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Core Java is a programming language and also a platform for computing. Core Java was first released by Sun Microsystems in 1995. There are lots of applications and websites that will not work unless Java is installed in them. Each day there are more and more applications created using Java. It is a very simple, powerful and robust programming language which provides a fast, secure, and reliable platform to work on. Java is being used everywhere, from laptops to data centers, game consoles to scientific supercomputers, and cell phones to the Internet.
Due to its ease of use and simplicity, Java is used almost everywhere in the IT sector. This means that the professionals who are skilled in Java are always required. If you are someone who is likely to attend a job interview that would test you on your Java skills, then please go through the list of Java interview questions and answers provided herein. You may also refer to them just to add on to your knowledge, but for dedicated knowledge enhancement, don't forget to check out JanBask’s Java online course.
For a Core Java enthusiast who is looking to attend a Core Java role interview anytime soon, here are the most popular interview questions and answers that might come your way. We have tried to include the top frequently asked Java interview questions as well as their answers to help freshers and experienced professionals with their job interviews. Here are some of the Java interview questions for experienced and freshers.
Ans:-
|
JDK |
JRE |
JVM |
|
JDK stands for Java Development Kit and is the tool used to compile, package and document Java programs. |
JRE stands for Java Runtime Environment and is a runtime environment that is a prerequisite to run java byte-code. |
JVM stands for Java Virtual Machine and is a specification that facilitates a run-time environment in which Java’s bytecode can be implemented. |
Ans:- Java is platform-independent because it provides the feasibility to run and compile the program in one platform and execute the program on any other platform of your choice.
Ans:- Yes, it would be correct to say that Java is not 100% Object-oriented because it utilizes eight kinds of primitive data types and they are - Boolean, byte, char, int, float, double, long, short. These data types are not objects.
Ans:- A constructor is a block of code that is used in a Java environment to initialize an object. It carries the same name that you had given to the class. A Java constructor is automatically called when an object is created.
Ans:- A constructor constructs the value, by providing data for the object. It is a special type of method that is used to initialize the object. The constructor has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. A method can be invoked using the dot operator and has its own name, and a return type. No, declaring the constructor as final is not possible.
Ans:- A class, in the environment of Core Java, are some templates that are used for object creation, and to define certain object data types and their methods.
Ans:-
Ans:- A final class is a constant value of a final variable. Extending A final class is not possible ie., final class may not be subclassed. A final method cannot be overridden when its class is inherited.
Ans:- The classes that “wrap” the primitive data type into an object of that class are called wrapper classes.
Here are the Core Java interview questions and answers for experienced professionals who have gained some experience and are looking to upgrade their career.
Ans:- Abstract serves as a template. In order to use an abstract class, it must be extended or subclassed. An abstract class may not be instantiated. Any class with an abstract method is abstract itself, hence is declared as such.
Ans:- Synchronization is the method to control the access of multiple threads to shared resources, with respect to multithreading. One thread can modify a shared variable when not in synchronization even when another thread is in the process of using or updating the same shared variable. This can lead to significant errors, which you shall learn more about as you proceed towards advanced java programming.
Ans:- The main purpose of garbage collection is to analyze a program, identify and discard the objects that are no longer needed. It is helpful as the resources can be reclaimed and reused. A Core Java object is subjected to garbage collection when the object is not reachable to the program in which it is used.
Ans:- The thread can be implemented by using runnable interface Inheriting from the Thread class. The use of the Runnable interface is more advantageous because when going for multiple inheritances, the only interface can help.
Ans:- When the main() method is declared as private, the program compiles but during runtime it shows “main() method not public.” Message. When the static modifier is removed from the signature of the main() method, the Program compiles but at runtime throws an error “NoSuchMethodError”.
Ans:- Unlike in C/C++ where the first element by default is the program name, the string array in main() has no element, the String array is empty. When you enter the wave of coding, you shall notice such differences.
Ans:- A Servlet in Core Java refers to the server-side technologies which are used to extend the competence of web servers by giving it support for a dynamic response as well as data persistence.
Ans:- Here is the life-cycle of a Core Java Servlet-
Ans:- Request Dispatcher is a Core Java interface that is used to forward a request to some other resource which can be HTML, JSP or any other Core Java servlet within the same application.
Ans:- Follow the given steps to connect to a database in Core java-
Ans:- JDBC Driver in Java refers to a software component that allows Java applications to relate with the database.
There are 4 types of JDBC drivers in Java-
Ans: Java interview questions like this may appear difficult, but the concept is simple. The transient keyword in Java is a field modifier used to indicate that a field should not be included when the object is serialized into a byte stream. Fields marked as transient are excluded from the persistent storage and are particularly useful when certain fields contain sensitive or unnecessary data that don’t need to be saved during serialization. This ensures data privacy and reduces unnecessary storage.
