1. What is Java?
Java is a high-level, object-oriented programming language used to create powerful and scalable applications. It is platform independent and designed to run on any device or operating system.

2. What is the difference between JDK and JRE?
JDK (Java Development Kit) is a software development kit that provides tools to develop and deploy Java applications. JRE (Java Runtime Environment) is an environment that allows you to run Java applications but does not contain development tools like compilers and debuggers.

3. What are the different data types in Java?
Java has two categories of data types: primitive and non-primitive (object reference) data types. Primitive data types include int, byte, short, long, float, double, boolean, and char. Non-primitive data types include classes, interfaces, arrays, etc.

4. What is the difference between “==” and “.equals()” in Java?
“==” is used to compare the references of objects, while “.equals()” is used to compare the content or values of objects. In Java, the default implementation of “.equals()” method compares the references, but it can be overridden to compare the contents of objects.

5. What is the purpose of the “final” keyword in Java?
The “final” keyword can be applied to variables, methods, and classes. When applied to a variable, it makes the variable constant and its value cannot be changed. When applied to a method, it prevents the method from being overridden in subclasses. When applied to a class, it prevents the class from being subclassed.

6. What are the different types of inheritance in Java?
Java supports single, multilevel, hierarchical, and multiple inheritances through interfaces. However, multiple inheritances of classes are not supported in Java.

7. What is the difference between abstract class and interface?
An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. A class can extend only one abstract class, but it can implement multiple interfaces.

8. What are the access modifiers in Java?
Java has four access modifiers: public, private, protected, and default (no keyword required). Public allows access from everywhere, private allows access only within the same class, protected allows access within the same package and subclasses, and default allows access within the same package.

9. What is method overloading in Java?
Method overloading allows the creation of multiple methods with the same name but different parameters in the same class. The method to be invoked is determined by the number, type, and order of the parameters.

10. What is method overriding in Java?
Method overriding allows a subclass to provide a specific implementation of a method that is already defined in its superclass. It is achieved by using the same method signature (name and parameters) in both the superclass and subclass.

11. What is a constructor in Java?
A constructor is a special method used to initialize objects in Java. It is called automatically when an object is created. It has the same name as the class and does not have a return type.

12. What is the “static” keyword in Java?
The “static” keyword in Java can be applied to variables, methods, and nested classes. It means they belong to the class itself, rather than to specific instances of the class. Static members can be accessed using the class name without creating an instance of the class.

13. What is the “this” keyword in Java?
The “this” keyword in Java refers to the current instance of the class. It is used to differentiate between instance variables and parameters or local variables with the same name. It can also be used to invoke other constructors in the same class.

14. What is method chaining in Java?
Method chaining is a technique in Java where multiple methods are called on the same object in a single statement. Each method returns an object, allowing the next method to be called on the same object.

15. What is the “try-catch-finally” block in Java?
The “try-catch-finally” block is used in Java to handle exceptions. The “try” block contains the code that may throw an exception, and the “catch” block catches and handles the exception. The “finally” block is always executed, regardless of whether an exception is thrown or caught.

16. What is the difference between checked and unchecked exceptions?
Checked exceptions are checked at compile-time and must be handled using try-catch or declared in the method signature using the “throws” keyword. Unchecked exceptions do not need to be explicitly handled or declared.

17. What is the role of the “finalize()” method in Java?
The “finalize()” method is called by the garbage collector when it determines that there is no more reference to the object. It allows the object to perform any necessary cleanup operations before being garbage collected.

18. What is the purpose of the “toString()” method in Java?
The “toString()” method is used to convert an object into a human-readable string representation. It is often overridden in classes to provide a meaningful string representation of the object.

19. What is the difference between StringBuilder and StringBuffer?
StringBuilder and StringBuffer are used to create mutable (modifiable) strings in Java. The main difference is that StringBuilder is not thread-safe, while StringBuffer is thread-safe.

20. What is the difference between static and instance variables?
Static variables are shared among all instances of a class, while instance variables have separate copies for each instance of the class. Static variables are defined using the “static” keyword, while instance variables are defined within the class but outside any method or constructor.

1. What is the difference between composition and inheritance in Java?
Composition is an object-oriented design technique where a class contains an instance of another class as one of its member variables. Inheritance, on the other hand, is a concept where a class inherits properties and methods from another class. Inheritance allows code reuse while composition provides better flexibility and loose coupling.

2. Explain the concept of Java Reflection.
Java Reflection allows the inspection and manipulation of classes, interfaces, fields, and methods at runtime. It provides the ability to obtain metadata about classes and objects, and invoke methods dynamically.

