook(String title, String publisher,float price){} //overloading
}

Q2) What is inheritance?

Ans) Inheritance allows a Child class to inherit properties from its parent class. In Java this is achieved by using extends keyword. Only properties with access modifier public and protected can be accessed in child class.

public class Parent {
  public String parentName;  
  public String familyName;

  protected void printMyName() {
    System.out.println(“ My name is “+ chidName+” “ +familyName);
  }
}
public class Child extends Parent {
  public String childName;
  public int childAge;
  //inheritance
  protected void printMyName() {
    System.out.println(“ My child name is “+ chidName+” “ +familyName);
  }
}

In above example the child has inherit its family name from the parent class just by inheriting the class. When child object is created printMyName() present in child class is called.

Q3) What is multiple inheritance and does java support?

Ans) If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes. The problem with with multiple inheritance is that if multiple parent classes has a same method name, the at runtime it becomes diffcult for compiler to decide which method to execute from the child class. To overcome this problem it allows to implement multiple Interfaces. The problem is commonly referred as What is Diamond Problem.

Q) What is difference between polymorphism and inheritance?

  • Inheritance defines parent-child relationship between two classes, polymorphism take advantage of that relationship to add dynamic behaviour in your code.
  • Inheritance helps in code reusability by allowing child class to inherit behavior from the parent class. On the other hand Polymorphism allows Child to redefine already defined behaviour inside parent class. Without Polymorphism it's not possible for a Child to execute its own behaviour while represented by a Parent reference variable, but with Polymorphism he can do that.
  • Java doesn't allow multiple inheritance of classes, but allows multiple inheritance of Interface, which is actually require to implement Polymorphism. For example a Class can be Runnable, Comparator and Serializable at same time, because all three are interfaces. This makes them to pass around in code e.g. you can pass instance of this class to a method which accepts Serializable, or to Collections.sort() which accepts a Comparator.
  • Both Polymorphism and Inheritance allow Object oriented programs to evolve. For example, by using Inheritance you can define new user types in an Authentication System and by using Polymorphism you can take advantage of already written authentication code. Since, Inheritance guarantees minimum base class behaviour, a method depending upon super class or super interface can still accept object of base class and can authenticate it.

Q4) What is an abstraction ?

Ans) Abstraction is a way of converting real world objects in terms of class. Its a concept of defining an idea in terms of classes or interface. For example creating a class Vehicle and injecting properties into it. E.g

public class Vehicle {
  public String colour;
  public String model;
}

Q5) What is Encapsulation?

Ans) The encapsulation is achieved by combining the methods and attribute into a class. The class acts like a container encapsulating the properties. The users are exposed mainly public methods.The idea behind is to hide how thinigs work and just exposing the requests a user can do.

Q6) What is Association?

Ans) Association is a relationship where all object have their own lifecycle and there is no owner. Let's take an example of Teacher and Student. Multiple students can associate with single teacher and single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently.

Q7) What is Aggregation?

Ans) Aggregation is a specialize form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object. Let's take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. We can think about "has-a" relationship.

Q8) What is Composition ?

Ans) Composition is again specialize form of Aggregation and we can call this as a "death" relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let's take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.

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