Java Interview Questions

Explore how Java achieves platform independence with the Java virtual machine, converting bytecode to machine code. Understand JVM, JRE, JDK, and system, extension, and bootstrap class loaders.

Learn why the main method starts a Java program and how static enables calling it without an instance. Explore overloading, program termination on exceptions, and why main cannot be overridden.

Explore how string immutability and the string pool determine object creation, compare string versus new string, and contrast string buffer with string builder for performance and thread safety.

Explore object class in Java, compare equals and the == operator, and learn hashCode, toString, clone, and finalize, including reflexive, symmetric, transitive, and consistent rules with getClass versus instance of.

Explore how wrapper classes wrap primitive types, compare them with primitives, demonstrate autoboxing and auto unboxing, discuss collection use, caching, and null pointer risks.

Explore core Java concepts such as function overloading and overriding, and how interfaces define responsibilities; learn that interfaces can include default methods since Java eight.

Explore runtime polymorphism in Java by demonstrating how a parent reference calls overridden child methods like eat, and how methods absent in the parent cannot be invoked through that reference.

Examine how default, public, and private access modifiers control visibility across packages and subclasses, noting that public is visible everywhere while private remains limited to the same class.

Explore abstract classes in Java, including how the abstract keyword prevents instantiation, how constructors run via subclass super calls, and how interfaces and static methods relate.

Explore inner classes in Java, comparing static and non-static variants, and learn that non-static inner classes need an outer instance while static inner classes instantiate directly.

Understand static variables shared across instances, and static methods that belong to the class and require no object. Learn that static blocks run on class load in order.

This lecture explains marker interfaces, including the random access and serializable interfaces, and how serialization and deserialization use object output and input streams.

Learn how to serialize Java objects by implementing Serializable, manage serialVersionUID, and use object streams, while noting static and transient fields and comparing Serializable with Externalizable.

Explore cloning in Java, compare shallow and deep copies, and implement clone, copy constructors, and serialization to duplicate objects like employee and department.

Explore the Java exception hierarchy, differences between checked and unchecked runtime exceptions, and the distinction from errors. Learn handling with try-catch-finally and using try-with-resources for automatic closing.

Navigate the Java collections hierarchy from iterable to map, covering list, set, and sorted set with implementations like vector, linked list, hash set, and tree map.

Explore differences between list implementations like array list and vector, and map and set varieties such as hash map, hash table, hash set, and tree set, including null handling.

Learn to remove duplicates using a set or java eight streams distinct, and survey linked list versus array list storage, plus navigable set methods like floor, lower, higher, and poll.

Explore how hash maps store entries in buckets, use equals and hashCode, handle collisions with linked lists, and perform get and put operations.

Learn to sort a list of objects in Java using Comparable and Comparator, with natural ordering via compareTo and custom orders via Comparator.

Explore how concurrent collections enable safe iteration while modifying a collection, avoiding concurrent modification exceptions, and compare them with synchronized and fail-fast collections, highlighting copy-on-write behavior.

Explore generics in Java, apply type restrictions to collections to enforce string-only lists, and leverage compile-time checks to prevent runtime ClassCastException while using generic classes and understand type erasure.

Explore the Java executor framework, creating thread pools (single, fixed, scheduled, cached), using runnable vs callable, and managing task results with Future and CompletableFuture for asynchronous execution.