Java OOPS Interview Questions and Answers

Java is an object-oriented programming language that is widely used in developing web and mobile applications. As such, it is important for developers to have a solid understanding of Java OOPS concepts. Here, we will provide a summary of some common Java OOPS interview questions that you may be asked during a job interview. Java OOPS questions can be divided into two main categories: beginner questions and advanced questions. Beginner Java OOPS technical interview questions are typically focused on basic concepts, such as inheritance and polymorphism. Expert OOPS concepts interview questions, on the other hand, are typically focused on more advanced topics, such as interfaces and abstract classes. Regardless of your level of expertise, we believe that these OOPS concepts in Java interview questions will help you prepare for your next assessment. No matter what level you're at, being able to answer these Java OOPS interview questions confidently will go a long way in landing the job you want. By reviewing these questions and answers before your interview, you'll be well-prepared to demonstrate your knowledge of OOPS and basic Java Training concepts. With a little practice, you'll be able to confidently answer any Java OOPS interview questions that come your way.

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In Java, abstraction is the process of hiding the implementation details of a class and exposing only the functionality to the outside world. By doing so, it provides greater flexibility and modularity in code design. There are two ways to achieve abstraction in Java: through abstract classes and interfaces. 

Abstract classes are classes that cannot be instantiated, meaning they can only be subclassed. An abstract class may contain both abstract and non-abstract methods (methods with implementation).

However, it is not mandatory for an abstract class to contain any abstract methods. On the other hand, an interface can only contain abstract methods. A class can implement multiple interfaces but can extend only one abstract class. 

When should abstraction be used? Abstraction should be used when you need to hide the complexity of your code from the outside world. For example, consider a car. The user does not need to know how the engine works in order to drive the car. All he needs to know is how to start it and how to use the pedals (accelerator and brake). The rest is taken care of by the car itself. In a similar way, abstraction allows you to hide the complexity of your code and expose only the functionality that is needed by the outside world. 

The answer to this question lies in the design philosophy of Java. The creators of Java wanted to keep the language simple and easy to learn. They felt that allowing operators to be overloaded would make the language too complex. In addition, they believed that operator overloading could lead to unexpected results.

For example, consider the + operator. In most programming languages, this operator can be used for both addition and concatenation. However, if we overloaded the + operator in Java, it would become ambiguous. Should it perform addition or concatenation? 

For example, consider the following code: 

int x = 10; 

int y = 5; 

x + y; //15 

x - y; //5 

x * y; //50 

x / y; //2 

Now, let's say we overloaded the + operator to add two strings together instead of two numbers. With this change, the meaning of the code above would be changed and could potentially lead to unexpected results. Therefore, by not supporting operator overloading, Java can avoid these potential issues and remain a simple and easy-to-use programming language. 

In Java, binding refers to the linking of a function with the object that calls it. Static binding occurs when this linking is done at compile time, while dynamic binding happens at runtime. Static binding is also often referred to as early binding.

There are several benefits to static binding. For one, it makes code easier to read and understand because you can see exactly which functions are being called for each object. It can also make code more efficient since the compiler can optimize the code more easily. Finally, it can help to catch errors early on, since the compiler will flag any type mismatches.

Dynamic binding is more flexible than static binding since it allows for different objects to call different functions at runtime. This can be useful in situations where you don't know ahead of time which object will be calling the function. However, dynamic binding can also lead to more errors since type mismatches may not be caught until runtime. In general, static binding should be used whenever possible for maximum clarity and efficiency.

In Object-Oriented Programming, method overloading is a feature that allows a class to have more than one method with the same name but different parameters. The different parameters can be of different data types or even number of parameters. This allows the class to have multiple methods with the same name that can be invoked with different arguments.

For example, a class could have two methods with the same name but with different numbers of parameters. One method could take an integer and return a string, while the other method could take two integers and return an integer. Method overloading is often used when creating methods that perform similar operations on different data types.

By overloading the method, the programmer can reuse the code without having to write separate methods for each data type. This also makes the code easier to read and understand as it is not necessary to remember which method performs which operation.

In order to overload a method, the programmer must first create a class with two or more methods with the same name but different parameters. The compiler will then identify the methods by their unique signatures, which include the name of the method and the number and type of parameters. Once the methods have been identified, the compiler will select the appropriate method to invoke based on the arguments that are passed to the method call. If no match is found, an error will be generated.

Method overloading is a powerful feature of Object-Oriented Programming that can make code more readable and easy to understand. By using method overloading, programmers can reuse code without having to write separate methods for each data type. This also makes code easier to read and understand as it is not necessary to remember which method performs which operation. 

