Every Java Freshers and Experienced should know this 50 Questions Before attend the Interview
1.What is Java?
Java is a high-level programming language and a platform for developing applications, particularly for mobile devices, embedded systems, and the web.
2.Difference between abstract class and interface?
- An abstract class can contain methods with implementations and fields, while an interface can only contain method signatures.
- A class can implement multiple interfaces, but can only inherit from one abstract class.
3.What is the difference between a private and a protected method in Java?
A private method can only be accessed within the same class, while a protected method can be accessed within the same class and its subclasses.
4.What is the purpose of the static keyword in Java?
The static keyword is used to create class-level variables and methods, which belong to the class rather than a specific instance of the class.
5.Difference between a ArrayList and a LinkedList in Java?
- An ArrayList is an implementation of a dynamic array, while a LinkedList is an implementation of a doubly-linked list.
- ArrayList has better performance for random access and retrieval of elements, while LinkedList has better performance for inserting and deleting elements.
6.Difference between a method and a constructor in Java?
A constructor is a special method that is called when an object is created, and it is used to initialize the object's state. A method is a named block of code that performs a specific task and can be called multiple times.
7.Difference between a for-each loop and a for loop in Java?
- A for loop uses a counter to iterate through a set of values, while a for-each loop directly accesses elements of an array or collection without using an index.
- The for-each loop is more concise and easier to read, but it cannot be used to modify the elements being accessed.
8.What is inheritance in Java and how does it work?
Inheritance is a mechanism in Java that allows a class to inherit properties and behaviors from a parent class. It enables code reuse and polymorphism, allowing objects of different types to be treated as objects of a common type.
9.What is the difference between a stack and a queue in Java?
A stack is a last-in, first-out (LIFO) data structure, while a queue is a first-in, first-out (FIFO) data structure. In a stack, the most recently added element is the first one to be removed, while in a queue, the first element to be added is the first one to be removed.
10.What is a Thread in Java?
A Thread is a lightweight unit of execution in Java, allowing multiple tasks to run concurrently within a single application. Threads can be used to improve performance by enabling multiple tasks to run in parallel.
11.What is the difference between a String and a StringBuffer in Java?
A String is an immutable object, meaning its value cannot be changed once it is created. A StringBuffer is a mutable object, meaning its value can be modified. Strings are more efficient for cases where their value is not expected to change, while StringBuffers are more efficient for cases where their value will change frequently.
12.What is the purpose of the final keyword in Java?
The final keyword is used to indicate that a variable, method, or class cannot be overridden or modified. This can be used to improve code stability and security by preventing unintended changes.
13.What is the difference between a HashMap and a Hashtable in Java?
Both HashMap and Hashtable are implementations of the Map interface, but HashMap is not synchronized while Hashtable is. This means that HashMap is not thread-safe, while Hashtable is. HashMap also allows for null values, while Hashtable does not.
14.Difference between .equals and == in Java?
The '==' operator compares object references to see if they refer to the same object in memory. The .equals() method compares the contents of the objects to see if they are equal, but the implementation may vary depending on the object's class.
15.What is a NullPointerException in Java?
A NullPointerException is an exception that occurs when an application tries to use a null reference as if it were an object reference. It usually indicates an error in the code where a variable or object that was expected to have a value is instead null.
16.Define exception in Java?
An exception in Java is an abnormality that occurs during programming. It is used to signal errors or abnormal conditions that cannot be handled within the normal flow of execution. Exceptions are thrown, caught and handled in Java to provide a way to deal with unexpected conditions in a controlled manner.
17.What is the use of the synchronized keyword in Java?
The synchronized keyword is used to control access to a shared resource in a multi-threaded environment. It ensures that only one thread can access the shared resource at a time, avoiding synchronization problems such as race conditions and data corruption.
18.What is a lambda expression in Java?
A lambda expression is a new feature in Java 8 that allows you to write anonymous functions. It is a compact and convenient way to represent a single method interface using an expression. A lambda expression is also known as a closure, and it enables functional programming in Java.
19.Difference between a wrapper class and a primitive data type in Java?
A wrapper class is a class that wraps or encloses a primitive data type. It is an object representation of a primitive data type, allowing it to be manipulated as an object. Primitive data types are basic data types such as int, double, etc., that are not objects and do not have methods associated with them.
20.Define abstract class in Java?
An abstract class is a class that cannot be expressed and is used as a base class for other classes. An abstract class can contain both abstract and concrete methods, and it is up to the subclasses to provide implementations for the abstract methods.
