1. What is Java?
Java is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It is platform-independent, meaning that Java programs can run on any device that has the Java Runtime Environment (JRE) installed. This is achieved through the use of Java Virtual Machine (JVM), which allows Java applications to run on various operating systems without modification.
2. Explain the concept of OOP (Object-Oriented Programming) in Java.
Object-Oriented Programming (OOP) is a programming paradigm based on the concept of objects, which contain data and methods. In Java, the four core principles of OOP are:
- Encapsulation: Wrapping data and methods into a single unit (class).
- Abstraction: Hiding the implementation details and showing only the essential features.
- Inheritance: A mechanism where one class acquires the properties and behaviors of another class.
- Polymorphism: The ability of one object to take many forms, allowing methods to be used in different contexts.
3. What is a Java class?
A class in Java is a blueprint for creating objects. It defines the properties (fields) and behaviors (methods) of the objects that belong to that class. A class does not occupy memory until an object is created from it.
Example:
class Car {
String model;
int year;
void start() {
System.out.println("Car is starting");
}
}
4. What is the difference between JDK and JRE?
- JDK (Java Development Kit): A software development kit that provides tools for developing Java applications. It includes the JRE, compilers, debuggers, and other tools needed to create Java programs.
- JRE (Java Runtime Environment): It is a part of JDK and provides libraries and JVM for running Java applications. It does not include development tools like compilers.
5. What is JVM?
The Java Virtual Machine (JVM) is responsible for running Java programs. It takes the bytecode generated by the Java compiler and interprets or compiles it into machine code that can be executed on the host machine. JVM provides platform independence by abstracting the underlying hardware and OS.
6. What are the different types of memory areas allocated by JVM?
The JVM allocates several memory areas during the execution of a Java program, such as:
- Heap Memory: Used for dynamic memory allocation of Java objects.
- Stack Memory: Used for storing method calls, local variables, and partial results.
- Method Area: Stores class-level data, including method code, field data, and constant pool.
- PC Register: Stores the address of the current method being executed.
- Native Method Stack: Stores native methods used in Java programs.
7. What is a constructor in Java?
A constructor is a special type of method in Java used to initialize objects. It has the same name as the class and is invoked when an object of that class is created. It does not have a return type.
Example:
class Person {
String name;
Person(String name) {
this.name = name;
}
}
Person p = new Person("John");
8. What is method overloading in Java?
Method overloading in Java allows a class to have multiple methods with the same name but different parameters (number or type). The compiler distinguishes between the methods based on the method signature.
Example:
class MathOperation {
int add(int a, int b) {
return a + b;
}
double add(double a, double b) {
return a + b;
}
}
9. What is method overriding in Java?
Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. The method signature must be the same in both the parent and child classes.
Example:
class Animal {
void sound() {
System.out.println("Animal makes a sound");
}
}
class Dog extends Animal {
@Override
void sound() {
System.out.println("Dog barks");
}
}
10. What is the difference between == and equals() in Java?
- == is used to compare object references, i.e., whether two references point to the same object in memory.
- equals() is a method in the Object class that compares the actual content of two objects, and you can override it to provide custom equality logic.
Example:
String s1 = new String("hello");
String s2 = new String("hello");
System.out.println(s1 == s2); // false
System.out.println(s1.equals(s2)); // true
11. What is an abstract class in Java?
An abstract class is a class that cannot be instantiated on its own and must be subclassed. It can have abstract methods (without implementation) that must be implemented by its subclasses, as well as concrete methods.
Example:
abstract class Animal {
abstract void sound();
}
class Dog extends Animal {
@Override
void sound() {
System.out.println("Bark");
}
}
12. What is an interface in Java?
An interface is a reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. A class implements an interface, providing the code for the methods declared in the interface.
Example:
interface Animal {
void sound();
}
class Dog implements Animal {
@Override
public void sound() {
System.out.println("Bark");
}
}
13. What is the difference between an interface and an abstract class?
- Interface: Can only declare methods without implementation (until Java 8, which added default methods). A class can implement multiple interfaces.
- Abstract Class: Can have both abstract and concrete methods. A class can inherit from only one abstract class.
14. What is the final keyword in Java?
The final keyword in Java can be used to:
- Final variable: The value of the variable cannot be changed once assigned.
- Final method: The method cannot be overridden by subclasses.
- Final class: The class cannot be subclassed.
15. What is a package in Java?
A package is a namespace used to organize classes and interfaces logically. Packages are used to avoid name conflicts and to control access to classes and methods.
Example:
package com.example;
class MyClass {
void display() {
System.out.println("Hello from MyClass");
}
}
16. What is the difference between ArrayList and LinkedList in Java?
- ArrayList: It is backed by a dynamic array, which allows fast random access to elements. However, it is slow when inserting or deleting elements in the middle of the list.
- LinkedList: It is implemented as a doubly linked list, allowing faster insertion and deletion of elements, but slower random access compared to ArrayList.
Example:
ArrayList<Integer> arrayList = new ArrayList<>();
LinkedList<Integer> linkedList = new LinkedList<>();
17. What is the difference between HashMap and TreeMap in Java?
- HashMap: It stores key-value pairs and does not maintain any order. It allows null values and keys.