In object-oriented programming, it is possible to call a method without creating an instance of the class. This is known as a static method. Static methods are typically used for utility functions, such as math operators or string manipulation functions. However, static methods can also be used to call base class methods. For example, consider the following Java code: 

public class BaseClass { 
 public static void main(String[] args) {
 System.out.println("Hello, world!");
public class DerivedClass extends BaseClass {
 public static void main(String[] args) {
 public static void printMessage() {
 System.out.println("Hello from DerivedClass!");

In this code, the DerivedClass class inherits from the BaseClass class. The DerivedClass class also has a static method called printMessage(). This static method can be called without creating an instance of DerivedClass. However, it can also be used to call the static main() method of BaseClass. To do this, the DerivedClass.main() method must first be invoked, and then the BaseClass.main() method can be invoked using the super keyword, like this: 

DerivedClass.main(); // First invoke DerivedClass.main()

super.main(); // Then invoke BaseClass.main() using the super keyword

When this code is run, it will first print “Hello from DerivedClass!” and then “Hello, world!” This shows that it is possible to call a base class method without creating an instance of the class. Of course, this only works if the base class method is static. If the base class method is not static, then an instance of the class must be created before it can be called. 

One of the key ideas in object-oriented programming is inheritance, which allows a child class to inherit methods and properties from a parent class. However, inheritance also has some limitations.

  • First, it can create complex class hierarchies. For example, if a child class inherits from a parent class, and the parent class inherits from a grandparent class, the child class will have two levels of inherited methods and properties. This can make it difficult to understand how a particular method or property is being used.
  • Second, inheritance can lead to duplicate code. For example, if two child classes inherit from the same parent class, they may both contain identical methods or properties. This can make it difficult to make changes to the code, as any changes made in one child class will need to be made in the other child class as well.
  • Finally, inheritance can cause problems with encapsulation. For example, if a child class inherits from a parent class, the child class will have access to all of the parent class's methods and properties. This could potentially allow a child class to access or change data that it should not have access to. As a result, inheritance should be used carefully in order to avoid these potential problems. 

In an object-oriented programming language, a pure virtual function or abstract function is a virtual function for which we don't provide any implementation, in other words, we only declare it. A class containing a pure virtual function is called an abstract class.  

We use pure virtual functions when we want to create a base class that defines an interface but we don't want to implement all the functions in the base class, instead, we want the derived classes to provide their own implementation as per their requirements.  

A class with at least one pure virtual function is an abstract class. We can't create an object of an abstract class because if we do so there is no guarantee that all the pure virtual functions of that class are implemented or not. Also, if a class has even a single pure virtual function then the destructor of that class must be virtual otherwise we get undefined behavior.

Coupling refers to the degree of interdependence between modules. A high degree of coupling indicates that a change in one module is likely to result in changes in other modules. Cohesion, on the other hand, refers to the degree to which the elements inside a module are related to each other. A highly cohesive module is one where all the elements are closely related to each other and work together to achieve a common goal. In general, it is desirable to have low coupling and high cohesion. This means that modules should be independent of each other and that the elements inside a module should be tightly integrated.

Static polymorphism is when a method is overloaded with different signatures but the signature used is determined at compile time. This is also known as early binding because the type of object is determined at compile time. An example of static polymorphism would be if there were two methods, both called add. One method could take in two integers and return the sum while the other could take in two strings and concatenate them. The correct add method would be selected at compile time based on the types of arguments being passed in.  

Static polymorphism is generally preferred over dynamic polymorphism because it can lead to more efficient code since the decision about which method to call is made during compilation rather than at runtime. Additionally, it can avoid potential errors that could occur if the wrong method were called at runtime.

In Java, methods may be defined as static. This means that the method is attached to the class, rather than to a specific instance of that class. As a result, it is not possible to override a static method in Java. Attempting to do so will result in a compiler error. However, it is possible to overload a static method. This means that multiple methods can have the same name, as long as they have different parameter signatures. 

For example, a class could have two static methods with the name "calculate," one that takes two integers as parameters and one that takes three integers as parameters. Overloading provides a way to make methods more flexible and easier to use. However, it is important to remember that static methods cannot be overridden.

The Observer design pattern is a way of structuring code so that one class can automatically update another class when it changes state. This is often used in GUI programming, so that when a button is clicked, for example, the display can be updated accordingly. The Observer design pattern has two main components: the Subject and the Observer. The Subject is the object being watched, and the Observer is the object doing the watching. In order for this to work, the Subject must provide a way for Observers to register themselves; this usually takes the form of an addObserver() method.  

