Java Interview Questions -Set 8

Java Interview Questions – Set 8

Java interviews often include questions that test your understanding of core language concepts as well as important features introduced in newer Java versions. This set focuses on object comparison, Java 8 features, the final keyword, JIT compilation, and string handling classes.

YT:- DecodeIT

Q1. What is the difference between == and equals() in Java?

Answer: The == operator and equals() method are used for different types of comparison.

• ==: When used with objects, it checks whether two references point to the same object.
• equals(): It is intended to compare the logical content of objects. The actual comparison behavior depends on how the class implements or overrides this method.

String s1 = new String("hello");
String s2 = new String("hello");

System.out.println(s1 == s2);      
System.out.println(s1.equals(s2));

Here, the two String objects contain the same text, but they are separate objects. Therefore, == checks their references, while equals() checks their content.

Q2. What are the major features introduced in Java 8?

Answer: Java 8 introduced several features that changed the way developers write Java applications. Some important additions include:

• Lambda Expressions: Allow behavior to be represented using compact expressions.
• Stream API: Provides a functional approach for processing collections of data.
• Default Methods: Allow interfaces to contain methods with implementations.
• Optional: Provides a way to represent potentially missing values and reduce direct null handling.
• Date and Time API: The java.time package provides modern classes for handling dates and times.

List<String> names = Arrays.asList(
    "John", "Jane", "Jack", "Doe"
);

names.stream()
     .filter(name -> name.startsWith("J"))
     .forEach(System.out::println);

The stream filters the list and prints only the names that begin with the letter J.

Q3. What is the purpose of the final keyword in Java?

Answer: The final keyword can be applied to variables, methods, and classes. Its effect depends on where it is used.

• Final Variable: Once assigned, its value cannot be reassigned.
• Final Method: A subclass cannot override the method.
• Final Class: The class cannot be extended by another class.

final int MAX = 100;

public final void display() {
    System.out.println("Cannot override");
}

public final class Utility {
    public static final int MAX_VALUE = 100;
}

Using final can therefore restrict reassignment, method overriding, or class inheritance depending on its position in the code.

Q4. What is a JIT Compiler in Java?

Answer: JIT stands for Just-In-Time compiler. It is a component of the Java Virtual Machine that can compile frequently executed Java bytecode into native machine instructions while the application is running.

This runtime compilation can improve execution performance because frequently used code can be optimized for the underlying system instead of being interpreted repeatedly.

Q5. What is the difference between String, StringBuilder, and StringBuffer?

Answer: All three classes are related to text handling, but they differ mainly in mutability and synchronization.

• String: Immutable. Once a String object is created, its contents cannot be changed.
• StringBuilder: Mutable and generally preferred when frequent string modifications are required in a single-threaded context.
• StringBuffer: Mutable and synchronized, making it suitable for situations where its synchronization characteristics are required.

Quick Revision

• == compares object references when used with objects.
• equals() is used for logical object comparison according to the class implementation.
• Java 8 introduced features such as lambdas, streams, default methods, Optional, and the modern Date/Time API.
• final can restrict variable reassignment, method overriding, or class inheritance.
• The JIT compiler can compile frequently executed bytecode into native machine code during runtime.
• String is immutable, while StringBuilder and StringBuffer are mutable.

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Conclusion

These questions cover several Java concepts that are useful for technical interview preparation. Instead of memorizing short definitions, practice explaining each concept with a small example and understand when it would be useful in an application.

For better interview preparation, combine conceptual revision with hands-on coding. Writing small Java programs based on these topics can help you become more comfortable explaining your approach during technical discussions.

FAQs

1. What is the difference between == and equals() in Java?
== checks whether two object references point to the same object, while equals() is generally used to compare the logical content of objects.

2. What are the main features introduced in Java 8?
Java 8 introduced Lambda Expressions, Stream API, default methods, Optional, and the modern Date and Time API.

3. What does the final keyword do in Java?
final can prevent a variable from being reassigned, a method from being overridden, or a class from being inherited.

4. What is a JIT Compiler in Java?
The Just-In-Time compiler converts frequently executed Java bytecode into native machine code during runtime to improve application performance.

5. What is the difference between String, StringBuilder, and StringBuffer?
String is immutable, while StringBuilder and StringBuffer are mutable. StringBuilder is generally preferred for frequent modifications in a single-threaded context, whereas StringBuffer provides synchronized operations.

6. Why is String immutable in Java?
String objects cannot be modified after they are created. This provides better security, supports string pooling, and makes strings safer to use across multiple parts of an application.

7. Can a final method be overridden in Java?
No. A method declared with the final keyword cannot be overridden by a subclass.

8. What is the difference between StringBuilder and StringBuffer?
Both are mutable classes used to modify strings. StringBuilder is generally faster because it is not synchronized, while StringBuffer provides synchronized methods for thread-safe operations.

9. What is the purpose of Lambda Expressions in Java?
Lambda expressions provide a concise way to represent a block of behavior, especially when working with functional interfaces, collections, and the Stream API.

10. What is the Java Stream API used for?
The Stream API provides a convenient way to process collections of data using operations such as filtering, mapping, sorting, and collecting results.

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