Ans: Basic Java interview questions like this one are a must-know for beginner developers who might want to try out Polymorphism. Method overloading in Java allows programmers to define two or more methods within the same class sharing the same name, as long as their parameter lists are different. This makes it easy for Java interview questions for freshers to expand APIs, improving code usability by allowing multiple ways to call a method with different argument combinations. The expansion to the JLS has a more detailed explanation.
Ans: When discussing basic Java interview questions, it's particularly important to understand method overriding in Java. It occurs when a subclass provides a specific implementation for a method that is already defined in its superclass. This allows the subclass to customize or extend the behavior of the inherited method. When the overridden method is called on an instance of the subclass, the subclass's implementation is executed instead of its superclass's.
Ans: Questions in Core Java viva questions may involve operator usage. In Java, the == operator is used for reference comparison. It checks whether two object references point to the same memory location. On the other hand, the .equals() method, when overridden by a class, compares the content or value of objects based on the class's implementation. This allows for custom comparison logic.
Ans: In Core Java interview questions experienced, the try-with-resources statement is significant. It is used to automatically manage and close resources that need explicit closure, such as files, sockets, or database connections. It simplifies resource management and reduces the risk of resource leaks and exceptions. The resources declared within the try-with-resources block are automatically closed when the block exits.
Ans: Design patterns might be explored in interview questions on Java. Method chaining in Core Java is a design pattern where multiple methods are called on an object in a single line, with each method returning the modified object. This pattern provides a way to produce code that is readable and concise since methods can be called in a chain, avoiding the need for intermediate variables.
Ans: When answering Java basic interview questions, it's vital to mention multiple inheritance. Core Java supports multiple inheritance using interfaces. A class can implement multiple interfaces, allowing it to inherit method signatures (a.k.a., contractual obligations) from multiple sources. This helps evade class inheritance’s associated ambiguity and complexities, since a class can only inherit from one superclass.
Ans: Core Java interview questions and answers often delve into synchronization. In Core Java, the volatile keyword is used to declare a variable as volatile, ensuring that read and write operations of the variable are atomic and immediately visible to all threads. It prohibits individual threads from caching the variable’s value, making it a good fit for variables accessed concurrently by multiple threads.
Ans: Core Java interview questions often involve data structures in Java. The ArrayList and LinkedList both store collections of objects, but they have different underlying data structures and characteristics. The ArrayList is backed by an array, so it’s efficient for random access (get and set operations), but less so for insertions and deletions in the middle of the list. The LinkedList is implemented as a doubly-linked list, so it’s efficient for insertions and deletions in the middle of the list, but less so for random access.
Ans: Core Java interview questions like these are important for any Core Java developer. The hashCode() method in Core Java is used to calculate a hash code value for an object, which is used by data structures like HashSet and HashMap to store and retrieve objects. The default implementation in the Object class returns the memory address of the object in an integer format, but classes can override the method to provide custom hash code implementation.
Ans: Reflection in Core Java is a powerful and advanced feature that allows you to inspect and manipulate classes at runtime even though classes (fields,methods,constructors etc.) are not identified at compile time. The ability to reflect on the code is pivotal for several advanced applications, running frameworks and tools which require to “get information about followed analysis and execution of methods”.
Ans: The Comparable interface is used for objects of each class that need to be ordered with a single logical ordering, through the use of the compareTo method. By contrast, the Comparator interface is used to define multiple possible orderings for objects by external implementations of the methods, allowing for the flexibility of custom ordering to be provided without modifying the actual class of the object. These concepts forms a critical part of a Core
Java online course that focuses on the data structures, particularly that of sorting algorithms.
Ans: Generics provide a much broader definition of objects than the Object class, which thereby permits a much broader range of functions to be performed on the data. By adding stability to the code, it will make more of programmers' bugs become discoverable at the compile time, rather than the execution time. The programmer may possibly, with a single method declaration, specify a set of related methods; with a single class declaration, specify with a set of related types.
Ans: Exception handling in Core Java is managed through the use of try, catch, finally blocks, and the throws keyword. The try block encloses the code that might throw an exception, while the catch block is used to handle the exception. The finally block contains code that is always executed after the try and catch blocks, regardless of whether an exception was thrown or caught. The throws keyword is used in method signatures to indicate that a method might throw one or more exceptions. Proper exception handling is crucial for building robust Java applications and is a common focus in both Core Java learning online platforms and Core Java courses.
Q35). What are the different types of memory areas for JVM?
Ans: JVM is divided into following types of memory areas:
a) Heap
b) Stack
c) Method Area
d) Program Counter (PC) Register
e) Native Method Stack
Heap – This is used for dynamic memory allocation for all the objects and JRE classes.
Stack – For every thread, there is one runtime stack which is created. This is used in a method to store local variables when defining a method.
Method Area – This one stores per-class structures such as the run-time constant pool, field and method data, and the code for methods.