The Subject will then notify all of its Observers whenever it changes state, typically by calling a method such as update(). The Observers can then use this information to update their own state accordingly. The Observer design pattern is a powerful tool, but it should be used with care. If too many Observers are registered with a Subject, then notification messages can start to pile up, potentially leading to performance problems. Therefore, it is important to consider whether using this design pattern is really the best solution for a particular problem before implementing it.

Operator overloading is a feature of some programming languages that allows operators to be overloaded so that they can work on data of different types. For example, in the Java programming language, the + operator can be used to add two numbers or concatenate two strings. In order to overload an operator, a programmer must define the new behavior of the operator in a class or structure.  

Operator overloading can be useful for making code more readable and concise, but it can also lead to code that is difficult to understand. When overloading an operator, it is important to choose a meaning that will be intuitive for users of the code. Otherwise, the code may be difficult to maintain and understand.

Java does not support multiple inheritance because it can lead to ambiguity. For example, if a class extends two classes that have the same method signature, it is not clear which method the subclass should inherit. This can lead to unexpected behavior and make code difficult to understand and maintain.  

Additionally, multiple inheritance can lead to the Diamond Problem, in which a subclass inherits conflicting methods from its superclasses. To avoid these issues, Java only supports single inheritance, meaning a class can extend only one other class. While this may seem limiting, it helps to keep code more robust and predictable. As a result, Java developers can be confident that their code will behave as expected.

In Java, association is a relationship between two classes that allows one class to access the members of the other class. There are majorly three types of association: aggregation, composition, and inheritance. Aggregation is a weak form of association where one class is simply associated with another class; there is no ownership involved. Composition is a stronger form of association where one class owns another class; if the owner class is destroyed, the owned class will also be destroyed. Inheritance is the strongest form of association, where one class inherits from another class; if the parent class is destroyed, the child class will still exist.  

In addition to these three types of association, there are also two subtypes of association: unidirectional and bi-directional. Unidirectional association means that only one class can access the members of the other class; bi-directional association means that both classes can access each other's members.

Java provides three access level modifiers for classes and members: public, private, and protected. The default access level is package-private, which means that the class or member can only be accessed by other classes in the same package.  

Public classes and members can be accessed by any other class, regardless of package. Private classes and members can only be accessed by other members of the same class. Protected members can be accessed by other members of the same class and its subclasses, regardless of package.  

The main difference between public, private and protected access modifier is that public modifier makes a class or member accessible from anywhere, private modifier makes a class or member only accessible within the declared class itself and protected modifier makes a class or member accessible within the declared class and its subclasses.

Constructor chaining is the process of one constructor calling another constructor in the same class, or in a derived class. The most common use for constructor chaining is to initialize inherited member variables to their proper starting values. When one constructor calls another, the first thing that happens is that the body of the called constructor is executed. This can lead to problems if the base class constructor has not yet finished initializing the object.  

To avoid this, Java provides the keyword super, which can be used to call the base class constructor from within a derived class constructor. By using super, we can ensure that the base class constructor is always called before the body of the derived class constructor is executed. In this way, we can be sure that our objects are always initialized properly.

There are three different types of inheritance in Java: single, multi-level, and hierarchical. Single inheritance is when a child class inherits from a parent class. Multi-level inheritance is when a child class inherits from a parent class, which itself inherits from another parent class. Hierarchical inheritance is when a child class inherits from multiple parent classes. Each type of inheritance has its own benefits and drawbacks, so it's important to choose the right one for your needs.  

Single inheritance is the simplest and most common type of inheritance, but it can lead to problems if the child class needs to inherit from more than one parent class. Multilevel inheritance can be useful for sharing code between classes that are closely related, but it can also lead to complex code that is difficult to maintain. Hierarchical inheritance is the most flexible type of inheritance, but it can be hard to keep track of all the different parent classes.

A copy constructor is a member function which initializes an object using another object of the same class. A copy constructor is called when a new object is initialized as a copy of an existing object. Copy constructors are usually implemented by calling the base class's copy constructor and then initializing the derived class's members with the appropriate values.  

However, sometimes it's necessary to write a custom copy constructor if the default behavior is not correct for the class. For example, if a class has pointers to dynamic data, then the default behavior of the copy constructor will simply result in both objects pointing to the same data. In this case, it's necessary to write a custom copy constructor that allocates new memory for the data and then copies the data from the existing object.