- TreeMap: It stores key-value pairs in a sorted order according to the natural ordering of the keys or by a custom comparator.
Example:
HashMap<Integer, String> hashMap = new HashMap<>();
TreeMap<Integer, String> treeMap = new TreeMap<>();
18. What is the try-catch block in Java?
The try-catch block is used to handle exceptions in Java. The code that might throw an exception is placed in the try block, and the code to handle the exception is placed in the catch block.
Example:
try {
int result = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Cannot divide by zero");
}
19. What is the throws keyword in Java?
The throws keyword is used to declare exceptions that a method may throw. It is used in the method signature to indicate that the method may throw one or more exceptions.
Example:
void myMethod() throws IOException {
throw new IOException("An I/O error occurred");
}
20. What is the finally block in Java?
The finally block is used to execute important code, such as closing files or releasing resources, regardless of whether an exception was thrown or not. It is always executed after the try block, even if an exception is not caught.
Example:
try {
int result = 10 / 2;
} catch (ArithmeticException e) {
System.out.println("Error");
} finally {
System.out.println("This will always execute");
}
21. What is the difference between StringBuilder and StringBuffer in Java?
- StringBuilder: It is used to create mutable strings. It is not synchronized, meaning it is faster than StringBuffer when used in a single-threaded environment.
- StringBuffer: It is similar to StringBuilder but is synchronized, making it thread-safe at the cost of performance.
Example:
StringBuilder sb = new StringBuilder("Hello");
StringBuffer sbf = new StringBuffer("World");
22. What are the access modifiers in Java?
Java has four main access modifiers:
- public: The member is accessible from anywhere.
- private: The member is accessible only within the same class.
- protected: The member is accessible within the same package or subclasses.
- default (no modifier): The member is accessible only within the same package.
23. What is a singleton class in Java?
A singleton class ensures that only one instance of the class is created. It is used to restrict the instantiation of a class to one object. The class has a private constructor and a static method to return the single instance.
Example:
class Singleton {
private static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
24. What is the difference between super and this in Java?
- super: Refers to the superclass of the current object. It can be used to access methods and fields of the superclass.
- this: Refers to the current instance of the class. It can be used to access fields and methods of the current object.
Example:
class Animal {
void sound() {
System.out.println("Animal makes a sound");
}
}
class Dog extends Animal {
void sound() {
super.sound(); // Calls the superclass method
System.out.println("Dog barks");
}
}
25. What is the hashCode() method in Java?
The hashCode() method is used to get a unique integer representation of an object, which is used by hash-based collections like HashMap. Two objects that are equal according to the equals() method must have the same hash code.
Example:
String str = "Hello";
System.out.println(str.hashCode());
26. What is the use of instanceof keyword in Java?
The instanceof keyword is used to test whether an object is an instance of a specific class or implements an interface. It returns true if the object is of the given type, otherwise false.
Example:
String s = "Hello";
System.out.println(s instanceof String); // true
27. What are the different types of loops in Java?
Java provides three types of loops:
- for loop: Used when the number of iterations is known in advance.
- while loop: Used when the number of iterations is unknown but continues as long as the condition is true.
- do-while loop: Similar to while, but it executes at least once before checking the condition.
Example:
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
28. What is a default method in Java interfaces?
Introduced in Java 8, a default method allows an interface to provide a default implementation for a method. This helps avoid breaking existing implementations when new methods are added to interfaces.
Example:
interface Animal {
default void sound() {
System.out.println("Animal makes a sound");
}
}
29. What is the difference between final and finally in Java?
- final: A keyword used to declare constants, prevent method overriding, and prevent class inheritance.
- finally: A block used to execute code after a try-catch block, regardless of whether an exception occurred.
30. What are Lambda expressions in Java?
Lambda expressions, introduced in Java 8, allow for a more concise way to express instances of functional interfaces (interfaces with a single abstract method). They enable functional programming features like passing behavior as arguments to methods.
Example:
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(name -> System.out.println(name));
31. What is the difference between throw and throws in Java?
- throw: It is used to throw an exception explicitly from within a method or block of code.
- throws: It is used in the method signature to declare that a method might throw one or more exceptions.
Example of throw:
void checkAge(int age) {
if (age < 18) {
throw new ArithmeticException("Underage");
}
}
Example of throws:
void readFile() throws IOException {
// code that may throw IOException
}
32. What is the String class in Java?
The String class in Java represents a sequence of characters. It is immutable, meaning once a String object is created, its value cannot be changed. Any modification creates a new String object.
Example:
String str = "Hello";
str = str + " World"; // A new String object is created
33. What is the purpose of clone() method in Java?
The clone() method in Java is used to create a copy of an object. The object being copied must implement the Cloneable interface; otherwise, it throws a CloneNotSupportedException.
Example:
class Person implements Cloneable {
String name;
Person(String name) {
this.name = name;
}
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
Person p1 = new Person("John");
Person p2 = (Person) p1.clone(); // p2 is a clone of p1
34. What is the super keyword in Java?
The super keyword refers to the superclass (parent class) of the current object. It is used to access members (fields and methods) of the parent class.
