OOPs Questions

Question 1

Four main principles of OOPS Concepts?

Answer 1

Inheritance
Polymorphism
Data Encapsulation
Abstraction

Question 2

What is inheritance?

Answer 2

The process by which one class acquires the properties and functionalities of another class is called inheritance

Question 3

Does Java support Multiple Inheritance?

Answer 3

When a class extends more than one classes then it is called multiple inheritance. Java doesn’t support multiple inheritance

Question 4

What is Polymorphism?

Answer 4

Polymorphism is the ability of an object to take many forms. The most common use of polymorphism in OOPs is to have more than one method with the same name in a single class.

Question 5

What are the types of Polymorphism?

Answer 5

static polymorphism and dynamic polymorphism

Question 6

What is method overriding in Java?

Answer 6

When a sub class (child class) overrides the method of super class(parent class) then it is called overriding. To override a method, the signature of method in child class must match with the method signature in parent class

Question 7

Can we override a static method?

Answer 7

No, we cannot override a static method in Java.

Question 8

What is method overloading?

Answer 8

When a class has more than one methods with the same name but different number, sequence or types of arguments then it is known as method overloading

Question 9

Does Java support operator overloading?

Answer 9

Operator overloading is not supported in Java.

Question 10

Can we overload a method by just changing the return type and without changing the signature of method?

Answer 10

No, We cannot do this. To overload a method, the method signature must be different, return type doesn’t play any role in method overloading.

Question 11

Is it possible to overload main() method of a class?

Answer 11

Yes, we can overload main() method in Java.

Question 12

What is the difference between method overloading and method overriding?

Answer 12

Overloading happens at compile-time while Overriding happens at runtime: The binding of overloaded method call to its definition has happens at compile-time however binding of overridden method call to its definition happens at runtime.

Question 13

What is static and dynamic binding in Java?

Answer 13

Binding refers to the linking of method call to its body. A binding that happens at compile time is known as static binding while binding at runtime is known as dynamic binding.

Question 14

What is Encapsulation?

Answer 14

Wrapping of the data and code together is known as encapsulation

Question 15

What is an abstract class in Java?

Answer 15

An abstract class is a class which can’t be instantiated (we cannot create the object of abstract class), we can only extend such classes. It provides the generalised form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. We can achieve partial abstraction using abstract classes, to achieve full abstraction we use interfaces.

Question 16

What is Interface in java?

Answer 16

An interface is used for achieving full abstraction. A class implements an interface, thereby inheriting the abstract methods of the interface.

Question 17

What is the difference between abstract class and interface?

Answer 17

1) abstract class can have abstract and non-abstract methods. An interface can only have abstract methods.
2) An abstract class can have static methods but an interface cannot have static methods.
3) abstract class can have constructors but an interface cannot have constructors.

Question 18

Name the access modifiers that can be applied to the inner classes?

Answer 18

public ,private , abstract, final, protected.

Question 19

What is a constructor in Java?

Answer 19

Constructor is used for creating an instance of a class, they are invoked when an instance of class gets created. Constructor name and class name should be same and it doesn’t have a return type.

Question 20

Can we inherit the constructors?

Answer 20

No, we cannot inherit constructors.

Question 21

Can we mark constructors final?

Answer 21

No, Constructor cannot be declared final.

Question 22

What is a default constructor?

Answer 22

Constructors with no arguments are known as default constructors, when you don’t declare any constructor in a class, compiler creates a default one automatically.

Question 23

What is a parameterized constructor?

Answer 23

Constructor with arguments are known as parameterized constructors.

Question 24

Can a constructor call another constructor?

Answer 24

Yes. A constructor can call the another constructor of same class using this keyword. For e.g. this() calls the default constructor.
Note: this() must be the first statement in the calling constructor.

Question 25

Can a constructor call the constructor of parent class?

Answer 25

Yes. In fact it happens by default. A child class constructor always calls the parent class constructor. However we can still call it using super keyword. For e.g. super() can be used for calling super class default constructor.

Note: super() must be the first statement in a constructor.

Question 26

THIS keyword?

Answer 26

The this keyword is a reference to the current object.

Question 27

Can this keyword be assigned null value?

Answer 27

No, this keyword cannot have null values assigned to it.

Question 28

Explain ways to pass the arguments in Java?

Answer 28

In java, arguments can be passed as call by value – Java only supports call by value, there is no concept of call by reference in Java.

Question 29

What is static variable in java?

Answer 29

Static variables are also known as class level variables. A static variable is same for all the objects of that particular class in which it is declared.

Question 30

What is static block?

Answer 30

A static block gets executed at the time of class loading. They are used for initializing static variables.

Question 31

What is a static method?

Answer 31

Static methods can be called directly without creating the instance (Object) of the class. A static method can access all the static variables of a class directly but it cannot access non-static variables without creating instance of class.

Question 32

Explain super keyword in Java?

Answer 32

super keyword references to the parent class. There are several uses of super keyword:

It can be used to call the superclass(Parent class) constructor.
It can be used to access a method of the superclass that has been hidden by subclass (Calling parent class version, In case of method overriding).
To call the constructor of parent class.

Question 33

Use of final keyword in Java?

Answer 33

Final methods – These methods cannot be overridden by any other method.
Final variable – Constants, the value of these variable can’t be changed, its fixed.
Final class – Such classes cannot be inherited by other classes. These type of classes will be used when application required security or someone don’t want that particular class

Question 34

What is a Object class?

Answer 34

This is a special class defined by java; all other classes are subclasses of object class. Object class is superclass of all other classes.

objectClone ()
boolean equals(Object obj)
finalize()
toString ()

Question 35

objectClone ()

Answer 35

to creates a new object that is same as the object being cloned.

Question 36

boolean equals(Object obj)

Answer 36

determines whether one object is equal to another.

Question 37

finalize()

Answer 37

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup.

Question 38

toString ()

Answer 38

Returns a string representation of the object.

Question 39

What are Packages in Java?

Answer 39

A Package can be defined as a grouping of related types (classes, interfaces, enumerations and annotations).

Question 40

What is the difference between import java.util.Date and java.util.* ?

Answer 40

The star form (java.util.* ) includes all the classes of that package and that may increase the compilation time – especially if you import several packages. However it doesn’t have any effect run-time performance

Question 41

Garbage collection in java?

Answer 41

Since objects are dynamically allocated by using the new operator, java handles the de-allocation of the memory automatically, when no references to an object exist for a long time. This whole process is called garbage collection. The whole purpose of Garbage collection is efficient memory management.

Question 42

Use of finalize() method in java?

Answer 42

finalize() method is used to free the allocated resource.