Top 10 Most Asked TCS Java Interview Questions and Answers (2025)

Top 10 Most Asked TCS Java Interview Questions and Answers (2025)

Getting ready for a Java interview, especially with a big company like TCS? Knowing your Core Java basics inside out is super important. Interviewers want to see that you understand the fundamentals, not just memorized answers.

This article covers the 10 most common Core Java questions you might face in 2025, explained in simple English with clear examples. Let's dive in!

1. What are the main principles of Object-Oriented Programming (OOP) in Java?

OOP is like building with smart LEGO blocks. Java is built around this idea. The four main "pillars" or principles are:

• Encapsulation (Bundling): This is like putting a medicine tablet inside a capsule. It means bundling data (information) and the methods (actions) that work on that data into a single unit (called a "class"). It helps hide the inner workings and protect the data.
  • Simple Example: A Car object has data like color and speed, and methods like startEngine() and accelerate(). You use the methods to interact with the car, but you don't need to know how the engine starts inside.
• Inheritance (Passing Down Traits): This is like how a child inherits traits from parents. In Java, a new class (child/subclass) can inherit properties and behaviors (methods) from an existing class (parent/superclass). It helps reuse code.
  • Simple Example: A Dog class can inherit from an Animal class. The Animal class might have a eat() method, and Dog automatically gets it.
• Polymorphism (Many Forms): This means "many forms." It allows objects to take on different forms or behave differently in different situations. It helps write flexible code.
  • Simple Example: An Animal object might have a makeSound() method. A Dog object could bark(), while a Cat object could meow(), even though both are Animals and call makeSound().
• Abstraction (Showing Only Essentials): This is like using a TV remote. You only see buttons like "Power," "Volume," "Channel." You don't need to know the complex internal circuits. Abstraction hides the complex details and shows only the necessary features.
• Simple Example: When you drive a car, you use the steering wheel and pedals. You don't need to know how the engine cylinders fire. The car abstracts away the complex engine details.

2. Explain JVM, JRE, and JDK.

These three terms are crucial for any Java developer:

• JVM (Java Virtual Machine): This is the "runner" of your Java code. It's a program that takes your compiled Java code (bytecode) and translates it into instructions your computer's specific hardware can understand. It's what makes Java "Write Once, Run Anywhere."
• JRE (Java Runtime Environment): This is the "environment" needed to run Java programs. It includes the JVM and all the necessary libraries and files to execute Java applications. If you only want to run Java programs (not write them), you only need JRE.
• JDK (Java Development Kit): This is the "developer's toolkit." It includes everything in the JRE, plus tools like the Java compiler (which turns your Java code into bytecode), debugger, and other development tools. If you want to write and compile Java programs, you need the JDK.

3. What is the difference between == and .equals() in Java?

This is a classic!

• **== (Double Equals Sign):** This is used to compare if two things are the exact same object in memory. For primitive types (like int, char, boolean), it checks if their values are equal. For objects, it checks if they point to the same location in memory.
• .equals() (Method): This is a method (a piece of code) that you use to compare if the contents or values of two objects are the same. Most classes in Java (like String) have their own .equals() method that checks if the actual data inside them is identical.

String s1 = "hello";

String s2 = "hello";

String s3 = new String("hello"); // Creates a new object

System.out.println(s1 == s2);       // true (often, due to String Pool optimization)

System.out.println(s1 == s3);       // false (s1 and s3 are different objects in memory)

System.out.println(s1.equals(s3));  // true (their content "hello" is the same)

4. What is Exception Handling in Java?

Exception handling is Java's way of dealing with unexpected problems that might happen while your program is running. Think of it as a safety net.

When something goes wrong (like trying to divide by zero or trying to open a file that doesn't exist), Java creates an "exception" object. You can then "catch" this exception and handle it gracefully, preventing your program from crashing.

We use try, catch, and finally blocks:

• **try:** This block contains the code that might cause an exception.
• **catch:** If an exception happens in the try block, the code in the catch block runs to handle it.
• **finally:** This block always runs, whether an exception occurred or not. It's often used for cleanup tasks like closing files.

try {

    int result = 10 / 0; // This will cause an ArithmeticException

    System.out.println(result);

} catch (ArithmeticException e) {

    System.out.println("Oops! You tried to divide by zero. Please don't!");

} finally {

    System.out.println("This message always appears, even after an error.");

}

5. What are abstract classes and interfaces in Java? How are they different?

Both abstract classes and interfaces are used to achieve "abstraction" (hiding details and showing essentials). They act like blueprints for other classes, but they have key differences:

Feature	Abstract Class	Interface
Purpose	Designed for inheritance, provides a base with some common behavior and some incomplete behaviors.	Defines a contract; a list of methods that a class must implement.
Methods	Can have both regular (with code) and abstract (without code) methods.	Before Java 8, only abstract methods. From Java 8, can also have default and static methods (with code).
Variables	Can have any type of variables (regular, final, static).	All variables are automatically public static final (constants).
Inheritance	A class can extend only one abstract class.	A class can implement multiple interfaces.
Constructor	Can have constructors.	Cannot have constructors.
"Pure" Abstraction	Can be 0% to 100% abstract (some methods might have code).	Was 100% abstract (until Java 8 default methods).

