Chapter-11: Past Interview Question Java-3 : Question: What is closable resource in Java?
Answer: In Java, a closable resource is any object that implements the 'java.lang.AutoCloseable' or 'java.io.Closeable' interface. These interfaces have a 'close()' method which is called to release the resource or clean up when the object is no longer needed. The introduction of the try-with-resources statement in Java 7 allows for automatic resource management where closable resources are automatically closed when exiting the try block.
Question: NullPointerException is a compile time or Runtime exception?
Answer: 'NullPointerException' is a runtime exception. It occurs at execution time if a program attempts to access or modify an object or call a method on an object when the reference variable is 'null'.
Question: When do you use throw and throws clause?
Answer: 'throw' is used to explicitly throw an exception from a method or any block of code. For instance, 'throw new IllegalArgumentException("Invalid input!");'. On the other hand, 'throws' is used in the method signature to declare that the method might throw one or more exceptions. The caller must handle these exceptions, either by catching them or by declaring them using its own 'throws' clause.
Question: What is checked and unchecked exceptions?
Answer: Checked exceptions are those that need to be either caught using a catch block or declared in the method signature using the 'throws' keyword. These exceptions extend 'java.lang.Exception' but not 'java.lang.RuntimeException'. Unchecked exceptions, also known as runtime exceptions, don't need to be explicitly caught or declared. They extend 'java.lang.RuntimeException' or 'java.lang.Error'.
Question: How can you change the memory parameters for the Java?
Answer: Memory parameters for a Java application can be adjusted using JVM flags when starting the Java application. For instance, using '-Xms' for the initial heap size and '-Xmx' for the maximum heap size.
Question: How can you bound your Java program to have minimum memory to 2GB and max memory upto 4GB?
Answer: You can use the JVM flags '-Xms2G' and '-Xmx4G' when starting your Java application. This will set the initial heap size to 2GB and the maximum heap size to 4GB.
Question: When do you use break and continue?
Answer: Both 'break' and 'continue' are control flow statements. 'break' is used to terminate a loop or switch statement prematurely. 'continue' is used to skip the current iteration of a loop and jump to the next iteration.
Question: What is the Java Scanner class?
Answer: The 'Scanner' class in Java is part of the 'java.util' package. It provides methods to read input of different types (like int, long, double, byte, etc.) from various sources, including input streams, files, and strings. It's commonly used to read user input from the console.
Question: What is the difference between class and Object?
Answer: A class is a blueprint or prototype from which objects are created. It defines a set of properties (attributes) and methods (functions) that an object will have. An object, on the other hand, is an instance of a class. It's a concrete realization of the class and has a state (the values of its properties) and behavior (as defined by its class's methods).
Question: What is your preference to set variable using constructor or setter method?
Answer: The choice between using a constructor or a setter method depends on the specific use-case:
- If a variable is essential for an object to be in a valid state, it's better to set it through the constructor. This ensures that the object is always created in a valid state.
- If a variable is optional or can be changed after the object's creation, using a setter method provides more flexibility. Setters also allow for better encapsulation and can include validation logic.
Question: What is the Override and Overloading?
Answer: Overriding and overloading are two concepts in Java related to methods:
- Override: This occurs when a subclass provides a specific implementation for a method that is already defined in its superclass. The overridden method in the subclass should have the same name, return type, and parameters as the one in the superclass. The '@Override' annotation is used to signify that a method is intended to override a method in the superclass.
- Overloading: This refers to defining multiple methods in the same class with the same name but with different parameters (either different types or a different number of parameters). The return type is not considered while differentiating overloaded methods.
Question: What is the difference between StringBuilder and StringBuffer, which one you use and when?
Answer: Both 'StringBuilder' and 'StringBuffer' are mutable sequences of characters. The key difference is in synchronization:
- StringBuffer: Every method in 'StringBuffer' is synchronized, making it thread-safe. However, this synchronization introduces overhead, making it slower in single-threaded scenarios.
- StringBuilder: It was introduced later in Java and is essentially like 'StringBuffer' but without synchronization, making it faster but not thread-safe.
Typically, if you're working in a single-threaded environment or managing synchronization yourself, use 'StringBuilder' for better performance. In multi-threaded scenarios where thread safety is a concern, use 'StringBuffer'.