Program Counter (PC) Register – Core Java Virtual Machine instruction uses this register.
Native Method Stack – To execute a native method this stack is used.
To optimize the Core application’s performance, you need to understand these areas very well and this is covered in Java interview questions for experienced professionals of our Java courses.
Ans: Fail-fast iterators immediately throw a ConcurrentModificationException if there is a structural modification of the collection while iterating over it except through the iterator’s own remove method. Fail-safe iterators don’t throw any exceptions if a collection is structurally modified while iterating over it because they work on a clone of the collection, not on the original collection itself. This is one of the most important and frequently asked questions in Core Java core interviews. This is generally asked in Core Java online courses at the basic level also.
Ans: The Double-Checked Locking idiom reduces the overhead of acquiring a lock by first testing the locking criterion (the lock is actually acquired for real if the locking criterion indicates the lock’s presence). This idiom is often used for lazily initializing the singleton object. It must be implemented with care in Core Java to be thread-safe. Generally it is implemented using volatile variables. It is an advanced concurrency pattern and covered in detail in many of the Core Java training programs which focuses on design patterns and concurrency.
Ans: The Core Java 9 Module System (Project Jigsaw) introduces a modular approach to Java application development. Modules make it easier for developers to organize their applications, with explicit dependencies where a module can explicitly declare itself to depend on specific other modules. This approach to scalability — turning the company’s myriad of Core Java projects into modules — is much more practical than creating a giant monolithic application. Modules make it easier to manage and turn large applications into a few much smaller applications where the only pieces of a framework that are used are the pieces actually required. This is one of the more significant topics in Core Java learning online because it significantly changes the way Core Java applications are organized and so should be designed and developed for much better organization and much improved runtime performance.
Ans: The default keyword in Core Java interfaces enables developers to provide method implementations directly within the body of the interface. This feature, introduced in Java 8, effectively allows interfaces to be extended without having to revise every implementing class. The benefit of this is that it permits developers to expand large codebases of interfaces without fear of breaking the old code. Default methods are a common topic of interest in Core Java courses and Core Java training, as they provide a new sense of flexibility and extensibility to what has always been a rigid outline of APIs.
Ans: The Core Java Stream API gives developers a way to process viewless collections of objects in a functional manner. The primary advantage of using the Stream API is the ease with which it can be parallelized, allowing for significant performance improvements in a multi-core system. By employing the parallelStream method, the developer can effortlessly transition between sequential and parallel execution modes without modifying the original code structure; the simple act of changing the method call makes the Stream API an effective tool for efficiently handling complex data processing, a theme that is emphasized in Core Java online courses which focus on performance optimization.
Ans: Type erasure is a process that is applied by the Core Java compiler in order to apply type constraints at compile time and then remove the generic type information replacing it with non-generic upper bounds (such as Object) and raw types. This means that the information that a generic type used is not available at runtime. The process was introduced to ensure binary backward compatibility with pre-existing versions of Core Java that did not have generics.
Ans: The volatile keyword in Core Java is used to ensure visibility of changes to variables across threads. When a variable is declared volatile, any write to that variable is immediately made visible to other threads. This prevents threads from caching the variables value, ensuring that each pthread has the most up-to-date value of a variable. It's important in concurrent programming as it helps avoid inconsistent states and is a fundamental concept covered in a Core Java course, particularly as it pertains to Core Java’s memory model and how volatile variables impact thread communication.
Ans: Core Java memory leaks occur when objects are no longer being used, but cannot be garbage collected because they are still being referenced. Over time, memory leaks can result in an application using an excessive amount of memory, causing performance degradation or crashes. Avoiding memory leaks involves being careful about how long a reference to an object is held, using weak references where appropriate, and always dereferencing an object when it’s no longer needed.
Ans: The @FunctionalInterface annotation in Core Java is used to indicate that an interface type declaration is intended to be a functional interface, as defined by the Core Java Language Specification. Function interfaces have exactly one abstract method. This is not required, but helps the compiler to understand the intention of the interface. That being said, this area is found to be usually heavily covered in a modern Core Java course, especially those introducing Lambda expressions or Core Java programming with regard to functional programming.
Ans: The @Override is used in Core Java to indicate that a method you are overriding is intended to actually be an override. That way you can be sure if you’re reading the code of the overriding method because the IDE and compiler can tell you for sure through the presence of the @Override annotation. This will allow for checking errors like misspelling the method name or not correctly matching the method signature, so the programmer can let the compiler find the error.
Ans: To achieve thread-safe singleton in Core Java, you can use Bill Pugh Singleton implementation or an enum based singleton. Bill Pugh uses a static inner helper class to hold the singleton instance, creating it only when requested and leveraging Core Java's class loading mechanism for thread safety. Enum based singleton ensures a single instance is created through Core Java's guarantee that any enum constant is instantiated only once in a Core Java program. These design patterns are fundamental to application efficiency and scalability, integral topics in advanced Core Java training.
