Digigen Technology IT Executive Interview Questions and Answers

Q1. Explain the difference between ArrayList and LinkedList in Java. When would you choose one over the other?
Ans. 
ArrayList uses a dynamic array for storage, while LinkedList uses a doubly linked list structure.
• ArrayList provides fast random access (O(1)) but slow insertions/deletions (O(n)). Example: accessing elements by index.

• LinkedList allows fast insertions/deletions (O(1)) but slower random access (O(n)). Example: adding/removing elements at the beginning.

• ArrayList is preferred when you need frequent access to elements and f...read more
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Q2. What are the advantages and disadvantages of using Java’s synchronized keyword for thread synchronization? Can you explain how the ReentrantLock compares to synchronized?
Ans. 
Java's synchronized keyword offers thread safety but has limitations compared to ReentrantLock.
• Advantages of synchronized: Simple to use and understand.

• Disadvantages of synchronized: Can lead to thread contention and performance issues.

• ReentrantLock allows more flexibility, such as tryLock() and timed lock attempts.

• ReentrantLock supports fairness policies, which can help avoid starvation.

• Synchronized blocks are tied to...read more
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Q3. What is the difference between == and .equals() in Java? When should each be used, and what issues can arise from improper usage?
Ans. 
In Java, '==' checks reference equality, while '.equals()' checks value equality. Use them appropriately to avoid bugs.
• == compares object references, checking if two references point to the same object in memory.

• Example: String a = new String('test'); String b = new String('test'); a == b returns false.

• .equals() compares the actual content of the objects, checking if they are logically equivalent.

• Example: a.equals(b) r...read more
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Q4. How does the Java garbage collector work? Can you describe the different types of garbage collection algorithms available in Java?
Ans. 
Java's garbage collector automatically manages memory by reclaiming unused objects, improving performance and preventing memory leaks.
• Java uses automatic garbage collection to manage memory, freeing developers from manual memory management.

• The main garbage collection algorithms in Java include: Serial GC, Parallel GC, Concurrent Mark-Sweep (CMS), and G1 GC.

• Serial GC is a simple, single-threaded collector suitable for s...read more
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Q5. What are the main features of Java 8? Can you explain how lambdas and the Stream API have changed the way Java applications are written?
Ans. 
Java 8 introduced lambdas, Stream API, and other features that enhance functional programming and simplify code.
• Lambdas: Enable concise representation of functional interfaces. Example: (x, y) -> x + y.

• Stream API: Facilitates functional-style operations on collections. Example: list.stream().filter(x -> x > 10).collect(Collectors.toList()).

• Default Methods: Allow adding new methods to interfaces without breakin...read more
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Q6. Describe the differences between checked and unchecked exceptions in Java. Provide examples and explain how to handle them properly.
Ans. 
Checked exceptions must be handled or declared, while unchecked exceptions do not require explicit handling.
• Checked exceptions are subclasses of Exception (excluding RuntimeException). Example: IOException.

• Unchecked exceptions are subclasses of RuntimeException. Example: NullPointerException.

• Checked exceptions must be either caught using try-catch or declared in the method signature with throws.

• Unchecked exceptions can...read more
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Q7. What is the Java Memory Model, and how does it affect multithreading and synchronization? How does volatile help ensure memory visibility?
Ans. 
The Java Memory Model defines how threads interact through memory, ensuring visibility and ordering of shared variables.
• The Java Memory Model (JMM) specifies how threads interact through memory and what behaviors are allowed.

• It defines rules for visibility, atomicity, and ordering of operations in a multithreaded environment.

• Without proper synchronization, threads may see stale data due to caching or compiler optimizat...read more
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Q8. Can you explain the difference between method overloading and method overriding in Java? Provide examples where each should be used.
Ans. 
Method overloading allows multiple methods with the same name but different parameters; overriding replaces a superclass method in a subclass.
• Method Overloading: Same method name, different parameters (type, number, or both).

• Example of Overloading: 'int add(int a, int b)' and 'double add(double a, double b)'.

• Use Overloading for convenience and readability when methods perform similar functions.

• Method Overriding: Redefi...read more
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Q1. What is a transaction in DBMS, what are ACID properties? Explain the difference between SQL and NoSQL databases. How would you optimize a slow running query?
Ans. 
A transaction in DBMS is a unit of work that must be executed as a whole. ACID properties ensure data integrity. SQL is relational, NoSQL is non-relational.
• A transaction in DBMS is a set of operations that must be executed as a single unit.

• ACID properties (Atomicity, Consistency, Isolation, Durability) ensure data integrity in transactions.

• SQL databases are relational and use structured query language, while NoSQL data...read more
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Q2. Define encapsulation and provide an example. Discuss the principles of inheritance and polymorphism.
Ans. 
Encapsulation is the concept of bundling data and methods that operate on the data into a single unit. Inheritance allows a class to inherit attributes and methods from another class. Polymorphism allows objects of different classes to be treated as objects of a common superclass.
• Encapsulation helps in data hiding and abstraction

• Example: A class 'Car' encapsulates data like make, model, and methods like start(), stop()

• ...read more
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Q3. Explain how to detect a cycle in a linked list.
Ans. 
Use Floyd's Tortoise and Hare algorithm to detect a cycle in a linked list.
• Start with two pointers, slow and fast, moving at different speeds.