Question: What class would you use to make String joins?
Answer: The 'String. join()' method can be used to join strings with a specified delimiter. Additionally, Java 8 introduced the 'StringJoiner' class in the 'java.util' package, which can also be used for joining strings with a prefix, delimiter, and suffix.
Question: What is the Java reflection and when do you want to use it?
Answer: Java Reflection is a feature in Java that allows inspection of, or interaction with, class properties and methods at runtime. With reflection, you can obtain class names, method names, field types, and other information without knowing them at compile time. It's commonly used in frameworks, libraries, and tools where there's a need to manipulate or analyze classes dynamically. However, it should be used judiciously as it can introduce performance overhead and security risks.
Question: What is Jagged Arrays in Java?
Answer: A jagged array, also known as a "ragged" array, refers to an array of arrays in which the member arrays can be of different lengths. In Java, since arrays are objects, a two-dimensional array is essentially an array of array objects, allowing each of these to have varying lengths.
Question: What is Pass by reference?
Answer: Pass by reference means that when passing a variable to a function, what's passed is the memory address of the variable, not the actual value stored in that memory location. As a result, changes made to the parameter inside the function affect the original value. Note: Java does not support true pass by reference; it uses pass-by-value, but when objects are passed, their reference values (memory addresses) are passed by value.
Question: What is Varargs?
Answer: Varargs (short for variable-length argument lists) in Java allow you to pass any number of arguments of a specified type to a method. It's denoted in method declarations by three dots ('...'). For instance, 'public void printNumbers(int... numbers)' would allow the method to accept any number of 'int' values.
Question: What is Covariant?
Answer: Covariance in Java refers to the ability to change the return type of a method to a subtype of the original return type when the method is overridden in a subclass. Since Java 5, covariant return types have been supported, allowing for more specific return types when overriding methods.
Question: When Default constructor is not created in Java?
Answer: In Java, if a class does not explicitly define any constructors, the compiler automatically provides a default no-argument constructor. However, if you provide any constructor (either parameterized or no-argument), the compiler will not generate a default constructor for you.
Question: What is the Copy Constructor in Java?
Answer: A copy constructor in the context of object-oriented programming is a constructor that creates an object by copying another object. Java doesn't provide a default copy constructor as languages like C++ do. In Java, you have to provide your own copy constructor if you need one, which would involve manually copying the properties from the source object to the new one.
Question: What is Constructor Chaining in Java?
Answer: Constructor chaining is the process of calling one constructor from another constructor within the same class or calling the parent class constructor from the child class. It is done using 'this()' (for calling a constructor in the same class) and 'super()' (for calling the superclass constructor).
Question: What is Constructor Overloading?
Answer: Constructor overloading in Java is a concept where a class can have multiple constructors with different parameter lists. The number of parameters, their type, and their order in the parameter list differentiate these overloaded constructors.
Question: What is the difference between private and protector method?
Answer:
- Private Method: Methods declared as private are only accessible within the class they are defined. They are not visible to subclasses or any other external classes.
- Protected Method: Protected methods are accessible within the class they are defined, as well as in subclasses residing in the same package or even in different packages. They are also accessible by other classes in the same package.
Question: How can you create User Defined Java Exception?
Answer: User-defined exceptions in Java are created by extending the 'Exception' class (for checked exceptions) or the 'RuntimeException' class (for unchecked exceptions). You can create a custom exception class, and then throw and catch instances of it just as with built-in exceptions.
Example:

Question: What is nested interface and classes in java?
Answer: A nested class or interface is one that is declared inside another class or interface. Nested classes can be static or non-static. A nested class that is declared static is called a static nested class. Non-static nested classes are also known as inner classes. Similarly, interfaces can also be nested within a class or another interface.
Question: Can a nested class considered as a subclass?
Answer: No, a nested class (whether static or inner) is not considered a subclass of the enclosing class. It is a separate and distinct class that simply resides within another class for scoping and organization.
Question: What is functional interface in Java?
Answer: A functional interface in Java is an interface that contains just one abstract method. It can contain multiple default or static methods. Functional interfaces are designed to be used in conjunction with lambda expressions. The '@FunctionalInterface' annotation is used to indicate that an interface is intended to be a functional interface.