Ans: Checked exceptions are checked by the compiler to ensure they are caught or declared in the method's throws clause, aiding in notifying the developer of error scenarios that may arise while the code is being written. Unchecked exceptions are often the result of programming defects and are usually not recoverable, breaking the normal flow of the application. They do not require to be declared on a method's throws clause. A strong understanding of this distinction, and proper usage, is expected; and can be reinforced through reinforcement of key concepts while you go through Core Java online course.
Ans: The Diamond Problem is a multiple inheritance problem that occurs when a class tries to inherit from two classes that have a method with the same signature. Core Java addresses this issue via interfaces, rather than by allowing multiple inheritance of classes. With default methods in interfaces, if a class implements two interfaces that have a default method with the same signature, the class is required to override the method to resolve the ambiguity. This is an important part of the Core Java online course curriculum for beginners, as it illustrates how Core Java deals with object-oriented design.
Ans: The assert keyword in Core Java is used for debugging, to test assumptions about the state of a program. An assertion will throw an AssertionError if the specified condition is false. Assertions are typically used during development and in testing to find and fix errors early in the life cycle. However, it is best practice to handle exceptions for error handling in production code. The use of assertions is a subject that is covered in detail in Core Java courses as developers learn to debug their applications effectively.
Ans: Method references in Core Java 8 are shorthand notations for lambda expressions that call existing methods. With method references, you can make your code more readable and concise by eliminating the need to include boilerplate code that just calls a method without modifying its arguments. Method references refer to static methods, instance methods of a specific object, constructors, or instance methods of an arbitrary object of a particular type.
Ans: In Core Java, the transient keyword prevents fields from being serialized. This protects sensitive or otherwise useless data from being serialized along with the rest of an object. The volatile keyword does not directly affect serialization, but it dictates that all threads running read the variable at the same time. In other words, volatile is used to ensure that any change to a variable is picked up by other threads immediately, making it important when using shared variables in a multi-threaded application. Both of these concepts are important to understand, and both feature heavily in a proper Core Java learning online course.
Ans: The NavigableSet interface extends the SortedSet interface to provide navigation methods for retrieving elements based on their position in the set. You can, for example, retrieve elements greater than or equal to, less than or equal to, or within a certain range. This interface is a particularly powerful way to work with ordered sets of data.
Ans: The forEach method, added in Core Java 8, gives a new, much improved way, to iterating collections using lambda expressions or method references. This provides a more readable, and concise way to iterate the elements of a collection. forEach is defined in the Iterable interface so that you can perform a sequence of elements on every Collection. As a result this new method gives a way to do functional-style operations on every element of a collection.
Ans: In Core Java, `static` methods belong to the class itself and can be called without creating an instance of the class. They can only access static data members and other static methods. Instance methods, on the other hand, require an object of the class to be created and can access both instance and static data members and methods. This distinction is critical for understanding object-oriented programming principles in Core Java, a common area of focus in core java interview questions.
Ans: The `interface` keyword in Core Java is used to define an abstract type that specifies a set of method signatures without providing their implementations. Classes can implement interfaces, thereby agreeing to provide implementations for the abstract methods declared within them. Interfaces support multiple inheritances in Core Java and are a cornerstone of Java's approach to polymorphism and abstraction, topics frequently covered in java interview questions for freshers.
Ans: The `super` keyword in Core Java is used within a subclass to refer to the superclass's variables, methods, and constructors. It is particularly useful for accessing and invoking the superclass's constructor methods, overriding methods, or when the names of global variables are hidden by local variables. Understanding `super` is fundamental for mastering Java's inheritance features, making it a popular topic in basic Core java interview questions.
Ans: In Core Java, a `final` variable can only be assigned once, either at the time of declaration or within the constructor. Once assigned, its value cannot be modified, making it effectively a constant. This immutability concept is critical for creating secure, thread-safe code, and is a concept that is often tested in Core Java viva questions.
Ans: Core Java packages are used to group related classes and interfaces, providing a controlled namespace mechanism that helps avoid name clashes. Packages can be imported into other Core Java files, allowing the classes and interfaces they contain to be easily reused. Understanding how to create and use packages is a basic yet essential part of Java programming, often discussed in core java interview questions.
Ans: The `public static void main(String[] args)` method is the entry point for a Core Java application. The `public` modifier makes it accessible from outside its class, `static` allows it to be invoked without creating an instance of its class, `void` indicates it doesn't return any value, and `String[] args` is an array that stores command-line arguments. This method's understanding is essential for all Core Java developers, making it a staple in Core Java interview questions.
