Question 1

What's the difference between == and .equals() in Java?

Accepted Answer

== compares object references (memory addresses) for objects, or primitive values directly. .equals() compares logical/content equality and can be overridden (e.g. String, Integer override it to compare values, not references).

Question 2

What is the difference between an interface and an abstract class?

Accepted Answer

An abstract class can have state (fields) and constructors, and a class can extend only one. An interface (pre-Java 8) had only method signatures and constants, can't hold instance state, and a class can implement many. Since Java 8, interfaces can have default/static methods, but still no instance fields.

Question 3

Explain the difference between ArrayList and LinkedList.

Accepted Answer

ArrayList is backed by a resizable array — O(1) random access, O(n) insert/remove in the middle. LinkedList is a doubly-linked list — O(1) insert/remove at the ends, O(n) random access. ArrayList is almost always the better default unless you specifically need frequent head/tail insertions.

Question 4

What is the purpose of the 'final' keyword?

Accepted Answer

On a variable, it makes the reference immutable (can't be reassigned). On a method, it prevents overriding in subclasses. On a class, it prevents inheritance entirely (e.g. String is final).

Question 5

What is garbage collection and can you force it?

Accepted Answer

Garbage collection automatically reclaims memory used by objects with no live references. You can call System.gc(), but it's only a hint to the JVM — there's no guarantee it runs immediately or at all.

Question 6

What's the difference between checked and unchecked exceptions?

Accepted Answer

Checked exceptions (extend Exception, not RuntimeException) must be declared with 'throws' or caught at compile time — e.g. IOException. Unchecked exceptions (extend RuntimeException) don't require this, e.g. NullPointerException, IllegalArgumentException.

Question 7

What is a HashMap and how does it handle collisions?

Accepted Answer

A HashMap stores key-value pairs using a hash function to compute a bucket index. When two keys hash to the same bucket (a collision), Java stores them as a linked list (or a balanced tree, since Java 8, once a bucket gets large) inside that bucket.

Question 8

Explain the difference between synchronized methods and synchronized blocks.

Accepted Answer

A synchronized method locks the entire method on the instance (or class, if static) monitor. A synchronized block lets you lock only a critical section on a chosen object, which is more granular and usually better for performance since it holds the lock for less time.

Question 9

What is the Java Memory Model's stack vs. heap?

Accepted Answer

The stack stores method call frames and local primitive variables/references — it's per-thread and fast. The heap stores actual objects and is shared across threads, managed by the garbage collector.

Question 10

What is the difference between String, StringBuilder, and StringBuffer?

Accepted Answer

String is immutable — every modification creates a new object. StringBuilder is mutable and NOT thread-safe (fast, use in single-threaded code). StringBuffer is mutable and thread-safe (synchronized methods, slightly slower).

Java Interview Questions

Java Interview Questions