Beginner6 min read
The equals() and hashCode() Contract
Why overriding equals() without hashCode() breaks every hash-based collection, what the five contract rules guarantee, and the mutable-key bug that silently loses data.
Read tutorialInterview Prep · Phase 1
Core Java & OOP Foundations
The questions every Java interview opens with, answered at the depth that separates a memorised answer from an understood one: the equals/hashCode contract, string interning, generics erasure, exception semantics and class initialisation order.
Beginner6 min read
Strings: Immutability, the Pool and StringBuilder
Why String is immutable and what that buys you, how the string pool and intern() really work, why == sometimes appears to work, compact strings, and when concatenation in a loop actually costs you.
Read tutorialBeginner6 min read
OOP Principles the Interviewer Actually Probes
The OOP questions behind the textbook four: static vs dynamic dispatch, Liskov violations that compile cleanly, abstract class versus interface, and composition over inheritance.
Read tutorialBeginner6 min read
Exception Handling, finally and try-with-resources
Checked versus unchecked and when each is right, how finally silently discards an exception or a return value, and suppressed exceptions in try-with-resources.
Read tutorialIntermediate6 min read
static, final and Class Initialisation Order
The exact order the JVM initialises a class, why a static final String can survive deleting the class that declared it, effectively final, and how two classes can deadlock while loading.
Read tutorialIntermediate7 min read
Generics, Type Erasure and Wildcards
What the compiler removes and what it inserts, why you cannot create an array of a generic type, PECS explained by what it enables, bridge methods, and heap pollution from unchecked varargs.
Read tutorialIntermediate6 min read
Immutable Objects & Defensive Copying
The five conditions for a genuinely immutable class, the leaked-collection bug that defeats final, why records are only shallowly immutable, and what final fields guarantee across threads.
Read tutorialIntermediate5 min read
Comparable, Comparator and Sorting Contracts
Natural order versus external order, the total-ordering contract and the exception thrown when you break it, comparator chaining, the integer-overflow bug, and TimSort stability.
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