Ans: In Core Java, `try-catch-finally` blocks are used for exception handling. Code that might throw an exception is placed in the `try` block. The `catch` block is used to handle the exception, and the `finally` block contains code that is executed after the `try` and `catch` blocks, regardless of whether an exception was thrown or caught. The `finally` block is typically used for resource cleanup activities. This concept is foundational for robust Core Java application development, frequently explored in Core Java interview questions for freshers.
Ans: In Core Java, `==` is a reference comparison operator that compares whether two references point to the same object in memory. In contrast, the `.equals()` method is intended for content comparison, checking if two objects have the same value. Not all classes override `.equals()`, so its behavior can vary unless specifically implemented. This distinction is crucial for understanding Core Java's comparison mechanisms, a common subject of basic Core java interview questions.
Ans: Garbage collection in Core Java is an automatic memory management process that frees up memory allocated to objects that are no longer in use, preventing memory leaks. The garbage collector determines which objects are not referenced by any running state or other objects and removes them to free up space. While developers do not control the garbage collection process directly, understanding its workings is important for optimizing application performance, a topic often included in Core Java viva questions.
Ans: Wrapper classes in Core Java provide a way to use primitive data types (int, char, etc.) as objects. The eight primitive data types have corresponding wrapper classes in the `java.lang` package (e.g., `Integer` for `int`, `Character` for `char`, etc.). These classes are essential for working with collections, which can only store objects. Understanding wrapper classes is fundamental for working with Core Java's object-oriented features, often covered in core java interview questions.
Here are the top Java interview questions and answers for experienced senior developers you should study.
Ans:- The applet is a Core Java program that is designed for transmitting the Core Java code over the internet. Ttion works automatically by Java-enabled Web Browser. The applet can respond to the user input immediately as it is dynamically programmed.
Ans:- Numeric promotions of a numeric operator are used for the conversion of the operands into a common type. In order to perform calculations easily, numeric promotion, conversion is performed. It is the conversion of a smaller numeric type to a larger numeric type so that integer and floating-point operations can be performed over it. Here byte, char, and short values are converted to int values. The int values are converted to long values, and the long and float values are converted to double values.
Ans:- On multi-core systems, false sharing is one of the well-known performance issues. Here each process has its local cache. When threads on a different processor, modify variables false sharing occurs, that resides on the same cache line as shown in the following images
As the thread may access different global variables completely, false sharing can be hard to detect. Finding this interesting? Well knowing everything about Java is essential, but the actual truth is, that you can never be a master, and there shall always be scope to learn more Java developer skills.
Ans:- Equals and hashcode methods are to be implemented In order to use any object as Key in HashMap, in Java.
Ans:- Core Java classes whose objects cannot be modified once they are created are known as Immutable classes. Any modification of Immutable object results formation of the new object.
Ans:- The only difference between StringBuffer and StringBuilder is that StringBuffer methods are synchronized while StringBuilder is not synchronized. StringBuilder in Java was introduced in Java 5.
Ans:- Abstract Factory provides one more level of abstraction. Consider different factories each extended from an Abstract Factory and responsible for the creation of different hierarchies of objects based on the type of factory. E.g. Abstract Factory extended by Automobile Factory, User Factory, Role Factory, etc. Each individual factory would be responsible for the creation of objects in that genre.
Ans:- We are going to share the Difference between the JAR & WAR File:
|
JAR FILES |
WAR FILES |
|
Full form of JAR files is Java Archive Files. |
Full form of WAR files is Web Archive Files. |
|
Aggregating many files into one is allowed in JAR files |
XML, Java classes, and JavaServer Pages are stored in WAR |
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The JAR is usually used to hold Java classes in a library. |
Mainly used for Web Application purposes.` |
Ans:- Just-In-Time(JIT) compiler is used to improve the performance. JIT compiles parts of the bytecode that has similar functionality which in turn reduces the amount of time needed for compilation. The term “compiler” here refers to a translator from the instruction set of a Core Java virtual machine (JVM) to the instruction set of a specific CPU. It's essential to know how long it shall take to learn Java, so you know on which Core Java basic interview questions to focus more.
Ans:- Multi-catch blocks make the code shorter and cleaner when every catch block has a similar code. We can catch multiple exceptions in a single catch block using this feature.
Ans:- The collection of related classes and interfaces grouped together is Package in Core Java. It enables developers to easily modularize codes. Packages help in providing easier access control on the code.
Ans:- Final is a special keyword in Core Java being used as a non-access modifier. It is used in various contexts like Final variables, Final Methods, and Final Class. Final class refers to a class being declared as final in Core Java.
Ans:- Core Java has four types of inheritance namely:
Ans:- The most important feature of Core Java 8 is its functional interface and static methods. When adding a functional interface to the object-oriented interface, programming becomes less complicated and more efficient. To know about the more Core Java 8 important features, you can opt for Java projects for beginners, and understand better through practical approach.
Ans:- Classpath is related to Core Java executables, used to locate class files. Path, on the other hand, exists in an operating system used to locate executables.