3. What are checked and unchecked exceptions in Java?
Checked exceptions are the exceptions that are checked by the compiler during compilation and require a catch or a throws statement. Unchecked exceptions are not checked by the compiler, and a catch or throws statement is not mandatory.

4. What is the purpose of the transient keyword in Java?
The transient keyword is used to indicate that a variable shouldn’t be serialized during object serialization. It is useful for excluding specific fields from being saved in a file or transferred over a network.

5. What is a singleton class in Java?
A singleton class is a class that allows only one instance to be created and provides a global point of access to that instance. It is commonly used in scenarios where only a single instance should exist, such as a database connection or a thread pool.

6. What is the purpose of the finalize() method in Java?
The finalize() method is called by the garbage collector before reclaiming the memory occupied by an object that is no longer referenced. It can be used to perform cleanup activities or release resources before the object is destroyed.

7. What is the purpose of the volatile keyword in Java?
The volatile keyword is used to indicate that a variable may be asynchronously accessed by multiple threads. It ensures that the value of the variable is always read from and written to the main memory, rather than being cached in a thread’s local memory.

8. Explain the concept of Java synchronization.
Java synchronization is used to control access to shared resources or critical sections of code in a multi-threaded environment. It ensures that only one thread can execute the synchronized code block or method at a time, preventing concurrent access and potential data inconsistencies.

9. What is the difference between the sleep() method and the wait() method in Java?
The sleep() method is used to make a thread pause for a specified amount of time. It does not release any locks or monitor resources. The wait() method, on the other hand, is used for inter-thread communication and causes the current thread to wait until another thread notifies it. It releases the lock on the object and enters a suspended state.

10. What is the purpose of the serialVersionUID field in Java serialization?
The serialVersionUID field is a unique identifier for the version control of serialized objects. It helps in determining whether the saved object is compatible with the current class definition during deserialization. If the serialVersionUID of the saved object doesn’t match the serialVersionUID of the class, an InvalidClassException will be thrown.

11. Explain the concept of Java generics.
Java generics allow the usage of type parameters to create reusable code that can work with different types. It enables the writing of type-safe collections, algorithms, and classes by specifying the type at compile-time. Generics provide compile-time type checking and eliminate the need for explicit typecasting.

12. What is the purpose of the finalize() method in Java 11 and later versions?
In Java 11 and later versions, the finalize() method is deprecated and has no specific purpose. It is no longer recommended for implementing cleanup or releasing resources. The Object class no longer has a finalizer, and its behavior doesn’t depend on the finalize() method.

13. What is the purpose of the try-with-resources statement in Java?
The try-with-resources statement is used to automatically close resources that implement the AutoCloseable interface. It ensures that the resource is closed even if an exception occurs within the try block. It eliminates the need for explicit finally blocks for resource cleanup.

14. What is the difference between an abstract class and an interface in Java?
An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. An abstract class can provide a default implementation for some or all of its methods, whereas an interface only provides a contract that implementing classes must follow. A class can only inherit from one abstract class but can implement multiple interfaces.

15. Explain the concept of method overloading in Java.
Method overloading allows a class to have multiple methods with the same name but different parameter types or a different number of parameters. The compiler selects the appropriate method based on the arguments passed during the method invocation. Method overloading helps in writing code that is easier to read and maintain.

16. What is the difference between method overriding and method overloading in Java?
Method overriding occurs when a subclass provides its own implementation of a method that is already defined in its superclass. Method overloading, on the other hand, occurs when a class has multiple methods with the same name but different parameters. Method overriding is related to inheritance, while method overloading is related to a single class.

17. What are lambda expressions in Java?
Lambda expressions are anonymous functions that allow the writing of shorter and more concise code. They enable the implementation of functional interfaces using a more compact syntax. Lambda expressions are primarily used for functional programming and working with collections in Java.

18. Explain the concept of functional interfaces in Java 8.
Functional interfaces are interfaces that have exactly one abstract method. They are used for functional programming and can be implemented using lambda expressions or method references. Functional interfaces provide a way to achieve functional programming paradigms in a Java codebase.

19. What are the benefits of using JDBC over ODBC in Java?
JDBC (Java Database Connectivity) is a Java API for connecting to and interacting with databases. The benefits of using JDBC over ODBC (Open Database Connectivity) include platform independence, better performance, improved security, and better integration with Java programming, as JDBC is specifically designed for Java applications.

20. How does garbage collection work in Java?
Garbage collection in Java is the process of automatically reclaiming memory occupied by objects that are no longer referenced by the program. It involves identifying unreferenced objects, marking them as eligible for garbage collection, and reclaiming their memory. The garbage collector executes in the background and uses different algorithms for memory management, such as mark-and-sweep, generational, and concurrent garbage collection.