Expert Java Training in Noida with Certification and Placement Support

What are the key features of Java?
Java is a high-level, object-oriented, platform-independent programming language. Key features include portability (Write Once, Run Anywhere – WORA), robustness (automatic memory management and exception handling), security (bytecode verification and sandboxing), multithreading, and scalability. It is also highly popular due to its vast standard libraries and APIs.

What is the difference between JDK, JRE, and JVM?
The Java Development Kit (JDK) provides the tools needed for Java development, including a compiler and debugger. The Java Runtime Environment (JRE) contains the libraries and JVM required to run Java applications but lacks development tools. The Java Virtual Machine (JVM) is the runtime engine that executes Java bytecode and provides features like garbage collection and memory management.

Explain OOP concepts in Java.
Java supports object-oriented programming with key principles:

Encapsulation (hiding data with access modifiers and exposing only necessary functionality via methods),
Inheritance (enabling code reuse by deriving classes from parent classes),
Polymorphism (method overloading and overriding), and
Abstraction (defining abstract classes and interfaces to focus on what a class should do without worrying about how it does it).

What is the difference between “==” and .equals() in Java?
The == operator compares memory references (addresses) of objects, meaning it checks if two objects are the same instance. The .equals() method, on the other hand, compares the content of objects. For example, in the case of Strings, s1.equals(s2) checks the content, while s1 == s2 checks reference equality.

What are Java collections, and why are they used?
Java Collections Framework (JCF) provides data structures like List, Set, Map, and Queue to store and manipulate groups of objects efficiently. For example, ArrayList provides dynamic arrays, while HashMap is used for key-value pair mappings. Collections are widely used to manage, sort, and retrieve data.

Explain the concept of exception handling in Java.
Java uses a try-catch-finally block for exception handling. The try block contains code that may throw exceptions, the catch block handles specific exceptions, and the finally block contains cleanup code. Checked exceptions (e.g., IOException) must be handled explicitly, while unchecked exceptions (e.g., NullPointerException) arise during runtime.

What is the difference between abstract classes and interfaces?
Abstract classes can have both abstract and concrete methods and support instance variables, while interfaces can only contain abstract methods (prior to Java 8) and static constants. From Java 8 onwards, interfaces can also include default and static methods. Abstract classes are used for partial abstraction, whereas interfaces achieve full abstraction.

What are Java’s access modifiers?
Java provides four access modifiers:

Public (accessible from anywhere),
Protected (accessible within the same package and subclasses),
Default (package-private) (accessible only within the same package), and
Private (accessible only within the class).

What is garbage collection in Java?

Java performs automatic memory management using garbage collection, which reclaims memory occupied by objects that are no longer referenced. The JVM uses algorithms like the Mark-and-Sweep method to identify unused objects. Developers can request garbage collection using System.gc(), though it is not guaranteed to execute immediately.

What are multithreading and thread lifecycle in Java?
Multithreading allows concurrent execution of multiple threads, improving the efficiency of CPU utilization. The lifecycle of a thread includes states such as New, Runnable, Running, Blocked/Waiting, and Terminated. Java provides the Thread class and Runnable interface to implement multithreading, where the run() method defines the thread’s task and start() begins its execution.