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Java Interview Questions
Both “ArrayList” and “LinkedList” implement the “List” interface, but they have different underlying implementations:
– “ArrayList” is backed by an array. It provides fast random access with O(1) complexity but can be slow in inserting or deleting elements in the middle since it may require shifting elements.
– “LinkedList” is implemented using a doubly-linked list. It provides faster insertion and deletion at O(1) complexity (if the node reference is known, such as with an iterator), but getting an element at a specific index has O(n) complexity.
An immutable object is one whose state cannot be changed after it is created. Making an object immutable can provide thread-safety and reduce potential side effects. Examples of immutable classes in Java are “String”, “Integer”, “Double”, “LocalDate”, and “LocalTime”.
Both are interfaces used for object comparison:
– “Comparable” is used for providing a single natural sorting order for the implementing class via its “compareTo” method.
– “Comparator” is used to define multiple different ways to compare instances of a class. It’s typically used when we want to sort a list of objects in ways other than their natural ordering.
Java provides automatic garbage collection. The JVM periodically runs the garbage collector (GC) to find and delete objects that are no longer reachable in the application. The main algorithms include generational collection where the heap is divided into Young and Old generations, with the idea that most objects quickly become unreachable.
Both are used to achieve synchronization in multithreading:
– A “synchronized” method locks on the object’s monitor (or the class’s monitor for static methods). When a thread enters a synchronized method, no other thread can access any synchronized methods on the same object.
– A “synchronized” block allows finer-grained locking by specifying exactly what object’s monitor to lock on. This can help in reducing contention by not locking the entire method.
The Java Memory Model (JMM) is an abstraction of the underlying computer architecture and provides a specification that guarantees visibility of changes to variables made by one thread to another. It defines rules and semantics for reading and writing volatile and non-volatile variables in a multithreaded environment.
In Java, the “volatile” keyword is used to indicate that a variable’s value might be changed by multiple threads simultaneously. It guarantees visibility of changes to a variable across threads. When a variable is declared as volatile, reads and writes to that variable are directly done on the main memory, avoiding local thread caches.
Introduced in Java 7, “try-with-resources” simplifies resource management. Any object that implements the “AutoCloseable” interface (like “InputStream” or “OutputStream”) can be used with this statement. When the try block is exited, either due to successful completion or because of an exception, the “close()” method on the resource is automatically called.
Type erasure is a process where the generic type information is removed during compilation, and the code is transformed to use the raw type (Object, for example). This is done for backward compatibility with older Java versions that didn’t have generics. The compiler inserts necessary casts in the bytecode to ensure type safety.
The “==” operator checks for reference equality, meaning it checks if two references point to the same memory location. The “equals()” method checks for content equality and needs to be overridden for custom objects to provide meaningful checks. By default, the “equals()” method in the “Object” class behaves the same as “==”.
Checked exceptions are those exceptions that need to be either caught or declared in the method signature using the “throws” keyword. They are used for scenarios where the client can take corrective action. Examples include “IOException” and “SQLException”. Unchecked exceptions, derived from “RuntimeException”, don’t need to be declared or caught, and they generally represent programming errors. Examples include “NullPointerException” and “ArithmeticException”.
– final: A keyword that can be applied to classes, methods, and variables.
– Classes: Prevents inheritance.
– Methods: Prevents method overriding in subclasses.
– Variables: Makes them immutable once assigned.
– finally: A block in a try-catch structure that ensures its contained code always runs, regardless of exceptions being thrown or not.
– finalize: A method in the “Object” class which is invoked by the garbage collector before discarding objects, giving an opportunity to release system resources or perform cleanup.
– Interface: It is a completely ‘abstract’ type that can be used to represent common behaviors of objects. Interfaces can’t contain any concrete methods (prior to Java 8, after which default and static methods were introduced).
– Abstract Class: It’s a class that cannot be instantiated on its own and can have both abstract (without implementation) and concrete (with implementation) methods.
In Java, exceptions are handled using “try”, “catch”, and “finally” blocks. We wrap the code that might throw an exception inside a “try” block and catch the exception in corresponding “catch” blocks. The “finally” block, if present, always executes regardless of an exception occurring.
Polymorphism in Java allows objects of different classes to be treated as objects of a common superclass. The most common use is when a parent class reference points to a child class object. This helps in writing generic and reusable code.
Threading in Java can be implemented by either extending the “Thread” class or implementing the “Runnable” interface. After that, the “start()” method is called to execute the thread.
“HashMap” is based on the hash table and provides constant-time performance for basic operations. “TreeMap” is based on a red-black tree and orders its keys based on their natural ordering or a custom comparator. So, “HashMap” offers faster operations but doesn’t maintain order, while “TreeMap” is slower but provides sorted key order.
Java security can be ensured by using Java’s built-in security features like the Java Security Manager, bytecode verification, and JAAS. Additionally, always validating input, using prepared statements for SQL, following best practices, and keeping libraries/frameworks up-to-date are essential.
A Servlet is a Java class that handles requests, processes them, and sends back a response. It works as an intermediary between the client and the server in a Java web application. Servlets receive the request, process it (like querying a database), and then generate a response.
The MVC (Model-View-Controller) pattern is a design pattern that breaks an application into three interconnected components, which makes it easier to manage, scale, and maintain applications, especially web applications. Spring’s web MVC framework is designed around the MVC pattern, and it integrates seamlessly with the Spring core container.
Here’s a brief overview of how the MVC pattern works in Spring:
- Model: Represents the data structure and business logic. It is usually composed of POJOs (Plain Old Java Objects) and can leverage Spring’s dependency injection to manage services, data sources, and other necessary components. It is responsible for retrieving and storing data, often from a database.
- View: Represents the presentation layer. In Spring, views are usually JSPs, Thymeleaf templates, or other templating systems, but Spring is flexible enough to integrate with various view technologies. The View takes the model data and displays it to the user.
- Controller: Acts as an interface between Model and View. It takes the user’s requests and inputs, processes them (with possible updates to the Model), and returns the appropriate View. In Spring, controllers are typically POJOs annotated with “@Controller”. The methods in the controller are mapped to URLs using annotations like “@RequestMapping”.
JPA (Java Persistence API) is a specification for object-relational mapping in Java. It allows Java objects to be mapped to database tables. Hibernate is an ORM framework that implements the JPA specifications. They help in simplifying database operations and interactions in Java applications.
JUnit is a widely-used testing framework in Java. It uses annotations like “@Test” for specifying methods that represent tests, “@BeforeEach” and “@AfterEach” for setup and teardown operations, and “@BeforeAll” and “@AfterAll” for one-time setup and teardown respectively. It allows automated testing of code units and ensures code reliability.