21.What is a JAR file in Java?
A JAR file is a Java archive file that is used to package multiple Java classes and resources into a single file. It is a way to distribute Java code and make it easy to share and reuse between projects. JAR files can be executed as standalone applications or included in other applications as libraries.
22.Difference between a private and a protected method in Java?
A private method is a method that can only be accessed within the same class in which it is declared. A protected method is a method that can be accessed within the same class and by subclasses in different packages.
23.Define Constructor in Java?
A constructor is a special type of method in Java that is used to create and initialize objects. It has the same name as the class and is called when an object of that class is created. If a class does not have a constructor, Java provides a default constructor automatically.
24.Difference between a instance and an Static method in Java?
A static method is a method that is associated with the class and not with individual instances of the class. It can be called directly on the class, without the need to create an instance of the class. An instance method is a method that is associated with individual instances of the class and requires an instance of the class to be created before it can be called.
25.What is a Java Bean?
A Java Bean is a Java class that is used to represent a reusable component in Java. It is a convention-based model that is used to create reusable and customizable components. Java Beans are required to follow a set of conventions, such as providing a no-arg constructor and exposing properties through getters and setters.
26.What is the difference between an Array and an ArrayList in Java?
An array is a fixed-size data structure that holds a collection of elements of the same data type. An ArrayList is a dynamic data structure that holds a collection of elements of the same data type and can be resized dynamically.
27.What is the difference between a Stack and a Queue in Java?
A Stack is a Last-In-First-Out (LIFO) data structure that holds a collection of elements and adds and removes elements from the top of the stack. A Queue is a First-In-First-Out (FIFO) data structure that holds a collection of elements and adds elements to the end of the queue and removes elements from the front of the queue.
28.What is a HashMap in Java?
A HashMap is a Map implementation in Java that stores key-value pairs and uses a hash table to implement the map. A hash table uses a hash function to calculate an index in the form of buckets or spaces, from which the desired value can be found. The HashMap class provides constant-time average performance for the basic operations (get and put), making it one of the most popular Map implementations in Java.
29.What is a Set in Java?
A Set is a collection in Java that holds a set of distinct elements and does not allow duplicate elements. The Set interface extends the Collection interface and adds a stronger contract on the behavior of the add, equals, and hashCode operations. The Java platform provides several Set implementations, including HashSet and TreeSet.
30.What is the difference between a HashSet and a TreeSet in Java?
A HashSet is a Set implementation in Java that uses a hash table to store the elements and provides constant-time average performance for the basic operations (add, remove, and contains). A TreeSet is a Set implementation in Java that uses a tree structure to store the elements and provides logarithmic time performance for the basic operations. A TreeSet also provides several ordering options for the elements, such as natural ordering or a custom comparator.
31.Difference between a while loop and a do-while loop?
A while loop in Java is a control flow statement that repeatedly executes a block of code as long as the specified condition is true. A do-while loop is similar to a while loop, but the block of code is executed at least once, and then the condition is evaluated. If the condition is true, the code block is executed again.
32.Difference between a for loop and a for-each loop?
A for loop in Java is a control flow statement that is used to execute a block of code a specified number of times. It uses a counter that is incremented on each iteration. A for-each loop is used to iterate over elements in an array or collection. The for-each loop provides a simple and concise way to iterate over elements in an array or collection.
33.What is an interface in Java?
An interface in Java is a blueprint of a class that defines a set of methods but does not provide any implementation. An interface can be used to achieve polymorphism and to define a common set of methods that must be implemented by multiple classes. Interfaces are useful for defining contracts between objects, and they are a key part of the Java programming language.
34.What is a lambda expression in Java?
A lambda expression in Java is a shorthand syntax for writing anonymous functions. A lambda expression is a block of code that can be passed as an argument to a method or used to define a functional interface. Lambda expressions were introduced in Java 8 and provide a way to write more concise and functional code in Java.
35.What is a functional interface in Java?
A functional interface in Java is an interface that defines exactly one abstract method. Functional interfaces are used as the basis for lambda expressions and provide a way to pass behavior as a method argument. The Java platform includes several functional interfaces, such as Runnable and Comparator, and users can also define their own functional interfaces.
36.What is a constructor in Java?
A constructor in Java is a special method that is used to create an instance of a class. It has the same name as the class and is called when an object of the class is created. Constructors can have parameters and can be used to initialize the state of an object when it is created.
