300 Core Java Interview Questions
300 Must-Know Java Interview Questions
Core Java is the fundamental and essential part of Java programming language that includes concepts such as data types, arrays, operators, control structures, classes, objects, inheritance, polymorphism, abstraction, encapsulation, and exception handling. As part of the interview process for a Java developer role, candidates are often asked a wide range of questions to assess their knowledge and proficiency in these core concepts. These questions cover various topics such as basics of programming, OOPs concepts, multithreading, collections, and file handling. These 300 core Java interview questions aim to evaluate a candidate's problem-solving skills, critical thinking, and ability to write efficient and optimized code. By asking these questions, interviewers can assess the candidate's understanding of Java and their ability to apply it in real-world scenarios. A thorough understanding of these core concepts can help a candidate excel in Java development and pave the way for a successful career in the field.
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1 - What is Java?
Java is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It is known for its write once, run anywhere (WORA) philosophy, meaning that Java code can run on any platform that has a Java Virtual Machine (JVM).
2) What are the features of Java?
The main features of Java include platform independence, object-oriented nature, robustness, portability, security, and multithreading.
3) What is a platform?
A platform is a hardware or software environment in which a program runs. Java is a platform-independent language, meaning that Java code can run on any platform that has a JVM.
4) What is a JVM?
A JVM is a Java Virtual Machine, which interprets Java bytecode and executes it on the underlying hardware platform.
5) What is the difference between a Java compiler and a JVM?
A Java compiler converts the source code written in Java into bytecode, while a JVM interprets this bytecode and executes it on the underlying hardware platform.
6) What is bytecode?
Bytecode is a binary format that is the result of compiling Java source code. It can be executed by a JVM.
7) What is the syntax for writing a simple Java program?
The syntax for writing a simple Java program is:
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
8) What is a package in Java?
A package is a grouping mechanism for related Java classes and interfaces. It helps to organize code and avoid naming conflicts.
9) What is the difference between import and static import in Java?
The import statement is used to import a specific class, interface, or package, while the static import statement allows you to access static members (variables and methods) of a class directly without using the class name.
10) What is the difference between a class and an object in Java?
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A class is a blueprint for creating objects, while an object is an instance of a class that contains its own unique state and behavior.
11) What is inheritance in Java?
Inheritance is a mechanism in Java where one class acquires the properties (methods and variables) of another class.
12) What is polymorphism in Java?
Polymorphism is the ability of an object to take on many forms. In Java, it is achieved through method overloading and method overriding.
13) What is encapsulation in Java?
Encapsulation is the process of hiding the internal workings of an object and only allowing access through a public interface. It helps to prevent external code from directly accessing or modifying the internal state of an object.
14) What is abstraction in Java?
Abstraction is the process of hiding implementation details and only exposing essential features to the user. This helps to manage complexity and improve code readability.
15) What is a constructor in Java?
A constructor is a special method used to initialize objects of a class. It has the same name as the class and is called automatically when an object is created.
16) What is the difference between a constructor and a method in Java?
A constructor is used to initialize an object while a method is used to perform some operation on a given object.
17) What is the difference between a class variable and an instance variable?
A class variable (also known as a static variable) is shared by all instances of a class, while an instance variable is unique for each instance of a class.
18) What is a static method in Java?
A static method belongs to a class rather than an individual instance of a class. It can be called directly using the class name without creating an object.
19) What is the use of ‘this’ keyword in Java?
The ‘this’ keyword refers to the current object within an instance method or constructor. It is used to differentiate between class variables and instance variables with the same name.
20) What is a wrapper class in Java?
A wrapper class is used to encapsulate primitive data types and provide methods to manipulate them as objects. Examples include Integer, Double, and Boolean classes.
21) What is an interface in Java?
An interface is a collection of abstract methods and constants that can be implemented by a class. It is used to achieve abstraction and multiple inheritance in Java.
22) What is the difference between an interface and an abstract class?
An interface can only have abstract methods and constants, while an abstract class can have both abstract and concrete methods. A class can implement multiple interfaces but can only inherit from one abstract class.
23) What is a package-private access modifier in Java?
Package-private is the default access modifier in Java. It allows access to classes, methods, and variables within the same package but not outside of it.
24) What is the difference between a deep copy and a shallow copy in Java?
A deep copy creates a new object with the same values as the original object, while a shallow copy creates a new object that references the same values as the original object. In other words, changes made to the shallow copy will affect the original object, while changes made to a deep copy will not.
25) What is a try-catch block in Java?
A try-catch block is used for exception handling in Java. Code that may throw an exception is written within the try block, and the catch block is used to handle the exception if it occurs.
26) What is the difference between checked and unchecked exceptions in Java?