Question: What is the Comparator and Comparable and which one you use and when?
- Comparable: It is an interface that should be implemented by any class that wishes to define a natural order for its instances. The class must override the 'compareTo()' method. It is useful when there's a single, general way to compare instances of a class.
- Comparator: It is an external interface with the 'compare()' method, allowing for defining custom orderings for objects. Multiple comparators can be defined for a single class. It is used when you need multiple ways to compare instances or when you want to compare instances of a class that you can't modify.
Use 'Comparable' when you have one logical way to sort. Use 'Comparator' when you need multiple ways or can't alter the class source.
Question: What is the Deque interface in Java, and when do you want to use it?
Answer: 'Deque' stands for "double-ended queue." It's an interface in Java that represents a linear collection supporting element insertion and removal at both ends. It's versatile and can be used as a queue (FIFO) or a stack (LIFO). You'd use a 'Deque' when you need more flexible data structures than a regular queue or stack.
Question: What is the difference between SortedSet and TreeSet in Java and which one you want to use and when?
- SortedSet: It's an interface that extends the 'Set' interface. It represents a set that maintains its elements in ascending order. 'SortedSet' provides methods for range view operations and endpoint operations.
- TreeSet: It's a concrete class that implements the 'SortedSet' interface (and 'NavigableSet' as well). It uses a Red-Black tree to store elements, ensuring log(n) time cost for most operations.
You would use 'SortedSet' as a type for variables, parameters, or return values when you want to ensure the contract of sorting without tying to a specific implementation. You'd use 'TreeSet' when you want to have a concrete implementation of a sorted set.
Question: Can you tell me what all are the Lifecycle states of the Java Thread?
Answer: Java thread can be in one of the following states:
- NEW: A thread that has not yet started is in this state.
- RUNNABLE: A thread executing in the Java virtual machine is in this state.
- BLOCKED: A thread that is blocked waiting for a monitor lock is in this state.
- WAITING: A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
- TIMED_WAITING: A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
- TERMINATED: A thread that has exited is in this state.
Question: Once Java object is GCied can it be resurrected?
Answer: Yes, an object can be resurrected during its first garbage collection by overriding the 'finalize()' method and making the object accessible again. However, the 'finalize' method is called only once for an object, so after the next round of garbage collection, the object will be collected and cannot be resurrected.
Question: Does Java Thread priority really helpful?
Answer: Thread priorities are a tool in Java to influence the thread scheduler. However, the actual impact is highly platform-dependent. While they can be useful in certain scenarios, relying solely on thread priority can be unpredictable, especially across different JVM implementations and OS platforms. It's typically better to rely on other synchronization mechanisms for guaranteed behavior.
Question: Can you write sample code to create JDBC connection?
Question: Can you write sample code to create JMS connection?

Question: What is singleton object, how can you create that?
Answer:

A singleton ensures that a class has only one instance and provides a global point to access it. Here's a simple way to create a thread-safe singleton using the Bill Pugh Singleton Design:

Question: What is the Factory design pattern and when do you want to use it?
Answer: The Factory design pattern provides an interface for creating instances of a class, with its subclasses deciding which class to instantiate. You'd use it when the exact type of the object to be created isn't known until runtime, or when the creation logic is complex and should be separated from the client code.
Question: What is Observer Design Pattern, can you give me example where you can use it?
Answer: The Observer pattern defines a one-to-many dependency between objects so that when one object (the subject) changes state, all its dependents (observers) are notified and updated automatically. It's used in situations like event handling systems. For example, in a GUI system, when a button is pressed (subject changes state), all its event listeners (observers) get notified.
Question: How can you prevent your singleton design pattern from Reflection, Serialization and Cloning?
- Reflection: Make the constructor throw an exception if it's already initialized.
- Serialization: Implement 'readResolve()' method in the Singleton class and return the existing instance from it.
- Cloning: Override 'clone()' method and throw an exception or return the same instance.
Question: What is the use of Decorator Design Pattern?
Answer: The Decorator pattern allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. It's used in situations where you need to extend functionalities of classes in a flexible and modular way. For example, in Java I/O classes like.
