Java and backend interview questions

The JVM is a virtual machine that executes Java bytecode, so the same compiled class files can run on any OS that ships a JVM.

The JVM is only the runtime engine that executes bytecode.

Bytecode is the platform-neutral instruction set produced by javac and stored in .class files.

Local variables and method call frames live on the stack of the current thread.

Garbage collection automatically reclaims heap objects that are no longer reachable from GC roots such as stack frames and static fields.

Java source compiles to bytecode, not to a single OS-specific binary.

The interpreter executes bytecode one instruction at a time, which is simple but slow.

The launcher starts a JVM, the classloader loads Main and its dependencies, and the JVM looks for public static void main(String[]).

Encapsulation hides state behind methods so callers cannot corrupt invariants.

Abstraction is about showing a simple interface and hiding complexity, for example List versus the array inside ArrayList.

Overloading is compile-time: same method name, different parameter lists in the same class.

Use an interface when you want a capability contract that many unrelated classes can implement, especially after Java 8 default methods.

If Payment is an interface and UpiPayment and CardPayment both implement pay(), a checkout service can accept Payment and call pay() without knowing the concrete type.

Inheritance couples you to a parent class and leaks implementation details into subclasses.

If two parents define the same method, the compiler would not know which one to pick, the diamond problem.

List keeps ordered elements and allows duplicates, like order line items.

ArrayList is the default: random access is O(1) and sequential scan is cache-friendly.

Hashtable is synchronized and does not allow null keys or null values, which made it the old thread-safe map.

HashSet is backed by a HashMap, so add and contains are average O(1) with no sorted order.

HashMap treats a null key as a special case and stores it in a dedicated bucket.