• If there is a cycle, the two pointers will eventually meet at some point.

• If there is no cycle, the fast pointer will reach the end of the list.

• Example: 1->2->3->4->5->2 (cycle at node 2), slow and fast pointers will meet at node 2.
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First round was based on logical questions and was taken on hacker earth

Q1. First question was based on rotating a matrix by 90 degrees
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Q1. Explain the difference between ArrayList and LinkedList in Java. When would you choose one over the other?
Ans. 
ArrayList is a resizable array, while LinkedList is a doubly linked list. Choose based on performance needs.
• ArrayList: Faster for random access (O(1)). Example: list.get(5);

• LinkedList: Faster for insertions/deletions (O(1)) at both ends. Example: list.addFirst('A');

• ArrayList: Uses less memory overhead compared to LinkedList.

• LinkedList: Better for frequent insertions/deletions in the middle of the list.

• ArrayList: Requir...read more
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Q2. What are the advantages and disadvantages of using Java’s synchronized keyword for thread synchronization? Can you explain how the ReentrantLock compares to synchronized?
Ans. 
Java's synchronized keyword provides thread safety but has limitations compared to ReentrantLock.
• Advantages of synchronized: Simple to use, built-in language feature.

• Disadvantages of synchronized: Can lead to thread contention, no timeout options.

• ReentrantLock allows more flexibility: supports tryLock(), lockInterruptibly().

• ReentrantLock can be more efficient in high contention scenarios.

• Example of synchronized: synchr...read more
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Q3. What is the difference between == and .equals() in Java? When should each be used, and what issues can arise from improper usage?
Ans. 
== checks reference equality, while .equals() checks value equality in Java. Use .equals() for content comparison.
• == compares object references (memory addresses). Example: String a = new String('test'); String b = new String('test'); a == b returns false.

• .equals() compares actual content of objects. Example: a.equals(b) returns true.

• Use == for primitive types (int, char, etc.) and .equals() for objects.

• Improper use of...read more
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Q4. How does the Java garbage collector work? Can you describe the different types of garbage collection algorithms available in Java?
Ans. 
Java's garbage collector automatically manages memory by reclaiming unused objects, improving performance and preventing memory leaks.
• Garbage Collection (GC) is the process of automatically identifying and disposing of objects that are no longer needed.

• Java uses several GC algorithms, including Serial, Parallel, CMS (Concurrent Mark-Sweep), and G1 (Garbage-First).

• The Serial GC is a simple, single-threaded collector sui...read more
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Q5. What are the main features of Java 8? Can you explain how lambdas and the Stream API have changed the way Java applications are written?
Ans. 
Java 8 introduced lambdas, Stream API, and other features that enhance functional programming and improve code readability.
• Lambdas: Enable concise representation of functional interfaces. Example: (x, y) -> x + y.

• Stream API: Allows processing sequences of elements (collections) in a functional style. Example: list.stream().filter(x -> x > 10).collect(Collectors.toList()).

• Default Methods: Interfaces can have me...read more
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Q6. Describe the differences between checked and unchecked exceptions in Java. Provide examples and explain how to handle them properly.
Ans. 
Checked exceptions must be declared or handled; unchecked exceptions do not require explicit handling.
• Checked exceptions are subclasses of Exception but not of RuntimeException.

• Example of checked exception: IOException, which must be caught or declared.

• Unchecked exceptions are subclasses of RuntimeException.

• Example of unchecked exception: NullPointerException, which does not need to be declared.

• Checked exceptions are t...read more
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Q7. What is the Java Memory Model, and how does it affect multithreading and synchronization? How does volatile help ensure memory visibility?
Ans. 
The Java Memory Model defines how threads interact through memory, ensuring visibility and ordering of shared variables.
• The Java Memory Model (JMM) specifies how threads interact with memory, ensuring consistency and visibility of shared variables.

• It defines rules for visibility, atomicity, and ordering of operations in a multithreaded environment.

• Without proper synchronization, threads may see stale or inconsistent da...read more
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Q8. Can you explain the difference between method overloading and method overriding in Java? Provide examples where each should be used.
Ans. 
Method overloading allows multiple methods with the same name but different parameters; overriding allows subclass methods to replace superclass methods.
• Method Overloading: Same method name, different parameter types or counts.

• Example of Overloading: 'int add(int a, int b)' and 'double add(double a, double b)'.

• Use Overloading for convenience and readability when performing similar operations.

• Method Overriding: Same met...read more
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Q9. What are functional interfaces in Java? How do they work with lambda expressions? Provide an example of a custom functional interface.
Ans. 
Functional interfaces in Java are interfaces with a single abstract method, enabling lambda expressions for concise code.
• A functional interface has exactly one abstract method.

• They can have multiple default or static methods.

• Common examples include Runnable, Callable, and Comparator.

