Java Interview Questions and Answers

The JVM runs Java bytecode, the JRE is the JVM plus the standard libraries needed to run Java programs, and the JDK is the JRE plus the development tools such as the compiler (javac).

== compares primitives by value and objects by reference (whether they are the exact same object), while equals() compares the logical content of two objects when the class overrides it.

String is immutable, StringBuilder is a mutable and fast but not thread-safe buffer, and StringBuffer is a mutable buffer with synchronized methods that is thread-safe but slower.

final is a keyword that prevents reassigning a variable, overriding a method or extending a class; finally is a block that always runs after try/catch for cleanup; finalize() was a method called by the garbage collector and is now deprecated.

An abstract class can have instance fields, constructors and a mix of implemented and abstract methods but supports only single inheritance, while an interface defines a contract that a class can implement multiple times and, since Java 8, can include default methods.

ArrayList is backed by a resizable array with O(1) index access and O(n) insertion in the middle, while LinkedList is a doubly linked list with O(n) index access and O(1) insertion once you hold the node; in practice ArrayList is faster for most workloads.

Checked exceptions (subclasses of Exception other than RuntimeException) must be caught or declared with throws, while unchecked exceptions (RuntimeException and Error) do not have to be handled and usually signal programming bugs.

Java 8 introduced lambda expressions, functional interfaces, the Stream API for declarative collection processing, Optional to represent missing values, default methods in interfaces, method references and the java.time date and time API.

A HashMap stores entries in an array of buckets; it computes the key's hash code to choose a bucket, resolves collisions with a linked list that becomes a red-black tree when a bucket grows large, and resizes when the load factor of 0.75 is exceeded.

HashMap is not thread-safe, Hashtable is thread-safe by locking the whole map on every method, and ConcurrentHashMap is thread-safe with much finer-grained locking and lock-free reads, so it scales far better.

The JVM divides memory into a per-thread stack, a shared heap (split into young and old generations) and Metaspace; the garbage collector frees heap objects that are no longer reachable, collecting short-lived young objects frequently and old objects less often.

synchronized gives mutual exclusion and memory visibility for a block of code, while volatile only guarantees that reads and writes of one variable are visible to all threads and does not make compound operations such as count++ atomic.