Introduction:
Top Java Interview Questions For Freshers
1. What is Java?
Java is a high-level, object-oriented programming language that was first released by Sun Microsystems in 1995.
It is designed to be platform-independent, which means that Java programs can run on any operating system without requiring any changes to the code. Java is widely used for developing desktop applications, web applications, mobile applications, and embedded systems.
2. What are the features of Java?
Java has several features that make it a popular programming language. Some of its key features are:
Object-oriented programming: Java is a fully object-oriented programming language, which means that it supports encapsulation, inheritance, and polymorphism.
Platform-independent: Java is designed to be platform-independent, which means that Java code can run on any platform without requiring changes to the code. This is achieved through the use of the Java Virtual Machine (JVM), which provides a runtime environment for Java programs.
Simple and easy to learn: The syntax of Java is simple and easy to understand, which makes it easier for developers to learn and use.
Secure: Java has built-in security features that protect against unauthorized access to sensitive data and prevent malicious code from running on the system.
Multi-threaded: Java supports multithreading, which allows multiple threads of execution to run concurrently, making it possible to write programs that can perform multiple tasks simultaneously.
3. What is the difference between JDK, JRE, and JVM?
JDK, JRE, and JVM are all important components of the Java platform, but they serve different purposes:
JVM (Java Virtual Machine): JVM is an abstract machine that provides a runtime environment for Java programs. It interprets compiled Java bytecode and executes it on the host machine. The JVM provides memory management, security, and other important services that make Java platform-independent.
JRE (Java Runtime Environment): JRE is a software package that includes the JVM, class libraries, and other components required to run Java applications. It is a subset of the JDK and is used to run Java applications on a user’s machine.
JDK (Java Development Kit): JDK is a software development kit that includes the JRE, a Java compiler, and other development tools required to develop Java applications. It is used by developers to write, compile, and debug Java code.
4. What is the main difference between abstract class and interface in Java?
In Java, an abstract class and an interface are both used to define a contract for a class to implement. However, there are some key differences between the two:
Method implementation: An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. This means that an abstract class can provide a default implementation for some of its methods, while an interface cannot.
Multiple inheritance: A class can implement multiple interfaces, but can only extend one abstract class. This means that interfaces are more flexible than abstract classes when it comes to multiple inheritance.
Access modifiers: An abstract class can have any access modifier for its members, while an interface can only have public methods and constants.
Object creation: An abstract class cannot be instantiated on its own, but can only be subclassed. An interface cannot be instantiated or subclassed, but can be implemented by a class.
5. What is a constructor in Java? How many types of constructors are there in Java?
In Java, a constructor is a special type of method that is used to initialize objects of a class. When an object of a class is created using the “new” keyword, the constructor is called automatically to initialize the object. The constructor has the same name as the class and does not have a return type.
There are two types of constructors in Java:
Default constructor: A default constructor is a constructor that is provided by the compiler if no constructor is explicitly defined in the class. It takes no arguments and initializes all instance variables to their default values (0 for numeric types, null for object types, false for boolean).
Parameterized constructor: A parameterized constructor is a constructor that takes one or more arguments and is used to initialize the instance variables of the object to specific values. It is defined explicitly in the class and can be overloaded to take different combinations of argument.
Here is an example of a parameterized constructor:
In this example, the Person class has a parameterized constructor that takes two arguments: name and age. When an object of the Person class is created using the new keyword, the constructor is called with the specified arguments to initialize the name and age instance variables.
6. What is a package in Java? Why is it used?
In Java, a package is a way to organize related classes and interfaces into a single namespace. A package is a directory that contains a collection of related Java classes and interfaces.
Encapsulate code: Packages allow you to encapsulate your code and provide a clear separation between different parts of your program. This makes it easier to maintain and update your code.
Reuse code: Packages allow you to reuse code across different projects. You can create a package that contains common functionality and reuse it in multiple projects.
7. What is the difference between ArrayList and LinkedList?
In Java, ArrayList and LinkedList are both implementations of the List interface, but they differ in how they store and access elements.
Storage: ArrayList stores its elements in a resizable array, while LinkedList stores its elements in a doubly-linked list.
Access: ArrayList provides constant-time access to elements using the get method, while LinkedList provides constant-time access to elements only at the beginning and end of the list using the getFirst and getLast methods, respectively. Accessing elements in the middle of a LinkedList requires traversing the list, which takes linear time.
Insertion and Deletion: Insertion and deletion of elements in an ArrayList require shifting the elements in the array, which takes linear time. In contrast, insertion and deletion of elements in a LinkedList can be done in constant time, as long as the position of the element is known.
Memory: ArrayList requires less memory than LinkedList, as it does not need to store pointers to the next and previous elements.
8. What is the difference between a static method and an instance method in Java?
In Java, a static method is a method that belongs to the class and not to any particular instance of the class, while an instance method is a method that belongs to an instance of the class.
Here are some key differences between static and instance methods in Java:
Access: Static methods can be accessed using the class name, while instance methods can be accessed using an object of the class.
Memory: Static methods are stored in memory once, regardless of how many instances of the class are created, while instance methods are stored in memory for each instance of the class.
Parameters: Static methods can only access static variables and cannot access instance variables or methods, while instance methods can access both static and instance variables and methods.
Overriding: Static methods cannot be overridden, only hidden, while instance methods can be overridden.
Polymorphism: Polymorphism is not possible with static methods, while it is possible with instance methods.
9. What is the difference between an abstract class and a concrete class?
In Java, an abstract class is a class that cannot be instantiated, while a concrete class is a class that can be instantiated.
Here are some key differences between abstract classes and concrete classes in Java:
Instantiation: Abstract classes cannot be instantiated directly, while concrete classes can be instantiated using the new keyword.
Implementation: Abstract classes can have abstract methods, which do not have an implementation and must be implemented by subclasses, while concrete classes have all their methods implemented.
Inheritance: Abstract classes can be used as a base class for inheritance, while concrete classes cannot be used as a base class for inheritance unless they are explicitly designed for it using the final keyword.
Objects: Abstract classes do not represent objects, while concrete classes represent objects.
Usage: Abstract classes are typically used as a way to define a common interface for a group of related classes, while concrete classes are used to create objects and perform operations on them.
10. What is the purpose of the ‘finalize’ method in Java?
In Java, the finalize() method is a method that gets called by the garbage collector when an object is about to be garbage collected. Its purpose is to give the object a chance to perform any necessary cleanup before it is destroyed.