Data abstraction is the process of hiding the details of a particular implementation from the user. In object-oriented programming, this is usually accomplished by creating an interface that presents only the essential features of the underlying implementation. Encapsulation, on the other hand, is the process of binding together data and code that manipulate that data. In OOP, this is typically accomplished by creating objects that contain both data and code. Encapsulation also allows for information hiding, which means that the internals of an object can be hidden from the outside world. This can make it easier to change the internals of an object without affecting code that uses that object.

A superclass is a class from which other classes can inherit methods and properties. In Java, every class has one superclass, with the exception of the Object class, which is the root of the class hierarchy. When a subclass inherits from a superclass, it inherits all of the superclass's non-private methods and members. However, a subclass can override certain methods and members, as well as define new ones.  

For example, if a superclass has a method named getName() that Returns the name of the object, a subclass could override this method to Return the full name of the object (e.g., "John Smith"). Similarly, if a superclass only has a default constructor (a constructor with no parameters), a subclass could define additional constructors with different parameters. Finally, a subclass can also add new methods and members that are not present in the superclass. By definition, therefore, a superclass is always more general than its subclasses.

There are several key differences between an abstract class and an interface. First, an abstract class can contain both abstract and non-abstract methods, while an interface can only contain abstract methods. Second, an abstract class can have instance variables, while an interface cannot. Third, an abstract class can have constructors, while an interface cannot. Finally, a subclass of an abstract class can only inherit from one superclass, but a class can implement multiple interfaces. Consequently, each type of structure has its own advantages and disadvantages that must be considered when designing a new system.

When it comes to method overloading, the general rule is that you can overload a method as long as the parameter list is different. This means that you can have two methods with the same name but different parameter lists. The compiler will then be able to distinguish between the two methods based on the number and type of parameters. As for method overriding, the rule is that you can only override a method if it is inherited from a superclass. So if you have a class A with a method foo(), and class B inherits from A, then B can override foo() as long as it has the same signature (i.e. same number and type of parameters). If B doesn't inherit from A, then it cannot override foo().

Exception handling is a method of responding to the occurrence of exceptional circumstances (“exceptions”) during the execution of a program. When an exceptional circumstance occurs, an exception is “thrown.” If the exception is not handled, it “terminates” the program—that is, the program stops running. To handle an exception, you write code that detects when an exception can occur and then takes appropriate action. This code is typically written in a “try/catch block.” A try/catch block consists of a try block followed by one or more catch blocks. The try block contains the code that might throw an exception. The catch blocks contain the code that handles the exceptions thrown by the try block.

When an exception is thrown, execution of the try block is halted and control transfers to the first catch block whose catch statement matches the type of exception thrown. If no match is found, the default exception handler is executed. The default exception handler displays a message and terminates the program. You can also write code that throws an exception explicitly—that is, code that causes an exception to be thrown intentionally. This code is typically written in a “throw statement.”

A throw statement consists of the keyword throw followed by an expression that evaluates to an object (the exception object). When a throw statement is executed, control is transferred to the first catch block whose catch statement matches the type of object thrown by the throw statement. If no match is found, the default exception handler is executed. The default exception handler displays a message and terminates the program. 

A finally block is a block of code that will always be executed, whether or not an exception is thrown. For example, if a method opens a file, it should place the file in a finally block to ensure that the file is always closed, even if an exception is thrown. A finally block is not used to handle exceptions; instead, it is used to perform cleanup tasks, such as closing open files or releasing resources that were acquired in the try block.  

Finally blocks are often placed after try/catch blocks, but they can also be used alone. When used alone, finally blocks are typically used to perform cleanup tasks that must always be executed, such as closing open files or releasing acquired resources.

The Decorator design pattern is based on the concept of inheritance. This pattern allows you to extend the functionality of an existing object without having to modify the code of the original object. The new functionality is added by creating a new object that wraps the original object. The new object has the same interface as the original object, but it also includes the new functionality. This allows you to seamlessly add new features to an existing object without breaking compatibility with existing code. The Decorator design pattern is a versatile and powerful tool that can be used to great effect in a wide range of situations.


different level of access that a class member may have. 

Public members are accessible to all other classes, regardless of whether they're in the same package or not. Private members, on the other hand, can only be accessed by other members of the same class. Protected members are accessible by members of the same class and subclasses, but not by classes that are not related. 

Access specifiers can be applied to both variables and methods. In general, it's good practice to make instance variables private and provide public getter and setter methods to allow other classes to access them. This encapsulation prevents outside classes from directly manipulating the data, which could lead to errors.