Question: What is Adapter Design Pattern and give an example use case?
Answer: The Adapter pattern works as a bridge between two incompatible interfaces. It involves a class that joins functionalities of independent or incompatible interfaces. Use Case: Imagine you have a legacy system that returns data in some old format, but your new system works on a different format. You can create an adapter that takes data from the legacy system and transforms it into a format that's compatible with your new system.
Question: What is Flyweight Design Pattern?
Answer: The Flyweight pattern is used to reduce the number of objects created, to decrease memory footprint and increase performance. It shares objects that are the same rather than creating new ones.
Question: What is composite Design Pattern?
Answer: The Composite pattern is used to treat individual objects and composites (group of objects) uniformly. It allows you to have a tree structure and ask each node in the tree structure to perform a task. For example, in graphic systems where there might be shapes made up of other shapes.
Question: What is Abstract factory design pattern?
Answer: The Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes. Essentially, it's a super factory which creates other factories.
Question: What is chain of Responsibility Design Pattern?
Answer: This pattern decouples the sender from receiver by allowing more than one object to handle a request. The request passes through a chain of potential handlers until either it's handled or reaches the end of the chain. An example could be event handling in GUI systems where an event could be handled by either a button, the panel containing the button, or the window containing the panel.
Quuestion-209: Can you give an example of Command Pattern?
Answer: The Command pattern encapsulates a request as an object, thereby allowing users to parameterize clients with different requests, queue requests, and support undoable operations. Example: In a generic remote control system, each button press can be a command, and the remote can execute these commands on a device.
Question: Can you give an example of Iterator Design Pattern?
Answer: The Iterator pattern provides a way to access elements of a collection object in a sequential manner without exposing its underlying representation. In Java, the 'java.util. Iterator' interface is a prime example, used with collections to provide sequential access without exposing details.
Question: Can you create an Immutable class?
Answer: Yes.
Steps:
- Make the class 'final' so it can't be extended.
- Make all fields private and final.
- Don't provide setter methods.
- If a field is a mutable object, don't allow it to be changed or accessed directly.
- Ensure exclusive access to any mutable components.
Question: What is the Difference between Singleton and object created using Static?
Answer: A Singleton ensures that a class has only one instance and provides a global point of access, while a static class (or class with static methods) does not represent an instance of anything, it's more of a global state and function holder. Singleton has an instance, can be lazily loaded, and can be overridden with mechanisms like inheritance, whereas static cannot.
Question: What do you mean by LRU and MRU?
Answer: LRU stands for "Least Recently Used." In computing, an LRU algorithm removes the least recently used items first. It's a common cache eviction strategy. MRU stands for "Most Recently Used." An MRU algorithm removes the most recently used items first. Both are policies to manage cache or other limited resources.
Question: What is the Lambda Expression and how does it helps?
Answer: Lambda expressions are a feature introduced in Java 8. They allow you to express instances of single-method interfaces (functional interfaces) in a much more concise, expressive, and readable way. They can be used primarily to define inline implementations of functional interfaces. Lambda expressions help in writing cleaner and more concise code, making it more readable. They are often used with the new Streams API for more declarative data processing.
Question: If your manager asked you to fix many code issues with tight timeline, what you will do?
Answer: I would prioritize the issues based on severity and business impact. If the timeline is too tight to fix all the issues, I'd communicate with my manager about the situation and suggest focusing on the most critical ones first. Additionally, I'd look for ways to increase efficiency, like seeking help from colleagues or using tools to automate some processes, but without compromising on the quality of fixes.
Question: What is the difference between Array and ArrayList?
- Arrays are of fixed size. Once they're created, you can't change their size.
- ArrayList is a part of the Java Collections Framework and can dynamically resize itself.
- Arrays can contain primitives or objects, whereas ArrayLists only store object references.
- ArrayList provides methods to manipulate the data, which Arrays don't.
Question: How would you find a missing number between 1 to n?
Answer: You can use the formula for the sum of the first n natural numbers: 'n(n+1)/2', and subtract from it the sum of the numbers in the given array. The result will be the missing number.
Question: How does Java Garbage Collection works?
