OOP is a programming paradigm based on the concept of “objects”, which can contain data and code: data in the form of fields (often known as attributes or properties), and code, in the form of procedures (often known as methods).
How does Java implement polymorphism?
Java implements polymorphism through method overloading and overriding. Overloading occurs when two methods in the same class have the same name but different parameters. Overriding happens when a subclass has a method with the same name and parameters as a method in its superclass.
What is encapsulation in Java?
Encapsulation is the mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit. In encapsulation, the variables of a class are hidden from other classes, accessible only through the methods of their current class. It’s achieved using access modifiers.
Can you differentiate between abstraction and encapsulation?
Abstraction is the concept of hiding the complex reality while showing the essential features. It focuses on what an object does. Encapsulation, on the other hand, focuses on how an object achieves its functionality, hiding the inner workings from outside interference.
What is inheritance and how is it implemented in Java?
Inheritance is a mechanism wherein a new class is derived from an existing class. In Java, inheritance is implemented using the extends keyword. The derived class inherits all the non-private members of the base class.
Explain the concept of interfaces in Java.
An interface in Java is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces cannot contain instance fields. The methods in interfaces are abstract by default.
What are abstract classes and how do they differ from interfaces?
Abstract classes are classes that cannot be instantiated and are declared with the abstract keyword. An abstract class can contain abstract and non-abstract methods. Interfaces can only contain abstract methods (until Java 8 introduced default and static methods in interfaces), and all methods in an interface are implicitly public and abstract.
How does Java handle multiple inheritance?
Java does not support multiple inheritance with classes to avoid the ambiguity caused by the diamond problem. However, it supports multiple inheritance through interfaces, allowing a class to implement multiple interfaces.
What is the Liskov Substitution Principle?
It’s a principle in object-oriented programming stating that objects of a superclass shall be replaceable with objects of a subclass without affecting the correctness of the program. Subclasses should extend the base classes without changing their behavior.
What is method overloading and method overriding?
Method overloading is when multiple methods have the same name but different parameters. Method overriding occurs when a subclass has a method with the same name and parameter list as a method in the superclass, offering a specific implementation of the method.
How does the super keyword function in Java?
The super keyword is used to access and call the parent class members (methods and constructors). It’s particularly used to call the superclass constructor from a subclass.
What is the final keyword and how can it be applied in Java?
The final keyword can be applied to classes, methods, and variables. A final class cannot be subclassed, a final method cannot be overridden, and a final variable’s value can only be assigned once.
How are this and super keywords different?
this is used to refer to the current object, while super is used to refer to the parent class object. this is often used for current class members access and constructor chaining, whereas super is used for accessing superclass members and constructors.
What are inner classes and why would you use them?
Inner classes are defined within the scope of other classes. They are used to logically group classes that are only used in one place, to increase encapsulation, and to create more readable and maintainable code.
How can you achieve encapsulation without using the private modifier?
Encapsulation can also be achieved using package-private (default) access modifier, which restricts access to the members of the class to other classes within the same package. Additionally, using interfaces and abstract classes can also contribute to encapsulation by hiding the implementation details from the user.
What is the difference between association, aggregation, and composition in Java?
Association is a relationship where all objects have their own lifecycle and there is no owner. Aggregation is a specialized form of association where all objects have their own lifecycle but there is ownership, and child objects can belong to another parent object. Composition is a more restricted form of aggregation where the child objects do not have their lifecycle and if the parent object is deleted, all its child objects will also be deleted.
How do you implement a singleton class in Java?
A singleton class is a class that can have only one object (an instance of the class) at a time. To design a singleton class:
Make constructor as private.
Write a static method that has a return type object of this singleton class. This method, called say getInstance(), manages the creation and lifetime of the instance.
What are the SOLID principles?
The SOLID principles are a set of design principles in object-oriented software development aimed at making software designs more understandable, flexible, and maintainable. They include Single Responsibility Principle, Open/Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle.
How can you prevent a class from being subclassed in Java?
By declaring the class as final, Java prevents the class from being subclassed.
Explain the concept of dependency injection and its benefits.
Dependency Injection is a design pattern that allows the removal of hard-coded dependencies, making it possible to change them, whether at runtime or compile time. This leads to more modular, testable, and maintainable code.
What is the significance of the @Override annotation?
The @Override annotation indicates that the method below it overrides a method declared in a superclass. It’s not required but helps to prevent errors by alerting the compiler to check the superclass for a matching method.
How does Java achieve runtime polymorphism?
Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime, which means the method that will be called is decided during the application’s runtime, depending on the object’s instantiation.
What is object cloning in Java?
Object cloning refers to the creation of an exact copy of an object. It is achieved by using the clone() method of the Object class. The class must implement the Cloneable interface, indicating that it allows for its objects to be cloned.
What are generics and how do they enhance Java’s type system?
Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. They provide a way to re-use the same code with different inputs and help in type checking at compile-time, enhancing type safety and reducing runtime errors.
How do you handle immutable objects in Java?
Immutable objects are those whose state cannot be changed once they are created. This can be achieved by declaring all the fields private and final, not providing setters, and if necessary, making deep copies of objects in the getters.
What are the advantages of using design patterns in Java?
Design patterns provide tested, proven development paradigms, improving code readability for coders and architects familiar with the patterns. They help to solve common problems with a defined approach, making code more robust and flexible.
How can OOP principles lead to better code reusability?
OOP principles like inheritance and polymorphism allow for the creation of a more generic code that can work with various data types or objects, promoting reusability. Encapsulation helps in creating a clear interface for parts of the code, making it easier to use and reuse.
What is method hiding in Java and how does it differ from overriding?
Method hiding occurs when a static method in a subclass has the same signature as a static method in the superclass. Unlike overriding, method hiding means that the version of the method that is called depends on the type of the reference, not the object.
How can you use composition to achieve polymorphic behavior in Java?
Composition involves creating classes that reference one or more objects of other classes in order to achieve polymorphic behavior. It’s a way to achieve flexibility in your designs by composing objects to perform tasks, allowing for polymorphism and code reuse.
What is the diamond problem in multiple inheritance and how does Java address it?
The diamond problem occurs in scenarios where a class inherits from two classes that have a common base class. This can create ambiguity as to which base class method the subclass should inherit if both base classes modify the same method. Java addresses this problem by not allowing multiple inheritances with classes and using interfaces instead, as interfaces do not hold state and can provide multiple inheritance safely.