When it comes to methods, making them public means that they can be called from anywhere. This is usually desirable for methods that need to be accessed by many different classes. On the other hand, private methods can only be called from within the defining class, which can be useful for hiding implementation details.

In summary, access specifiers help to control the visibility of class members and protect data encapsulation. They're an important part of Object-Oriented Programming in Java and should be used carefully to maintain a well-designed codebase. 

In computer programming, a garbage collector (GC) is a form of automatic memory management. The garbage collector attempts to reclaim memory that is no longer needed by the program. This process is known as garbage collection. 

Garbage collectors are most often used in programming languages that use a managed runtime environment, such as the Java platform. In these environments, the programmer does not have full control over when and how memory is allocated and freed. Instead, the runtime environment manages these aspects of memory management automatically. The garbage collector is one part of this runtime environment. 

The garbage collector runs periodically within the managed runtime environment. When it runs, it attempts to identify objects that are no longer needed by the program and reclaims the memory used by those objects. In some cases, the garbage collector may also compact memory to reduce fragmentation.

There are several different algorithms that can be used for garbage collection. Some of the more common algorithms are reference counting, mark-sweep, and tracing. Each of these algorithms has its own strengths and weaknesses.

Reference counting, for example, is simple to implement but can suffer from memory leaks if certain types of objects are not properly handled. Mark-sweep is more complex but can provide better performance in terms of memory usage and execution time. Tracing is even more complex but can provide the best performance of all the algorithms.

Which algorithm is used for garbage collection can have a significant impact on the performance of a program. Therefore, it is important for programmers to understand the basics of how garbage collectors work and the different algorithms that are available. With this knowledge, programmers can make informed choices about which algorithms to use for their programs. 

One of the key concepts in OOPS is inheritance, which allows programmers to create relationships between classes and reuse code. However, inheritance has some limitations that developers should be aware of.

  • First, inheritance creates what is known as a tight coupling between classes, which can make code difficult to maintain and refactor.
  • Second, inheritance can lead to unexpected behavior if super and subclasses are not carefully designed.
  • Finally, inheritance can make code harder to understand, as the relationship between super and subclasses may not be immediately apparent.

Despite these limitations, inheritance is still a powerful tool that can be used to great effect by experienced programmers. 

In object-oriented programming, polymorphism refers to the ability of an object to take on multiple forms. There are two main types of polymorphism: compile-time polymorphism and runtime polymorphism.

Compile-time polymorphism is also known as static binding. An example of compile-time polymorphism would be method overloading, which allows a class to have multiple methods with the same name but different signatures. The compiler is able to determine which method to call at compile time based on the number and type of arguments passed by the caller.

Runtime polymorphism, on the other hand, is achieved through inheritance and dynamic binding. Dynamic binding means that the code associated with a call is not resolved until runtime. An example of runtime polymorphism would be overriding a method in a subclass. In this case, even though the subclass method has the same signature as the superclass method, it is still considered distinct. When an overridden method is invoked at runtime, the JVM will dynamically choose which version of the method to execute based on the type of the object being referred to.

So there are two types of Polymorphism: Static Polymorphism and Dynamic Polymorphism. Static Polymorphism is implemented during compilation time using function overloading whereas Dynamic Polymorphism is implemented during run time using function overriding.

An important thing to note about both types of polymorphism is that they can only be achieved through Inheritance or Interfaces (since multiple inheritance is not supported in Java). In other words, if a class does not inherit from another class or implement an interface, it will not be able to utilize either type of polymorphism.

Finally, it’s worth mentioning that while some languages (like C++) support both compile-time and runtime polymorphism, Java only supports runtime polymorphism. This is because Java uses a single dispatch model, meaning that only one method can be called per object per message. In contrast, C++ uses a multiple dispatch model, which allows for multiple methods to be called per object per message. 

No, a Java application cannot be run without the implementation of OOPs. The Java programming language is built on the OOPS concepts and hence it is mandatory to use OOPS concepts while developing a Java application. However, it is possible to write a Java application without using any objects and classes. But doing so would not make much sense as the main purpose of using Java is to develop Object-Oriented applications. Moreover, such an application would not be very maintainable or extensible in the long run. Hence, it is advisable to always use OOPS concepts while developing a Java application.