Checked exceptions are checked at compile-time, which means they must be handled by the programmer, while unchecked exceptions are not checked at compile-time and can be handled at runtime.
27) What is a finally block in Java?
A finally block is used to execute code that must be executed regardless of whether an exception is thrown or not. It is often used to release resources or close files.
28) What is a deadlock in multithreading?
A deadlock occurs when two or more threads are waiting for each other to release resources, causing all threads involved to become stuck and unable to proceed.
29) What is synchronization in Java?
Synchronization is the process of controlling access to shared resources among multiple threads to prevent data corruption.
30) What is the difference between HashMap and Hashtable in Java?
HashMap is not synchronized, while Hashtable is synchronized. Additionally, null values are allowed in HashMap, but not in Hashtable.
31) What is Collection framework in Java?
The Collection framework is a set of interfaces, implementations, and algorithms used to represent and manipulate collections of objects in Java.
32) What is the difference between ArrayList and LinkedList in Java?
ArrayList is implemented using an array, while LinkedList is implemented using a doubly-linked list. This means that ArrayList is more efficient for random access operations, while LinkedList is more efficient for insertions and deletions.
33) What is the difference between a string and a StringBuilder in Java?
A string object is immutable, meaning it cannot be modified once it is created. A StringBuilder object, on the other hand, is mutable and can be modified without creating a new object.
34) What is the difference between a StringBuilder and a StringBuffer in Java?
StringBuilder is not synchronized, while StringBuffer is synchronized. This makes StringBuilder more efficient, but StringBuffer is safer for use in multithreaded applications.
35) What is the use of hashcode() and equals() methods in Java?
HashCode() is used to generate a unique integer value for an object, and equals() is used to compare two objects for equality, typically based on their hash codes.
36) What is the difference between a deep clone and a shallow clone in Java?
A deep clone creates a new object with the same values as the original object, while a shallow clone creates a new object that references the same values as the original object. In other words, changes made to the shallow clone will affect the original object, while changes made to a deep clone will not.
37) What are annotations in Java?
Annotations are markers that provide additional information about classes, methods, and variables to the compiler. They can be used for documentation, code optimization, and runtime processing.
38) What is the difference between an interface and a class in Java?
An interface only contains abstract methods and constants, while a class can contain both abstract and concrete methods. A class can also have instance variables, while an interface cannot.
39) What is method overloading in Java?
Method overloading is when multiple methods in a class have the same name but different parameters. This allows you to reuse the same method name for operations with different input types or number of parameters.
40) What is method overriding in Java?
Method overriding is when a subclass overrides the implementation of a method defined in its superclass. This allows the subclass to provide its own implementation of the method.
41) What is the garbage collector in Java?
The garbage collector is a built-in mechanism in Java that automatically frees up memory by destroying objects that are no longer referenced by the program.
42) What is the String Pool in Java?
The String Pool is a special area of the heap memory that is used to store String literals. This allows the program to reuse String objects, reducing the memory usage and improving performance.
43) What is the difference between an abstract class and an interface in Java?
An abstract class can contain both abstract and concrete methods, while an interface can only contain abstract methods. Additionally, a class can implement multiple interfaces but can only inherit from one abstract class.
44) What is the difference between a HashSet and a TreeSet in Java?
HashSet is implemented using a hash table, while TreeSet is implemented using a balanced binary tree. This means that HashSet has faster insertion and lookup times, but TreeSet is sorted and iterates over its elements in a specific order.
45) What is the difference between a final, finally, and finalize keyword in Java?
The final keyword is used to declare a constant variable, method, or class. The finally block is used to execute code that must be executed regardless of whether an exception is thrown or not. The finalize() method is called before an object is destroyed by the garbage collector.
46) What is reflection in Java?
Reflection is a feature in Java that allows you to inspect and modify the structure and behavior of classes, interfaces, methods, and variables at runtime.
47) What is the difference between ArrayList and Vector in Java?
ArrayList is not synchronized, while Vector is synchronized. Additionally, Vector is thread-safe, meaning that it can be used in multithreaded environments, while ArrayList is not.
48) What are the types of exceptions in Java?
There are two types of exceptions in Java: checked exceptions and unchecked exceptions. Checked exceptions must be handled by the programmer, while unchecked exceptions can be handled at runtime.
49) What is the difference between public, private, protected, and default access modifiers in Java?
Public is the most accessible, allowing access from any class or package. Private is the least accessible, only allowing access from within the same class. Protected allows access from the same package and subclasses, while default (package-private) allows access from the same package only.
50) What is an enum in Java?
An enum is a special data type in Java used to define a set of constants. Enums can also have methods and constructors like regular classes.
51) What is the difference between a static member and an instance member in Java?
