Top Java Interview Topics — Summary, Flashcards & Quiz Flashcards

Front

JVM

Back

Java Virtual Machine executes compiled Java bytecode on the host platform and provides memory management and runtime services. It abstracts the underlying operating system so Java bytecode can be platform-independent.

Front

JRE

Back

Java Runtime Environment supplies the libraries and JVM required to run Java applications but does not include development tools like a compiler. It is what you install on user machines to execute Java programs.

Front

JDK

Back

Java Development Kit includes the JRE plus development tools such as javac, jar, and debugging utilities. Developers use JDK to write and build Java applications.

Front

ClassLoader

Back

A ClassLoader loads class bytecode into the JVM at runtime, following a delegation model by default. It enables dynamic loading and isolation (e.g., webapp classloaders) and can be extended for custom behavior.

Front

Garbage Collection

Back

Garbage Collection reclaims heap memory occupied by unreachable objects automatically, helping prevent memory leaks. Different collectors (G1, CMS, Parallel) offer trade-offs between latency and throughput.

Front

transient

Back

The transient modifier marks a field to be ignored during serialization so it is not persisted into the serialized form. Use transient for non-serializable or sensitive data that should not be stored.

Front

Serializable

Back

Serializable is a marker interface that allows an object's state to be converted into a byte stream for storage or transmission. Implementing it signals that default serialization is acceptable unless custom handling is provided.

Front

equals/hashCode

Back

equals() determines logical equality of objects while hashCode() provides an integer used in hash-based collections. Both must be consistent: equal objects must have equal hash codes to function correctly in HashMap/HashSet.

Front

ArrayList

Back

ArrayList is a resizable array implementation of the List interface providing fast random access and amortized O(1) append. It is inefficient for frequent middle insertions or removals compared to linked structures.

Front

LinkedList

Back

LinkedList implements List and Deque using a doubly-linked node structure, offering cheap insertions and removals at ends or when you have a node reference. It has higher memory overhead and slower random access than ArrayList.

Front

HashMap

Back

HashMap provides key-value mapping with average O(1) performance for get/put using hashing and buckets. It allows null keys and values and is not synchronized by default, so concurrent use requires external synchronization or ConcurrentHashMap.

Front

ConcurrentHashMap

Back

ConcurrentHashMap is a thread-safe Map providing high concurrency without locking the entire map for reads/writes. It uses internal partitioning and non-blocking algorithms to reduce contention compared to synchronized maps.

Front

synchronized

Back

synchronized is a keyword that enforces mutual exclusion on an object's intrinsic lock for a block or method, ensuring one thread at a time executes the protected code. Excessive synchronized usage can lead to contention and reduced scalability.

Front

volatile

Back

volatile ensures visibility of reads/writes to a variable across threads by preventing caching and reordering for that variable. It does not provide atomicity for compound actions like incrementing.

Front

Deadlock

Back

Deadlock is a situation where two or more threads wait indefinitely for resources held by each other, causing a system hang. Prevent deadlocks by ordering locks consistently, using timeouts, or attempting lock acquisition with tryLock.

Front

Lambda

Back

A Lambda expression is an anonymous function that provides a concise way to implement a functional interface, enabling functional-style operations on collections with Streams. It improves expressiveness and supports parallel execution of streams.

Front

Stream API

Back

Stream API provides a fluent, declarative way to process collections and supports intermediate and terminal operations, including parallel processing. Streams emphasize transformations and reductions rather than explicit iteration.

Front

Optional

Back

Optional is a container object that may or may not contain a non-null value, designed to reduce null-checks and NullPointerExceptions. It provides methods like orElse, orElseGet, and ifPresent for safe handling.

Front

Dependency Injection

Back

Dependency Injection is a design pattern where an external container supplies required dependencies to objects, promoting loose coupling and testability. Spring supports constructor and setter injection, with constructor injection favored for mandatory dependencies.

Front

Hibernate

Back

Hibernate is an ORM framework mapping Java objects to relational database tables, managing entity lifecycle (transient, persistent, detached) and providing HQL and criteria APIs. It handles caching, lazy loading, and transaction integration with frameworks like Spring.

Front

Singleton

Back

Singleton is a creational pattern ensuring only one instance of a class exists in the application, often used for global configuration or resource managers. Implement thread-safe singletons using enum type or properly synchronized lazy initialization.