There are several advantages to using abstraction and inheritance in Java programming. First, abstraction allows for the development of more flexible and reusable code. By abstracting away certain details, code can be written in a way that is more general and thus can be applied to a wider range of situations. Second, inheritance provides a way to share common code between different classes, which can lead to more efficient code overall. Finally, abstraction and inheritance can make code easier to understand and maintain, as they help to break down complex problems into smaller, more manageable pieces. Consequently, these two features are often considered essential tools for any Java programmer.

Constructors are used to initialize an object, while methods are used to perform actions on an object. In other words, constructors create objects and methods define what they do. Another key difference is that constructors are called when an object is created, while methods are called when an action is performed on an object.  

For example, if you were creating a Dog class, the constructor would be called when you create a new Dog object, while a method would be called when you tell the Dog to sit or fetch. Finally, constructors have the same name as the class they're contained in, while methods can have any name. This naming convention helps to distinguish between the two.

In Java, an interface is a reference type that is used to specify a set of methods that a class must implement. In other words, an interface defines a contract that a class must adhere to. However, an interface does not provide any implementation for the methods it defines—that is the responsibility of the class that implements the interface. Interfaces are often used in Java to achieve polymorphism, which is the ability to execute different code depending on the type of object being dealt with.  

For example, a method could be written that takes an Object as a parameter. This method could then be invoked with different types of objects, and each time, different code would be executed depending on the actual type of object that was passed in. By using interfaces, Java developers can create flexible and reusable code that can be adapted to work with new types of objects without having to make any changes to the code itself.

Manipulators are a type of object that enables a programmer to modify the values of an underlying object without having direct access to that object. In object-oriented programming, manipulators are typically used to encapsulate data so that it can be safely accessed and manipulated by other objects in the system. By using manipulators, programmers can effectively control how data is accessed and modified, making it easier to maintain the integrity of the underlying data. Additionally, manipulators can provide a level of abstraction, allowing programmers to work with data without needing to understand the details of its implementation. Ultimately, manipulators can help to make code more modular and easier to understand.

When two methods in a class have the same name but different parameter types, it is known as method overloading. However, if a subclass overrides a method with a different return type, it is known as covariant method overriding. In Java, this is only possible if the return type of the subclass method is a subtype of the return type of the superclass method. For example, consider the following two classes: 

public class A { 
public Object m1() { ... } 
public class B extends A { 
public String m1() { ... } // return type is a subtype of Object 

In this example, the m1() method in class B covariantly overrides the m1() method in class A. This is allowed because the return type of the subclass method (String) is a subtype of the return type of the superclass method (Object). If the subclass had used a different return type (such as int), it would not have been considered covariant overriding, and an error would have been generated. 

When a new object is created, the Java virtual machine allocates memory for it and initializes the instance variables to their default values. The new keyword is used to create a new object. When you create an object using new, the Java virtual machine calls the constructor of the class to initialize the state of the object. The use of new also allows you to create objects by using a constructor that takes arguments. This allows you to initialize the fields of the object with different values when it is created. If you don't use new, the fields will all have their default values.  

For example, if you create a Point object without using new, its x and y fields will both be set to 0. If you create a Point object using new and pass in values for x and y, those values will be used to initialize the fields of the object.

A constructor in Java is a block of code that is executed when an instance of an object is created. The constructor initializes the state of the object by setting member variables to their desired values. There are a few rules to keep in mind when creating a constructor: 

  • The name of the constructor must be the same as the name of the class. 
  • A constructor cannot have a return type (void, int, etc.). 
  • A constructor can be overloaded, just like any other method. This means that you can create multiple constructors with different signatures.
  • If a class does not have a declared constructor, then the Java compiler will automatically generate a default constructor for the class. However, if you do declare at least one constructor for a class, then the compiler will not generate a default constructor.
  • Constructors can be invoked from other constructors using this keyword. This is useful for creating multiple constructors that initialize state in different ways.
  • Constructors can also call other methods in order to perform some initialization logic before returning control to the caller. This allows you to encapsulate all of your object's initialization logic in one place.

In Java, all parameters are passed by value. This means that when a method is called, the value of the parameter is copied into a new location in memory, and this copy is passed to the method. The main difference between pass by value and pass by reference is that with pass by value, the copied value is independent of the original value, while with pass by reference, the copied value is linked to the original value. This means that if you change the copied value, the original value will also be changed. With pass by value, changing the copied value will not affect the original value.

A class doesn't occupy any memory when it's created. Memory is only occupied when we create an instance of that class (i.e. an object of that class). When we create an object, memory is allocated to store the object. How much memory is required to store an object depends on how many instance variables the class has and the data type of those instance variables.