Answer: Java Garbage Collection is a process to identify and remove the objects from the heap memory which are no longer in use. When an object is no longer referenced by any variable, garbage collector considers it eligible for removal. Java uses several garbage collector algorithms like Serial Garbage Collector, Parallel Garbage Collector, CMS Garbage Collector, and G1 Garbage Collector.
Question: In Hibernate where the Objects types are mentioned?
Answer: In Hibernate, object-relational mapping is usually specified in XML configuration files or using annotations. The configuration specifies how the Java classes are mapped to the database tables.
Question: What is Apache CXF and Apache Axis?
Answer: Both Apache CXF and Apache Axis are frameworks that allow developers to create web services in Java.
- Apache Axis (and its successor Axis2) is older and provides both SOAP and RESTful web services.
- Apache CXF, which originated from the merger of the XFire and Celtix projects, is a more modern framework that also supports both SOAP and RESTful web services.
Question: Do you know, what is Scrum, Sprint and Standup call?
- Scrum: An agile framework within which people can address complex adaptive problems, delivering products iteratively and incrementally.
- Sprint: A set period during which specific work is completed and made ready for review.
- Standup call (or Daily Stand-up): A short meeting (usually 15 minutes) in which team members briefly share updates on what they did the previous day, what they plan to do today, and any blockers they have.
Question: Which build tool you are using in your recent projects?
Answer: I'm a virtual assistant, so I don't work on projects. However, common build tools used in the Java ecosystem include Maven, Gradle, and Ant.
Question: What is the yield method do?
Answer: The 'yield' method is a static method of the 'Thread' class. When invoked, it signals that the currently executing thread is willing to yield its current use of a processor so other threads can run. It's a hint to the thread scheduler, but there's no guarantee the scheduler will act upon it. The purpose is to promote better thread scheduling, though its behavior can be platform-dependent.
Question: What is MVC?
Answer: MVC stands for Model-View-Controller. It's a design pattern used in software engineering aimed at separating an application into three interconnected components:
- Model: Manages the data, logic, and rules of the application.
- View: Displays the data; the user interface.
- Controller: Accepts inputs and converts them to commands for the Model or View.
Question: What is SDLC?
Answer: SDLC stands for Software Development Life Cycle. It's a systematic process for planning, creating, testing, deploying, and maintaining software applications or systems. The main stages of SDLC include: Requirement Analysis, Planning, Design, Implementation/Development, Testing, Deployment, and Maintenance.
Question: Write an Algorithm to implement Fibonacci Pattern?
Answer: Check steps below.

Question: Given a sentence, Capitalize each word in the sentence?
Answer: Check steps below.

Question-Question: Describe Polymorphism?
Answer: Polymorphism, derived from Greek where "poly" means many and "morph" means forms, is a concept in OOP that allows objects of different classes to be treated as objects of a common super class. The most common use is when a parent class reference is used to refer to a child class object. It promotes flexibility and reusability in code. There are two types of polymorphism in Java: compile-time (method overloading) and runtime (method overriding).
Question: Describe four main pillars of OOPs?
Answer: The four main pillars of Object-Oriented Programming (OOP) are:
- Encapsulation: Bundling the data (attributes) and methods (functions) that operate on the data into a single unit or class. It also restricts direct access to some of the object's components.
- Abstraction: Hiding the complex reality while exposing only the necessary parts. It reduces complexity by displaying only essential features of an object.
- Inheritance: A mechanism where a new class inherits properties and behavior (methods) from another class.
- Polymorphism: The ability of a single function or method to work in different ways based on the object it's acting upon.
Question: Clearly give the difference between encapsulation and abstraction?
- Encapsulation: It's about bundling the data (attributes) and methods (functions) that operate on the data into a single unit or class and restricting direct access to some of the object's components. It’s more about how you hide the details.
- Abstraction: It's about hiding the complex implementation details and showing only the essential features of an object. It reduces complexity and increases efficiency. It’s more about what you hide.
Question: When do you prefer to use Thread class and Runnable interface to create a thread?
- Use the Thread class when you want to override other Thread methods along with the 'run()' method.
- Use the Runnable interface when you just want to implement the 'run()' method, or when you want to create a class that can extend another class as well because Java doesn’t support multiple inheritance of classes.