Ans:- Both serialization and deserialization are crucial to stream handling in Core Java. Through serialization, you convert an object into a byte stream, and through deserialization, you simply undo the process.
Ans:- A ternary operator is a conditional operator in Core Java used to decide what values to be assigned to a variable.
Ans:- Platform independence is when you design a program that runs on any operating system making things more accessible. This is the reason why Java is ahead of other programming languages.
Ans:- As of Sep 2023, Java 21 is the latest release. Also, it is the next long-term support version (LTS) after Core Java 11.
These are some of the most asked interview questions you should prepare for your next Core Java Interview. Read our next section for the most essential interview tips.
Ans:- C++ is not platform-independent; the main concept behind it, “write once, compile anywhere.” Since the byte code generated by the Core Java compiler is platform-independent, it can run on any machine, Java programs are written once and run everywhere.
Most of the high-level languages are compatible with C++ but with Java they are incompatible.
Ans:- Some of the significant features of Core Java Programming Language are:
Ans:- Here is the difference:
1. HashSet: It is implemented through a hash table and permits the null object.
TreeSet: implements SortedSet Interface that uses trees for storing data.
2. HashSet: It permits the null object.
TreeSet: It does not allow null objects.
3. HashSet: It does not maintain elements in an ordered way.
TreeSet: It maintains elements in a sorted way.
Ans:- It is a collection of related classes and interfaces which are bundled together. It allows developers to easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused.
Ans:- It is a runtime API used to inspect and change the behavior of methods, classes, and interfaces. Core Java Reflection is a powerful tool that allows you to analyze classes, interfaces, fields, and methods during runtime without knowing what they are called at compile time. It also helps in creating new objects, call methods, and getting/set field values.
Ans:- A collection of Strings stored in heap memory is called Core Java String Pool. Whenever a new object is formed, first of all, the String pool identifies whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable; otherwise, a new object is created.
Ans:- A map in Java is an interface of the Util package that maps unique keys to values. The Map interface is not a subset of the main Collection interface and hence it performs a little differently from the other collection types.
Ans:- Inheritance in Core Java allows for the reuse of code
Ans:- Java is dynamic in nature, it is designed to adapt to an evolving environment. It includes a large amount of runtime information that is used to solve access to objects in real-time.
Ans:- JSP stands for Java Servlet Page. The JSP page contains two types of text- Static Data and JSP elements.
Ans:- System. out and System. err both represent the monitor by default and so you can use both to send data to the monitor.
System. out is used to display normal messages and results, System.eerr for error messages, and System.in represents InputStream object which by default represents standard input device, i.e., keyboard.
Ans:- These are the four different options for authentication in servlet:
Ans. Interview questions on Core Java often explore OOP principles. Object-oriented programming in Java is based on four main principles: encapsulation (hiding data and providing access methods), inheritance (reusing and extending classes), polymorphism (multiple forms of a class), and abstraction (representing real-world entities as objects). These principles promote code organization, modularity, and flexibility.
Ans. In basic Core Java interview questions, understanding the finalize() method is important. The finalize() method is a method provided by the Object class, which can be overridden by a class to perform cleanup operations before the object is garbage collected. However, it's not recommended for critical resource management due to uncertainty about when it will be called.
Ans. Lambda expressions are a new and important feature included in Core Java SE 8. A lambda expression is a concise way to represent an anonymous function. It does not have a name; it is used in functional programming; and it can be passed as a parameter to a functional interface like Runnable or Comparator.
Ans. If you are currently taking a Java course, you may want to learn about object cloning, which is an advanced topic in this area. Object cloning refers to the process of creating an exact copy (clone) of an existing object. It can be accomplished by implementing the Cloneable interface and by providing a custom clone() method. Cloning allows you to create independent copies of objects and is useful when you need to preserve the state of an object. However, cloning must be done carefully to maintain data integrity.
Ans. The static keyword in Java indicates that a specific field or method is a class field. With no objects necessary you can call them. A static field or method is created once, and is shared among all instances of the class. It can contribute to effective memory management in large applications, particularly in the context of Java learning online, where static fields can improve manageability. They are used judiciously as they promote reusability and avoid redundancy of instance variables and instance methods.
Ans. Singleton design pattern in Java ensures that a class has just one instance and provides a global point of access to that instance. It is used in scenarios like a single database connection shared by multiple objects, and is usually used in scenarios like a single database connection shared by multiple objects in the pool. To implement a Singleton pattern in Java, you should make its constructor private, disable cloning and copying and create a static variable to hold the instance, and create a static method to get and maintain a global access. Singleton design pattern is one of the fundamental concepts in many Java design patterns and really helps to build efficient Java applications, which can be easily maintained and scaled.