A static member (variable or method) belongs to the class, while an instance member belongs to a specific object of the class. This means static members can be accessed without creating an object, while instance members can only be accessed through an object.
52) What is the difference between a byte stream and a character stream in Java?
A byte stream is used for input and output of binary data, while a character stream is used for input and output of character data. Character streams use a character encoding such as ASCII or Unicode, while byte streams do not.
53) What is the difference between a File and a Directory in Java?
A File represents a single file on the file system, while a Directory represents a folder or directory that can contain multiple files and subdirectories.
54) What is try-with-resources in Java?
Try-with-resources is a feature introduced in Java 7 that allows you to automatically close resources after they are used within the try block, eliminating the need for a finally block.
55) What is the difference between a constructor and a factory method in Java?
A constructor is used to create objects of a class, while a factory method is a static method used to create objects of a class without specifying the exact class of the object.
56) What is a stream in Java?
A stream is a sequence of data that can be processed one element at a time. It allows for efficient and parallel processing of large amounts of data.
57) What is the difference between an iterator and a list iterator in Java?
An iterator allows you to iterate over a collection in a forward direction, while a list iterator allows you to iterate over a list in both directions and modify the list during iteration.
58) What is the difference between a reader and an input stream in Java?
A reader is used for reading character streams, while an input stream is used for reading byte streams. Character streams use a character encoding such as ASCII or Unicode, while byte streams do not.
59) What is a daemon thread in Java?
A daemon thread is a thread that runs in the background and can be terminated automatically when the main thread terminates. They are typically used for background tasks such as garbage collection.
60) What is the difference between a transient and a volatile keyword in Java?
Both transient and volatile keywords are used in multithreaded programming. Transient is used to indicate that a variable should not be serialized, while volatile is used to indicate that a variable's value may be modified by multiple threads.
61) What is the difference between a class and an object in Java?
A class is a blueprint for creating objects, while an object is an instance of a class that contains its own unique state and behavior.
62) What is a lambda expression in Java?
A lambda expression is a concise way of representing an anonymous method. It is used to make code more readable and maintainable.
63) What is the difference between a stack and a heap in Java?
A stack is used for storing method calls and local variables, while a heap is used for storing objects.
64) What is the difference between a weak reference and a soft reference in Java?
A weak reference allows an object to be collected by the garbage collector if it is only referenced by weak references. A soft reference is like a weak reference but is only collected by the garbage collector when memory is low.
65) What is the difference between a thread-safe and a non-thread-safe class in Java?
A thread-safe class is designed to be used in a multithreaded environment, meaning that it can be accessed by multiple threads simultaneously without causing any issues. A non-thread-safe class, on the other hand, may have issues when accessed by multiple threads at the same time.
66) What are the different types of class loaders in Java?
The three types of class loaders in Java are Bootstrap class loader, Extension class loader, and System class loader.
67) What is the difference between an applet and an application in Java?
An applet is a small program that runs within a web browser, while an application is a standalone program that runs on the local machine.
68) What is the difference between a deep copy and a shallow copy in Java?
A deep copy creates a new object with the same values as the original object, while a shallow copy creates a new object that references the same values as the original object. In other words, changes made to the shallow copy will affect the original object, while changes made to a deep copy will not.
69) What is the difference between a break and a continue statement in Java?
A break statement stops the execution of the loop and continues with the code after the loop, while a continue statement skips the current iteration and continues with the next iteration of the loop.
70) What is enum with constructor in Java?
An enum with constructor allows you to specify additional properties for each constant value in the enum.
71) What is the difference between a wait() and a sleep() method in Java?
The wait() method is used to pause the execution of a thread and wait for a signal from another thread, while the sleep() method is used to pause the execution of a thread for a specified amount of time.
72) What is the difference between an interface and a class in Java?
An interface can only contain abstract methods and constants, while a class can contain both abstract and concrete methods. A class can also have instance variables, while an interface cannot.
73) What is the use of the ‘implements’ keyword in Java?
The ‘implements’ keyword is used to indicate that a class is implementing an interface. It is used in the class declaration before the name of the interface being implemented.
74) What is a deadlock in Java?
A deadlock occurs when two or more threads are waiting for each other to release resources, causing all threads involved to become stuck and unable to proceed.
75) What is the difference between a static and a non-static nested class in Java?
A static nested class is a static member of the outer class, meaning it can be accessed without creating an instance of the outer class. A non-static nested class (also known as inner class) is a member of the outer class and can only be accessed through an instance of the outer class.
76) What is the difference between a deep clone and a shallow clone in Java?
A deep clone creates a new object with the same values as the original object, while a shallow clone creates a new object that references the same values as the original object. In other words, changes made to the shallow clone will affect the original object, while changes made to a deep clone will not.
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