Core Java Interview Questions and Answers

Main Topics Covered

Return Statement in try/catch: Even if a return statement is executed, the finally block still runs.
Using try with finally without catch: It's possible to handle cleanup without a catch block to propagate exceptions.
Performance Impact: Using try, catch, and finally can slightly impact performance due to exception handling overhead but is generally minimal.
Exceptions:
- finally block won’t execute if JVM exits via System.exit() during try or catch execution.
- Multiple finally blocks are not allowed within a single try-catch-finally structure.
Handling Multiple Exceptions: Use a single catch block separated by a pipe | symbol to handle multiple exceptions with the same logic.

Running Java Applications: Cannot run without JRE as it contains essential tools and libraries.
- Alternative tool: JLink in newer Java versions (from Java 9) allows bundling Java applications with their own JRE.
- GraalVM: Can build a native image that doesn't need JRE to run.
JDK without JRE: Not possible, as JDK includes JRE containing essential components for running and developing Java applications.
Different Garbage Collection Algorithms: Mark-Sweep, Mark-Compact, Generational Copying are used depending on the collector chosen.
Memory Leaks: Can occur even with automatic garbage collection when objects are no longer needed but still referenced.

Java is Partially Object-Oriented: Uses primitive types like int, char which are not objects.
Advantages of Being Partially Object-Oriented:
- Simple non-object types (like int) help Java run faster and use less memory.
- Allows Java to work well with technologies and systems not fully object-oriented.
Use in Enterprise Projects: Object-oriented programming (OOP) helps organize code better, making it easier to update and scale, saving time and effort.
Access Modifiers: public static void main(String[] args) is the entry point of Java applications.
- public: Accessible from anywhere.
- static: Doesn’t need an object instance to be called.
- void: Doesn’t return any value.
- main: Method name.
Primitive Data Types: Cannot be null. Default values are provided (e.g., 0 for int, false for boolean).
Wrapper Classes: Enable storing primitives in collections by treating them as objects.

Overloading: Using the same method name with different inputs in the same class.
Overriding: Occurs when a subclass has a method with the same name, return type, and parameters as in its parent class.
Dynamic Method Dispatch: Determines which method to use at runtime when methods are overridden.

Abstract Classes: Cannot create objects directly; must be extended by other classes.
Interfaces: List methods a class should have without detailing how they work.
- Can include static and default methods but no state.
- Used for achieving polymorphism.

Singleton Pattern: Commonly used to manage database connections ensuring shared instance reuse efficiently.
Thread Safety in Singleton: Ensures only one thread can access shared data structure at a time.
Concurrent Programming: Immutable objects are useful because they can be shared without needing synchronization.

Encapsulation: Keeps important data hidden and safe, ensuring security and integrity.
Static Block: Can throw an exception but must handle it within the block.
Functional Interface: Allows only a single abstract method but can have inherited static and default methods.
Java Collections: Uses different algorithms for sorting (dual-pivot quicksort for primitives, timsort for objects).
- HashMap in Java 8: Handles collisions using a balanced tree for efficient searching.
Java Server Creation: Possible without frameworks using Java SE API (ServerSocket for TCP, HttpServer for HTTP).