37.What is the difference between a private and a protected method in Java?
A private method in Java is a method that can only be accessed within the same class and cannot be accessed from outside the class. A protected method in Java is a method that can be accessed within the same class, as well as in subclasses and classes in the same package. Protected methods can be overridden in subclasses, which allows for the implementation to be changed in a subclass.
38.What is a synchronized method in Java?
A synchronized method in Java is a method that is used to ensure that only one thread can execute the method at a time. The synchronized keyword is used to indicate that a method is synchronized, and it is used to prevent race conditions in multi-threaded applications. When a synchronized method is called, the thread acquires a lock on the object and blocks other threads from executing the method until the first thread has completed its execution.
39.What is the difference between an abstract class and an interface in Java?
An abstract class in Java is a class that can contain both abstract methods (methods without an implementation) and concrete methods (methods with an implementation). An abstract class is used to provide a common base class for subclasses that can share common behavior. An interface in Java is a blueprint of a class that defines a set of methods but does not provide any implementation. An interface can be used to achieve polymorphism and to define a common set of methods that must be implemented by multiple classes.
40.What is the difference between a try-catch block and a try-with-resources statement in Java?
A try-catch block in Java is a construct used to handle exceptions. It consists of a try block that contains the code that might throw an exception, and a catch block that contains the code to handle the exception. A try-with-resources statement is a try block that is used to manage the automatic release of resources, such as file handles or database connections, that are opened in the try block. The resources are automatically closed at the end of the try block, regardless of whether an exception was thrown or not.
41.What is an object in Java?
An object is an instance of a class. An object has a state and behavior, and it represents a real-world entity or concept. Objects can be created from classes, and they can interact with other objects through method calls. Objects are the fundamental building blocks of object-oriented programming in Java.
42.What is inheritance in Java?
Inheritance in Java is a mechanism that allows a class to inherit properties and behavior from another class. The class that inherits from another class is called a subclass, and the class that is being inherited from is called the superclass. Inheritance provides a way to reuse code and to create a hierarchy of classes that share common behavior.
43.What is polymorphism in Java?
Polymorphism in Java is the ability of an object to take on many forms. Polymorphism is achieved by using interfaces, abstract classes, and method overloading and overriding. Polymorphism allows objects to be treated as objects of their superclass, even though they are instances of a subclass. This allows for more flexible and reusable code, as well as for greater code clarity.
44.What is the difference between == and equals in Java?
The == operator in Java is used to compare primitive values or references to objects. The == operator compares two references to determine if they refer to the same object. The equals method in Java is used to compare the values of objects, rather than their references. The equals method is defined in the Object class and can be overridden in subclasses to provide a custom implementation for comparing the values of objects.
45.What is a static method in Java?
A static method in Java is a method that is associated with the class, rather than with an instance of the class. Static methods can be called without creating an instance of the class, and they can only access static variables and other static methods. Static methods are often used for utility methods that do not need access to instance variables or methods.
46.What is a final class in Java?
A final class in Java is a class that cannot be extended or subclassed. A final class is defined using the final keyword, and it is used to prevent subclassing and to ensure that the behavior of the class cannot be changed by a subclass. Final classes are often used for classes that define constants or utility methods.
47.What is a final method in Java?
A final method in Java is a method that cannot be overridden in a subclass. A final method is defined using the final keyword, and it is used to prevent subclassing and to ensure that the behavior of the method cannot be changed by a subclass. Final methods are often used for utility methods or methods that perform critical operations.
48.What is the difference between a final class and a final method in Java?
A final class in Java is a class that cannot be extended or subclassed, while a final method is a method that cannot be overridden in a subclass. A final class ensures that the class and its behavior cannot be changed by a subclass, while a final method ensures that the behavior of the method cannot be changed by a subclass.
49.What is a wrapper class in Java?
A wrapper class in Java is a class that wraps a primitive data type and provides an object-oriented representation of the data. The wrapper classes in Java are Integer, Double, Float, Short, Long, Byte, and Character. Wrapper classes are used to convert primitive data types to objects and vice versa, and they provide additional functionality, such as parsing strings to primitive values or converting values to strings.
50.What is a package in Java?
A package in Java is a namespace that groups related classes and interfaces together. Packages are used to organize code and to prevent naming conflicts between classes with the same name. Packages also provide access protection, and classes in one package can be made inaccessible to classes in other packages. Packages in Java are defined using the package keyword, and they are imported into other classes using the import keyword.
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