Ans. The Stream API introduced in Java 8 is a major advancement in the Java programming language, enabling more expressive and, perhaps more importantly, more efficient data processing. Streams represent a new abstraction on collections, supporting a more expressive and declarative approach to collections processing (including support of operations like filter, map, reduce, and collect). This allows developers to write more readable and more concise code for common operations that involve collections. For this reason, Stream API tends to be the focus of many articles, blogs, and Java training courses for advanced Java learning online. In addition to promoting the functional programming principles of Java, it can offer significant performance improvements to Java applications through parallelization of operations on collections.
Ans. The Optional class was introduced in Java 8 and is a container object that is used to contain not-null objects. Optional is a great way of avoiding NullPointerException and it has a lot of other useful methods to help in handling different situations such as 'isPresent()', 'ifPresent()', and 'orElse()' that simplifies the code and prevents the application from breaking. This is also one of the main topics of Java training or Java online course and you should explore more to learn it.
Ans. Memory leaks in Java can be managed by ensuring that objects are no longer referenced when they need to be. You might use tools such as 'jmap', 'jhat', 'jstack', and 'jconsole' to identify objects that are not being garbage collected. As these objects do not show up in the heap finder, these tools can be very useful to find where possible memory leaks are within the Java application. Also object finalization in Java can be used to remove unnecessary object references. other techniques involve using a profiler and writing efficient code; using a weak reference can also be handy. Also remember that class loader is a potential source of memory leaks in J2EE. You can explore more with Java online courses. And it is definitely one of the favorite Java interview questions.
Ans. The ExecutorService is a framework provided by the Java concurrency API that simplifies the execution of tasks in asynchronous mode. It provides a pool of threads for executing Runnable and Callable tasks. On the other hand, the Fork/Join Framework, introduced in Java 7, is designed to efficiently handle a large number of tasks. It uses the divide-and-conquer strategy in which a task keeps on dividing itself into further sub-tasks until it is a manageable size and your most important work to be done is done primarily on merging the no of sub-tasks. This approach suits the problems that can be broken down into smaller tasks that can be executed concurrently.
In general ExecutorService is more general with the motive to solve all general problems. While the Fork/Join acts as specialization of ExecutorService, that is designed to handle eval if the tasks can be recursively divided in smaller sub-tasks and then executed, and hence why would it not take advantage of it to provide the performance benefits. You will find these concepts in detail in a Java online course focused on Concurrency and Parallelism.
Ans. The volatile keyword in Java is used as an indicator to tell the JVM that a variable will be modified by different threads. The value of this variable will never be cached in a register and all writes to this variable will be written back to the main memory immediately. Understanding volatile keyword is critical for core Java interviews and can be a tough nut to crack, especially the concept of memory visibility and instruction reordering. Similarly experienced Java developer would also be aware of the volatile keyword for preventing compiler optimization and reorder of instruction.
Ans. Ensuring thread safety in Java collections is achieved typically through several approaches. One common way is to use the synchronized wrappers provided by the Collections class, such as Collections.synchronizedList() or Collections.synchronizedMap(). Java also provides specific thread-safe collections such as ConcurrentHashMap, CopyOnWriteArrayList, and BlockingQueue. These classes are designed to allow concurrent access and modifications without the need for external synchronization, and are covered in many Advanced Java training courses, pointing out their usage patterns and performance characteristics.
Ans. Java uses garbage collection (GC) as a way to track memory leaks and free up memory that is no longer accessible by your application objects. While this is a good thing, GC can also impact the performance of your Java application, especially during GC pause times. To optimize the performance of Java applications, a developer would look at tuning the GC configuration, selecting different GC algorithms, and minimizing the creation of objects that are unnecessary. Advanced Java training typically covers GC behavior and GC optimization techniques so developers can handle the potential performance issues that can occur.
Ans. Annotations provide a way to add metadata to Java code. This metadata can then be analyzed by the compiler, development tools, or even runtime libraries. They can be used to specify compile-time instructions to the compiler, they can specify runtime processing, and more. The end result is that you can add this information directly to your code and you don't need to maintain it in external configuration files, resulting in a codebase that is cleaner and easier to maintain. Annotations are a covered topic in our Java online course and are a powerful tool for framework developers and can be utilized to reduce the amount of “boilerplate” code that is written, as well as help developers to understand the intention of code.
Ans. Key differences between interface and abstract class are:
Ans. ThreadLocal class allows creating thread local variables to store state isolated from other threads. Some usages are:
Thread safety gets achieved without explicit synchronization.
Ans. Java enum declares set of related constants avoiding duplicates or invalid values. Common is to define enum singleton instance pattern:
public enum OrderStatus {
PLACED, APPROVED, DELIVERED;
}
Accessing values then is just:
OrderStatus status = OrderStatus.APPROVED;
This adds type-safety over plain constants.
Ans. The Java Heap space comprises of key regions:
Tuning these subspaces for sizing/ratio is key for efficiency, which you can further learn about in our remaining Java interview questions.