For example, if a class has two instance variables of type int, then 8 bytes of memory will be allocated for each object (4 bytes for each int). If the class has one instance variable of type double, then 12 bytes will be allocated for each object (8 bytes for the double + 4 bytes for a pointer to the next object in memory). The total memory required for a class is therefore the sum of the sizes of all its instance variables.

However, this is only an approximation because some classes have static fields and methods which are not included in this calculation. Classes can also have references to other objects, so the actual memory footprint of a class may be larger than this calculation suggests. Nevertheless, this should give you a general idea of how much memory is required for a given class. 

Operator overloading is a feature of some programming languages that allows operators to have different behaviors depending on the data types they are applied to. For example, in most programming languages, the + operator is used for addition, regardless of the type of data it is being applied to.  

However, in a language with operator overloading, the + operator could be used for addition when applied to numbers, but it could be used for string concatenation when applied to strings. This can provide a more natural and expressive syntax for some operations. However, it can also lead to confusion, as the same operator can have different meanings depending on the context. As a result, operator overloading is generally only used when it provides a clear benefit.

Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

Compile-time polymorphism is performed during compilation time and it can be defined as the ability to create a class with multiple methods that have the same name but different signatures. Method overloading comes under compile-time polymorphism.

Dynamic binding(runtime) means that the code associated with a given call to a method is not known until runtime. Static binding(compile time) means that the code associated with a given call to a method is known at compile time. An example of static binding would be calling a non-virtual function(function which can't be redefined in derived classes). An example of dynamic binding would be calling a virtual function(function which can be redefined in derived classes). 

Polymorphism allows us to invoke functions without knowing their addresses and type information, early binding cannot do this. For example, consider the following program written in C++:

In this program, the function Area() is invoked by simply using its name, without any type or address information. This illustrates polymorphism because depending upon the object for which Area() is invoked, a different version of Area() executes. If we had invoked Area() using its address, as illustrated here:

Or if we had specified its type explicitly, as illustrated here:

Then only one version of Area() would have been executed regardless of the object for which it was invoked. In order for our program to work correctly, the compiler must generate code that invokes Area() dynamically (that is, uses only its name), because it won’t know at compile time which version of Area() needs to be executed. This late/dynamic decision about which code to execute is what makes polymorphism possible; if this decision were made early/statically (at compile time), then we would not have polymorphism. 

There is a big difference between the assignment operator and copy constructor in Java. The assignment operator only copies the value of the object, whereas the copy constructor creates a new object with the same value as the original.

The assignment operator is used when an already existing object is assigned to a new variable. For example, if you have a variable called "x" that contains the value "5", and you want to create a new variable called "y" that also contains the value "5", you would use the assignment operator.

The copy constructor is used when you want to create a new object that is an exact copy of an already existing object. For example, if you have an object called "x" that contains the value "5", and you want to create a new object called "y" that also contains the value "5", you would use the copy constructor.

The main difference between the two is that the assignment operator only copies the value of the object, whereas the copy constructor creates a new object with the same value as the original. 

In object-oriented programming, inheritance is used to promote code reuse. It is a mechanism for creating new classes from existing classes by extending them. Hybrid inheritance is a combination of two or more types of inheritance. The most common types of inheritance are single, multilevel, and hierarchical. In hybrid inheritance, a derived class inherits from more than one base class. This can be accomplished by combining more than one type of inheritance in a single program.  

For example, a derived class could inherit from a base class and an interface. Hybrid inheritance is often used in situations where multiple inheritance is not possible or practical. For example, Java does not support multiple inheritance, but it does support hybrid inheritance. As a result, hybrid inheritance can be seen as a way to achieve the benefits of multiple inheritance while avoiding its potential problems.

Java does support operator overloading, but with some limitations. For example, you cannot overload the standard arithmetic operators (+,-,*,/). However, you can overload the + operator for string concatenation. You also cannot change the precedence of operators, and all overloaded operators must have at least one operand of a user-defined type. Despite these limitations, operator overloading can be a useful tool for increasing the readability and flexibility of your code.  

For instance, you could use operator overloading to create a ComplexNumber class that supports addition and subtraction using the + and - operators. When used judiciously, operator overloading can help to make your code more expressive and easier to understand.

There are a few different scenarios where the Singleton design pattern can be useful in Java. One common use case is when you need to ensure that only one instance of a given class is created. This might be important, for example, if you're working with a resource that can only be used by one object at a time.