Example:
class Animal {
void sound() {
System.out.println("Animal makes a sound");
}
}
class Dog extends Animal {
void sound() {
super.sound(); // Calls the superclass method
System.out.println("Dog barks");
}
}
35. What is the this keyword in Java?
The this keyword is used to refer to the current object in a method or constructor. It helps differentiate between class fields and parameters with the same name.
Example:
class Person {
String name;
Person(String name) {
this.name = name; // 'this' refers to the instance variable
}
}
36. What are static variables and methods in Java?
- static variables: A static variable belongs to the class, rather than to instances of the class. All instances share the same value of the static variable.
- static methods: A static method can be called without creating an instance of the class. It can only access static variables and other static methods.
Example:
class Counter {
static int count = 0;
static void increment() {
count++;
}
}
37. What is the purpose of finalize() method in Java?
The finalize() method is called by the garbage collector before an object is destroyed. It gives an opportunity to clean up resources like closing files or database connections. However, it is not recommended for general resource management, as its execution is not guaranteed.
Example:
protected void finalize() throws Throwable {
System.out.println("Object is being destroyed");
}
38. What are the different types of exceptions in Java?
There are two main types of exceptions in Java:
- Checked exceptions: Exceptions that are checked at compile-time. The programmer is required to handle them using try-catch or
throws. Example: IOException, SQLException. - Unchecked exceptions: Exceptions that are not checked at compile-time, also known as runtime exceptions. Example: ArithmeticException, NullPointerException.
39. What is the purpose of the instanceof keyword in Java?
The instanceof keyword is used to check whether an object is an instance of a specific class or implements an interface. It is commonly used for type checking before performing casting.
Example:
String s = "Hello";
System.out.println(s instanceof String); // true
40. What is the difference between String, StringBuilder, and StringBuffer in Java?
- String: Immutable, meaning its value cannot be changed after creation.
- StringBuilder: Mutable, faster than StringBuffer for single-threaded applications.
- StringBuffer: Mutable, thread-safe, but slower than StringBuilder.
Example:
String s = "Hello";
StringBuilder sb = new StringBuilder("Hello");
StringBuffer sbf = new StringBuffer("Hello");
41. What is the use of the notify() and notifyAll() methods in Java?
The notify() method is used to wake up one thread that is waiting on an object’s monitor, while notifyAll() wakes up all the threads waiting on that object’s monitor. These methods are used in synchronization to control thread execution.
Example:
synchronized (obj) {
obj.notify(); // Wake up one waiting thread
}
42. What is the difference between wait() and sleep() in Java?
- wait(): Used in synchronized blocks to make a thread release the lock and go into a waiting state until notified.
- sleep(): Pauses the current thread for a specified time without releasing the lock.
Example:
synchronized (obj) {
obj.wait(); // The thread will wait until notified
}
Thread.sleep(1000); // Pauses the current thread for 1 second
43. What is the for-each loop in Java?
The for-each loop is a simplified version of the for loop. It is used to iterate over elements in an array or a collection. It is more readable and eliminates the need for an index variable.
Example:
int[] numbers = {1, 2, 3, 4};
for (int num : numbers) {
System.out.println(num);
}
44. What is the Collections class in Java?
The Collections class is a utility class in Java that provides static methods to operate on or return collections such as List, Set, Queue, etc. It includes methods like sorting, reversing, and searching.
Example:
List<String> list = Arrays.asList("Apple", "Banana", "Cherry");
Collections.sort(list); // Sorts the list in ascending order
45. What are the differences between ArrayList and Vector in Java?
- ArrayList: A dynamic array that grows as elements are added. It is not synchronized.
- Vector: Similar to ArrayList, but it is synchronized, making it thread-safe, though slower.
46. What is the purpose of Thread class in Java?
The Thread class represents a thread of execution in Java. It provides methods like start(), run(), and sleep() to control the execution of a thread.
Example:
class MyThread extends Thread {
public void run() {
System.out.println("Thread is running");
}
}
MyThread t = new MyThread();
t.start(); // Starts the thread
47. What is the volatile keyword in Java?
The volatile keyword is used to indicate that a variable’s value may be changed by multiple threads. It ensures that any read/write operation on the variable is directly performed on the main memory rather than a local thread cache.
Example:
private volatile boolean flag;
48. What is the difference between Array and ArrayList in Java?
- Array: A fixed-size data structure, meaning its size cannot be changed once created.
- ArrayList: A dynamic array that can grow or shrink in size as elements are added or removed.
49. What is a Runnable interface in Java?
The Runnable interface represents a task that can be executed by a thread. It has a single method, run(), which contains the code to be executed.
Example:
class MyRunnable implements Runnable {
public void run() {
System.out.println("Task is running");
}
}
Thread t = new Thread(new MyRunnable());
t.start();
50. What is an Enum in Java?
An enum is a special class in Java used to represent a group of constants. It can be used to define a set of predefined values, making the code more readable and type-safe.
Example:
enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }