Question 1

What is the `Comparator` interface?

Accepted Answer

The `Comparator` interface provides a way to sort objects without changing their class, unlike `Comparable`. You implement `compare()` to define custom ordering—like sorting by age instead of name. It’s more flexible and often used with lambdas in modern Java.

Question 2

What is the `transient` keyword used for?

Accepted Answer

The `transient` keyword marks a field to be excluded from serialization, so its value isn’t saved when an object is serialized. It’s useful for temporary data or sensitive info like passwords that shouldn’t persist—like a secret you keep in memory only.

Question 3

What is multithreading in Java?

Accepted Answer

Multithreading lets a Java program run multiple tasks concurrently using threads—lightweight processes within a program. You can create threads via the `Thread` class or `Runnable` interface. It boosts performance for I/O-bound or parallel tasks, but requires careful synchronization.

Question 4

What is a Java marker interface?

Accepted Answer

A marker interface has no methods—like `Serializable` or `Cloneable`—and just tags a class for special treatment by the JVM or libraries. It’s a signal, like a flag, telling the system to handle the class in a certain way.

Question 5

What is a Java inner class?

Accepted Answer

An inner class is a class defined inside another class, giving it access to the outer class’s members—even `private` ones. It’s useful for grouping related logic, like event handlers. Types include static, non-static, local, and anonymous.

Question 6

What is polymorphism in Java?

Accepted Answer

Polymorphism lets objects of different classes be treated as objects of a common superclass or interface. It comes in two flavors: compile-time (method overloading) and runtime (method overriding). It’s what makes Java flexible—think of a `Dog` barking differently than a `Cat` through an `Animal` reference.

Question 7

What is the `Path` interface in Java?

Accepted Answer

The `Path` interface (from `java.nio.file`) represents a file system path—like a file or directory location. You create it with `Paths.get()` and use it with `Files` for operations. It’s more robust than `File` for modern I/O.

Question 8

What is a Java classpath?

Accepted Answer

The classpath tells the JVM where to find user-defined classes and libraries—like a search path for `.class` files or JARs. You set it with the `-cp` flag or `CLASSPATH` variable. Mess it up, and you’ll get a `ClassNotFoundException` fast.

Question 9

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

Accepted Answer

All three handle strings, but `String` is immutable (can’t change after creation), `StringBuilder` is mutable and not thread-safe (faster), and `StringBuffer` is mutable and thread-safe (slower). Use `String` for fixed text, `StringBuilder` for single-threaded edits, and `StringBuffer` for multi-threaded scenarios.

Question 10

What is the `Vector` class in Java?

Accepted Answer

The `Vector` class is a synchronized, growable array—like an old version of `ArrayList`. It’s thread-safe but slower due to synchronization on every operation. Stick to `ArrayList` unless you need legacy code or built-in thread safety.

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