Another scenario where Singleton can be handy is when you want to make sure that all instances of a particular class share the same state. In this case, using a Singleton ensures that any changes to the state will be reflected across all objects. Finally, another advantage of the Singleton pattern is that it can help to improve performance by avoiding the need to create multiple instances of an object.

In general, then, the Singleton design pattern can be a helpful tool in a number of different situations. If you need to make sure that only one instance of a class is created, or if you want to ensure that all objects share the same state, then using a Singleton may be the right choice. Additionally, the performance advantages of avoiding multiple object instantiations can also make this pattern worthwhile in some cases. 

In object-oriented programming, the three terms association, aggregation, and composition have distinct meanings. Understanding the difference between them is essential for correctly using these concepts in your code. 

Association represents a simple relationship between two objects, where one object uses another object. For example, a Person class might use a Car class, indicating that each person has a car. Association is usually represented by a line between the two classes. 

Aggregation represents a more complex relationship, where one object contains another object. For example, a Department class might contain a Person class, indicating that each department has several people. Aggregation is usually represented by a line with an arrowhead pointing to the container class. 

Composition is a special type of aggregation that indicates that the container object cannot exist without the container object. For example, a Person class might contain an Heart class, indicating that each person has only one heart and that the heart cannot exist without the person. Composition is usually represented by a line with a filled-in arrowhead pointing to the container class. 

Java supports four types of arguments: primitive data types, objects, arrays, and Strings. Primitive data types include int, float, and double. Objects are instances of classes, and can be used to call methods and access fields. Arrays are groups of data that are organized in a specific order. Strings are a special type of array that contains characters instead of numbers.  

Each type of argument has its own set of rules that must be followed when passing it to a method or function. For example, primitive data types can only be passed by value, while objects can be passed by reference. Arrays can be passed by value or reference, depending on the situation. And finally, Strings can only be passed by reference. Understanding how each type of argument works is essential for passing the correct arguments to methods and functions in Java.

Tokens are the smallest unit of a programming language. In Java, tokens include keywords, identifiers, literals, and operators. Keywords are reserved words that cannot be used as identifiers. They are used to identify the type of data or operation being performed. For example, int is a keyword that indicates an integer value. Identifiers are used to name variables, methods, and classes.  

Literals are Fixed values that are assigned to variables. They cannot be changed during program execution. For example, the number 10 is a literal. Operators perform operations on operands. They can be either unary or binary operators. Unary operators operate on a single operand; binary operators operate on two operands. For example, the + operator is a binary operator that adds two operands together.

The try-catch block is a programming construct that allows you to handle errors gracefully. It consists of two parts: the try block, which contains the code that might throw an error, and the catch block, which contains the code that will be executed if an error is thrown. For example, consider the following code: 

try { 
// Code that might throw an error 
} catch (Exception e) { 
// Code to handle the error 

If an error is thrown in the try block, execution will immediately jump to the catch block. The exception object (e) will contain information about the nature of the error, and you can use this information to take appropriate action. In many cases, simply logging the error message can be sufficient. Once the catch block has finished executing, execution will resume at the point where the try-catch block was called. Try-catch blocks can be nested, allowing you to create multiple levels of protection against errors. 

In Java, a class can inherit the constructor of its base class by using the keyword super. The syntax for doing this is as follows: 

class DerivedClassName extends BaseClassName {
// constructors
DerivedClassName() {

When a derived class inherits the constructor of its base class, the compiler automatically inserts a call to the base class constructor at the beginning of the derived class constructor. This ensures that the base class is always initialized before the derived class. Note that if a class does not explicitly inherit a base class constructor, then the default constructor (which takes no arguments) will be used instead. 


Java programming is based on the concept of object-oriented programming (OOP), which entails organizing code around objects rather than actions. This approach to programming has a number of benefits, including improved code reusability and self-contained units of code that are easier to debug. As a result, Java developers are often asked questions about OOPS concepts for interviews. 

The concepts of objects, classes, inheritance, abstraction, encoding, and polymorphism in Java interview questions form the basis of your knowledge and understanding. We are proud to offer the Java OOPS interview questions for beginners and experienced. We believe that this will help you to better understand the concept of object oriented programming and improve your chances of success in your next interview. We will start with the basics of Object Oriented Programming, and then move on to more advanced topics. We will also discuss some common design patterns used in Java programming. 

In order to make the most of this resource, we suggest that you take the time to read through all of the questions and think about your answers before you proceed to the next question. We also recommend that you attempt to answer each question as if you were in an actual interview setting. If you're new to object-oriented programming, you can also use KnowledgeHut’s Programming courses to brush up on the basics before your interview. 

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