Why `String` Is Immutable in Java — Security, Hashing, the String Pool, and Thread Safety

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/why-string-is-immutable-in-java

String vs StringBuilder vs StringBuffer

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/string-vs-stringbuilder-vs-stringbuffer-java

String Pool and Interning

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/string-pool-interning

`String.intern()` and the String Pool — How They Work and When to Use `intern()`

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/string-intern-method-java-explained

`String.equals()` vs `equalsIgnoreCase()` vs `contentEquals()` — Which to Use When

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/string-equals-vs-contentequals-java

Exception Hierarchy — Throwable, Error, Exception, RuntimeException

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/exception-hierarchy-java

Checked vs Unchecked Exceptions in Java

See full structured answer at https://interviewexplainer.com/core-java-interview-questions/checked-vs-unchecked-exceptions-java-when-to-use

Java Language & Core·Interview Q&A

Core Java & OOP interview questions

55+ curated questions with the same answer shape every time: quick recap, deep dive, pitfalls, and follow-ups. Coverage includes string handling, exception handling, generics wildcards, java io nio, and more.

55questions

7topics

15364difficulty

Open question 1 How to read this module

Suggested path

Read Core Java & OOP from question 1 to 55

Topics follow a teaching order. Each answer ends with a clear Next question control so you can move forward without hunting the list.

1Start with question 1 — it sets up the rest of the module.
2Use Continue · Next question at the bottom of every page (and the arrows in the sticky bar when you scroll).
3Jump to any topic in the full catalog — every row is numbered in reading order (Q1–Q55).

Open question 1 Jump to Q1 in list Scroll to full catalog

About this page

Core Java interviews test the language features you use every day and the edge cases interviewers love. This module is an elevation arc: it opens with string handling (immutability, pool, StringBuilder — the #1 opener), then exception handling (checked vs unchecked, try-with-resources), then generics and wildcards (type erasure, PECS), I/O and NIO, and reflection and annotations. It closes with the comparison canon and scenario-based questions where the features combine.

Reviewed by senior engineers·Updated for 2026·Free · No sign-up required

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High-signal Core Java & OOP questions

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Topics are ordered for learning. Within each topic, rows are numbered 1–55 globally so you always know where you are. Collapse topics if you want a lighter view — use Expand all when you need the full list again.

Full catalog · reading order

Core Java & OOP

Numbers 1–55 follow the order we recommend — click any row to open the full answer, or use the sticky Next link at the bottom of each page to go Q1 → Q55.

55 questions·7 topics

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Interviewers rarely ask Core Java & OOP in isolation. These are the modules that most often come up in the same loop.

Most relevant next step52+ Q&A

Java OOP & SOLID Interview Questions

Java OOP interview questions are the single most common opener in any backend interview, from junior to staff. This module is organized as an elevation arc so a learner reads top-to-bottom and feels the level rise. T0 Fundamentals grounds you in what OOP is and why it exists as a response to procedural programming's failures, then walks the four pillars. T1 Core Mechanics covers the daily building blocks - this/super, constructor chaining, overloading vs overriding, static. T2 Internet-Famous is the canon nearly every interview asks: SOLID (the #1 most-asked design-principle set), abstract class vs interface, composition vs inheritance, cohesion and coupling, the Law of Demeter, and the equals/hashCode contract. T3 Applied Patterns moves to code you actually ship - default/static interface methods, the diamond problem, association/aggregation/composition, enums, immutability. T4 Production & Edge Cases separates a working developer from someone who's shipped: shallow vs deep copy, marker interfaces, SerialVersionUID, inner classes, anonymous vs lambda, the Object class methods. Every answer comes with a short code snippet, the trade-off the interviewer is listening for, and the follow-up they are likely to ask next.

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Java Collections & Algorithms

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Java Concurrency & Multithreading

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JVM Internals & Performance

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FAQs about Core Java & OOP interview prep

QHow many Core Java & OOP interview questions are on this page?

This module bundles 55+ Core Java & OOP interview questions, grouped by 7 topics. Every answer follows the same structure — one-liner summary, structured deep-dive, common pitfalls, and realistic follow-ups an interviewer might ask.

QIs this Core Java & OOP content up to date for 2026?

Yes. Every answer is reviewed by senior engineers and updated for current best practices, version-specific APIs, and the scenario questions hiring managers are asking today.

QWhich difficulty level are these Core Java & OOP questions?

The set spans all three levels — 15 easy, 36 medium, and 4 hard questions. Junior candidates should focus on the conceptual questions; senior candidates should drill the scenario and trade-off questions in every topic group.

QWhich question should I read first?

Start with question 1 in the ordered list below ("Core Java Deep Dive Fundamentals") — it is the recommended entry point. Then use the Next question link at the bottom of each answer page to read straight through to Q55.

QWhat else belongs in a Java Language & Core prep plan?

Core Java & OOP is one module inside the broader Java Language & Core hub — open it to drill the full set of sibling modules with the same structured answer format.

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