• Lambda expressions provide a clear and concise way to implement functional interfaces.

• Example of a custom functional interface: @Functiona...read more
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Q10. What is a Java Stream, and how does it differ from an Iterator? Explain how Streams can be used to process collections efficiently.
Ans. 
Java Streams provide a functional approach to processing sequences of elements, unlike Iterators which are imperative.
• Streams are part of the Java 8+ API, enabling functional-style operations on collections.

• Unlike Iterators, Streams do not store data; they process data on-the-fly.

• Streams support operations like map, filter, and reduce, allowing for concise and readable code.

• Example: List<String> names = Arrays.as...read more
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Q11. Explain the concept of immutability in Java. How does the String class achieve immutability, and what are the advantages of immutable objects?
Ans. 
Immutability in Java means objects cannot be modified after creation, enhancing security and performance.
• 1. Immutability: Once created, an object's state cannot be changed.

• 2. String Class: Strings in Java are immutable; any modification creates a new String object.

• 3. Example: String s1 = "Hello"; s1 = s1 + " World!"; // s1 now points to a new String object.

• 4. Advantages: Thread-safe, easier to cache, and can be used as...read more
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Q12. What is the difference between final, finally, and finalize in Java? Provide examples to illustrate their usage.
Ans. 
final, finally, and finalize serve different purposes in Java: variable declaration, exception handling, and garbage collection respectively.
• final: Used to declare constants. Example: final int MAX_VALUE = 100;

• finally: Block that executes after try-catch, regardless of exceptions. Example: try { ... } catch { ... } finally { ... }

• finalize: Method called by the garbage collector before an object is removed. Example: pro
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Q13. Explain the Singleton design pattern in Java. How can you implement it safely to ensure thread safety?
Ans. 
The Singleton pattern restricts instantiation of a class to one object, ensuring controlled access to that instance.
• 1. The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

• 2. Common implementations include lazy initialization, eager initialization, and double-checked locking.

• 3. Lazy initialization: Create the instance when it is needed, using synchronized method for th...read more
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Q14. What are Java annotations, and how are they used in frameworks like Spring? Explain the difference between built-in and custom annotations.
Ans. 
Java annotations provide metadata for classes, methods, and fields, enhancing functionality in frameworks like Spring.
• Annotations are metadata that provide information about the program but are not part of the program itself.

• In Spring, annotations like @Component, @Service, and @Controller are used for defining beans and their roles.

• Built-in annotations include @Override, @Deprecated, and @SuppressWarnings, which serve...read more
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Q15. How do Java Streams handle parallel processing? What are the potential pitfalls of using parallel streams, and how can they be mitigated?
Ans. 
Java Streams enable parallel processing for efficient data handling but come with potential pitfalls that need careful management.
• Java Streams can be processed in parallel using the 'parallelStream()' method, which divides the workload across multiple threads.

• Parallel streams utilize the Fork/Join framework, allowing tasks to be split and executed concurrently, improving performance for large datasets.

• Potential pitfall...read more
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Q16. Explain the difference between ArrayList and LinkedList in Java. ArrayList is implemented as a dynamic array, while LinkedList is a doubly linked list. ArrayList provides fast random access (O(1) complexi...read more
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Q17. What are the advantages and disadvantages of using Java’s synchronized keyword for thread synchronization? The synchronized keyword ensures that only one thread can access a block of code at a time. It pr...read more
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Q18. What is the difference between == and .equals() in Java? == checks for reference equality, meaning it compares memory addresses. equals() checks for value equality, which can be overridden in user-defined...read more
Ans. 
== checks reference equality; .equals() checks value equality, can be overridden for custom comparison.
• == compares memory addresses: new String("hello") == new String("hello") returns false.

• .equals() compares actual content: "hello".equals("hello") returns true.

• Override equals() when logical equality differs from reference equality, e.g., in custom classes.

• When overriding equals(), also override hashCode() to maintain ...read more
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Q19. How does the Java garbage collector work? Garbage collection in Java automatically reclaims memory occupied by unused objects. The JVM has different types of GC algorithms, including Serial, Parallel, CMS...read more
Ans. 
Java's garbage collector automatically manages memory, reclaiming space from unused objects through various algorithms.
• Garbage collection in Java is automatic, freeing developers from manual memory management.

• The JVM uses different GC algorithms: Serial, Parallel, CMS, and G1, each with unique characteristics.

• Memory is divided into Young Generation (short-lived objects) and Old Generation (long-lived objects).

• Minor GC ...read more
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Q20. What are the main features of Java 8? Java 8 introduced lambda expressions, enabling functional-style programming. The Stream API allows efficient data processing with map, filter, and reduce operations. ...read more
Ans. 
Lambda expressions enhance Java code readability and maintainability by simplifying syntax and promoting functional programming.
• Concise Syntax: Lambda expressions reduce boilerplate code. For example, instead of writing an anonymous class for a Runnable, you can use: `Runnable r = () -> System.out.println("Hello");`

• Improved Readability: Code becomes more expressive. For instance, using `list.forEach(item -> Syste...read more
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