6. What is the Java Collections Framework? Name some common classes.

The Java Collections Framework is a set of pre-built "containers" or "structures" that help you store and manage groups of objects efficiently. Think of it as a toolkit with different types of boxes or lists for your data.

Some common classes (types of containers) you'll use a lot:

• ArrayList: Like a resizable array. Good for lists where you often add/remove items at the end or access items by their position (index).
• LinkedList: Good for lists where you often add/remove items from the beginning or middle, but slower for direct access.
• HashSet: Stores a collection of unique items. It's fast for checking if an item exists or adding/removing.
• HashMap: Stores data as "key-value" pairs (like a dictionary). You give it a "key" (e.g., a person's name) and it gives you back a "value" (e.g., their phone number). Super fast for looking things up!

import java.util.ArrayList;

import java.util.HashMap;

public class CollectionsExample {

    public static void main(String[] args) {

        // ArrayList example: A list of names

        ArrayList<String> names = new ArrayList<>();

        names.add("Alice");

        names.add("Bob");

        names.add("Alice"); // ArrayList allows duplicates

        System.out.println("Names in ArrayList: " + names);

        System.out.println("First person: " + names.get(0));

        // HashMap example: Phone numbers

        HashMap<String, String> phoneBook = new HashMap<>();

        phoneBook.put("Alice", "123-456-7890");

        phoneBook.put("Bob", "987-654-3210");

        System.out.println("Bob's number: " + phoneBook.get("Bob"));

    }

}

7. What is String immutability in Java?

In Java, String objects are immutable, which means once you create a String, you cannot change its content. If you try to change a String (like adding text to it), Java actually creates a brand new String object in memory with the updated content, leaving the original String untouched.

• Why is this important? It makes Strings very safe for things like security (you don't want a password string to suddenly change), and it's good for multi-threaded programs where many parts of your code might be using the same string.
• 
  

8. What is the final keyword used for in Java?

The final keyword is a powerful way to make things "fixed" or "unchangeable":

• final variable: Once given a value, you cannot change it. It becomes a constant.
  final int MAX_SPEED = 200; // Cannot change MAX_SPEED later
  

• final method: A method declared final cannot be overridden (changed) by any subclass. This ensures its behavior remains consistent.
• final class: A class declared final cannot be inherited by any other class. This prevents other classes from changing its behavior or adding new ones. (Example: String class is final).

9. Explain static keyword in Java.

The static keyword means something belongs to the class itself, not to a specific object created from that class.

• static variable: This variable is shared by all objects of that class. If one object changes it, all other objects see the change.
  Java

  public class Counter {
      static int count = 0; // Shared by all Counter objects
      public Counter() {
          count++;
      }
  }
  // If you create 3 Counter objects, Counter.count will be 3.
  

• static method: You can call this method directly using the class name, without needing to create an object of that class. main method is a classic example!
  Java

  public class MathUtils {
      public static int add(int a, int b) { // Call as MathUtils.add(5, 3)
          return a + b;
      }
  }
  

10. What is Method Overloading and Method Overriding?

These are two common concepts in OOP that sometimes confuse beginners:

• Method Overloading (Same Name, Different Arguments): This happens within the same class. You have multiple methods with the same name but different "signatures" (meaning they take a different number of arguments, or different types of arguments). The computer decides which method to call based on the arguments you provide.
  • Simple Example:
    public class Calculator {
        int add(int a, int b) { return a + b; }
        double add(double a, double b) { return a + b; } // Overloaded method
    }
    

• Method Overriding (Child Changes Parent's Behavior): This happens between a parent class and a child class. The child class provides its own specific implementation for a method that is already defined in its parent class. The method name, return type, and arguments must be exactly the same.
  • Simple Example:
    class Animal {
        void makeSound() { System.out.println("Animal makes a sound"); }
    }
    
    class Dog extends Animal {
        @Override // Tells Java we are overriding
        void makeSound() { System.out.println("Dog barks: Woof Woof!"); }
    }

Mastering these core Java concepts will give you a strong foundation for your TCS interview and a solid career in Java development. Good luck!