Java Interview Questions 2026

Tip: == compares references (whether two variables point to the same object), while .equals() compares logical value. For String literals interned in the string pool == may appear to work, but always use .equals() for content comparison. Override equals() and hashCode() together for custom classes.

Tip: The stack stores method frames, local primitives, and object references — it is per-thread and freed when a method returns. The heap stores all objects and is shared across threads and managed by the garbage collector. Class metadata lives in the Metaspace (post-Java 8, replacing PermGen).

Tip: HashMap is unsynchronised and allows one null key. HashTable is fully synchronised (legacy, slow) and disallows nulls. ConcurrentHashMap uses bucket/segment-level locking (CAS + synchronized bins since Java 8) for high-concurrency reads/writes without locking the whole map — prefer it over HashTable.

Tip: Encapsulation (private fields + getters/setters), Inheritance (extends, single class inheritance), Polymorphism (method overriding + dynamic dispatch, overloading at compile time), and Abstraction (abstract classes and interfaces). Mention that Java supports multiple interface inheritance but single class inheritance.

Tip: An abstract class can hold state (fields), constructors, and a mix of concrete and abstract methods; a class extends only one. An interface defines a contract — since Java 8 it can have default and static methods, and a class can implement many. Use an interface for "can-do" capabilities, an abstract class for shared base state.

Tip: GC reclaims unreachable objects using generational collection (Young/Eden + Survivor, then Old gen). Algorithms include G1 (default since Java 9) and ZGC for low pause times. A leak still occurs when objects remain reachable but are no longer needed — e.g. static collections, unremoved listeners, or unclosed resources holding references.

Tip: Checked exceptions (extend Exception) must be declared or handled at compile time — e.g. IOException. Unchecked exceptions (extend RuntimeException) signal programming errors like NullPointerException and need not be declared. Do not swallow exceptions; either handle meaningfully or propagate.

Tip: volatile guarantees visibility — reads/writes go to main memory, preventing stale cached values — but not atomicity. synchronized provides both mutual exclusion (atomicity) and visibility via the happens-before relationship. Use volatile for simple flags; use synchronized or AtomicInteger for compound read-modify-write operations.