Question: What is a cyclic barrier?
Answer: A CyclicBarrier is a synchronizer in Java that allows a set of threads to wait for each other to reach a common barrier point. Once the given number of threads reach this barrier, an action can be executed. It's cyclic because it can be reused after the waiting threads are released.
Question: What is the use of Semaphore?
Answer: A Semaphore is a concurrency tool in Java that is used to control access to resources. It maintains a set of permits, and a thread can acquire a permit (if available) or release a permit. Semaphores can be used to implement resource pools or to impose concurrency limits. For instance, if you have a resource pool with 5 resources, a semaphore can ensure that no more than 5 threads can access those resources at a time.
Question: What is the Dependency injection?
Answer: Dependency injection (DI) is a design pattern used in software engineering where an object receives its dependencies from an external source rather than creating them itself. This promotes the principle of inversion of control, which makes systems more modular and easier to test. DI can be implemented manually, or with the aid of DI frameworks like Spring, Guice, and Dagger.
Question: Explain what is Kafka Offset?
Answer: In Apache Kafka, an offset is a unique identifier for a record within a partition. It denotes the position of the record within the partition. Kafka maintains a numerical offset for each record in a partition. Consumers use these offsets to keep track of the records that have been consumed and to resume consumption in case of failures. It's worth noting that offsets are committed by the consumer, which means the consumer can decide when to commit an offset and therefore has control over when a record is considered "processed. "
Question: What is Object Oriented programming?
Answer: Object-Oriented Programming (OOP) is a programming paradigm based on the concept of "objects," which are instances of classes. These objects can contain data, in the form of fields or attributes, and code, in the form of methods. The four main principles of OOP are Encapsulation, Abstraction, Inheritance, and Polymorphism. It promotes code reuse, modularity, and a clearer structure for software development.
Question: What is Auto Closable in Java?
Answer: Introduced in Java 7, 'AutoCloseable' is an interface which classes can implement to ensure that their instances get closed automatically when used within a try-with-resources statement. Classes that implement 'AutoCloseable' must provide an implementation for the 'close()' method, which gets invoked automatically at the end of the try block. This feature is useful for ensuring that resources like files, network connections, or database connections are properly closed after their operations are completed.
Question: How do you make a singleton Java Object across Java processes?
Answer: Creating a singleton object across multiple JVMs or processes is non-trivial. In a single JVM, we use private constructors and controlled access. However, for multiple JVMs:
- Database Approach: Use a database to ensure a single shared instance across processes. The database can act as a synchronization mechanism.
- File Locking: Use a common file as a locking mechanism. A process locks the file when it creates an instance. Other processes must wait or can't acquire the lock and hence can't create another instance.
- Distributed Systems Tools: Use tools/frameworks like ZooKeeper which can help in creating distributed locks ensuring only one instance of a singleton across processes.
Question: What is the difference between HashMap and Hashtable?
- Synchronization: 'Hashtable' is synchronized, meaning it is thread-safe. In contrast, 'HashMap' is not synchronized and is not thread-safe.
- Null Keys and Values: 'HashMap' allows one null key and multiple null values, whereas 'Hashtable' doesn’t allow any null key or value.
- Performance: Since 'Hashtable' is synchronized, it's generally slower than 'HashMap'.
- Superclass: 'Hashtable' extends 'Dictionary' class which is quite old, whereas 'HashMap' extends 'AbstractMap' class.
- Iterating: 'HashMap' provides an iterator, while 'Hashtable' provides both an enumerator and an iterator.
Question: What is the use of Visitor design, can you give an example where you have used or you can use it?
Answer: The Visitor design pattern allows you to add further operations to objects without having to modify them. It is useful when dealing with an established object structure that needs new operations without modifying the objects themselves.
Example: Consider a computer system made up of various parts (keyboard, monitor, mouse, etc.). If you want to allow different operations on this system, like "display" or "price," without altering the classes of these parts, you can use the Visitor pattern. Each part can have a "accept" method that takes a visitor as an argument. The visitor will then implement "display" and "price" operations for each part. This way, if you want to add another operation in the future, you can simply add a new visitor without altering the existing parts' classes.