Ans. Java string pool refers to cache of literal string instances allowing reuse. As String objects are immutable, thread safe with reuse they are preferred over StringBuffer which would create new string buffer objects per invocation. String pool saves heap memory costs.
Ans. Yes, threads can share common objects like:
These provide inter-thread communication safely avoiding synchronization costs.
Ans. Some best practices around Java exceptions are:
Ans. Garbage collection automatically manages deallocation of unused objects determining unreachable ones and clearing them. Tuning aspects involve:
Tuning collectors for throughput or latency is key. To know more, you should explore our other core Java interview questions.
Ans. Java annotation provides metadata about the program embedded in code. Annotations have no direct effect on operations. Common uses are:
Objects live in Heap while execution contexts allocate associated stack with threads. Stack contains method specific variables, arguments, registers and return address. Stacks follow LIFO order to keep track of method calls ensuring cleanup. Heap handles objects providing more dynamic allocation whose lifecycle is disjoint from stack frames.
Ans. IDEs like Eclipse, IntelliJ provide shortcuts to generate boilerplate code like getters/setters by right click on field and selecting options like Generate Getters and Setters. Libraries like Lombok reduce this effort using annotations transforming code on compilation. Using IDE productivity features is highly encouraged.
Ans. Java access modifiers provide scope access for classes, constructors, variables and methods. The levels are:
Encapsulation gets enforced appropriately. To know more about this concept, feel free to check out our Core Java online training course.
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Ans. The foreach loop hides access to the iterator directly. To conditionally skip iterations if required logic is complex, prefer enhanced for loop:
for(Temperature temp : temperatures) {
if(temp.getValue() < 20 xss=removed>
This allows control via standard looping constructs.
Ans. Reflection API provides inspection abilities of classes, interfaces, methods fields during runtime discoverying them programmatically without needing compile time knowledge. This helps:
Ans. Yes, unlike constructors - finalize methods can throw exceptions which will just be logged and ignored by GC granting deference to cleanup in finalization. However exceptions should be avoided in finalizers expecting them to run interrupting GC. Instead resources should be closed correctly via try finally constructs.
Ans. Java Heap region handles dynamic object allocation at runtime. Stacks contain thread specific method execution contexts and tracking containing local variables, arguments, registers, return addresses to keep track of method calls similar to a stack data structure. This is one of the major Java interview questions that you can't ignore at any cost.
Ans. Arrays in Java by default initialize to null for references, and default values for primitives. To avoid NullPointerErrors, arrays can be initialized proactively:
String[] teams = new String[10];
Arrays.fill(teams, "Unknown");
This avoids unexpected null interpretations later.
Ans. Java API specifications provide standard definition of classes, interfaces serving as reference implementation to ensure inter operability between Java runtime environments. The specifications are defined and updated in coordination by the Java Community Process (JCP) in tandem with OpenJDK as a reference model open source implementation providing these APIs.
Ans. Shallow copy constructs new instance and inserts references to the same objects member fields refer. Shallow copies are cheap but may reflect changes later.
Deep copy iterates fields deeply creating duplicate nested object graphs fully decoupled from original objects. Immutability patterns help share buffers.
Ans. A marker interface does not declare any members itself but provides runtime type information to the compiler and JVM. Some examples are:
This allows building metadata for marked objects. To know more about this, check our our Java training course.
Ans. String is immutable causing retention in memory until Garbage Collected. Whereas char array state can be overwritten quickly after usage, clearing sensitive data deterministically. Strings also get output in heap dumps which can be probed later. Hence char array is a better choice for security reasons.
Ans. Both are index based iterable interfaces backed by dynamic arrays. Key differences are:
Choosing between them depends on synchronization needs.
Ans. CopyOnWriteArrayList, ConcurrentHashMap and ConcurrentSkipListSet are highly optimized concurrent collections relying on snapshots, clever sequencing avoiding locks allowing parallel reads improving throughput while changes reflected later. Comparatively synchronized collections use stricter mutual exclusion lowering scalability.
Ans. Instant represents timestamp with precision upto nanoseconds representing point on timeline (epoch/UTC) without timezone info.
Whereas LocalDateTime encapsulates date and time keeping timezone aspects making it more appropriate for common use cases like database columns, UI display etc.
Ans. Break statement inside try block immediately transfers control outside enclosing try-catch statement similar to return avoiding rest of try or catch blocks execution.
This helps avoid further unnecessary processing like cleanup on failures confined within the try block. If you are interested to know more about such core Java interview questions, feel free to let us know.
We are sure that these 39 Java interview questions and answers and our preparation tips will help you put your best foot forward.
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Core Java is a very popular programming language that can be found in the technology stack of every company. No matter if you are heading for the Java developer interview or full-stack developer interview, knowing Interview Questions for Java with their answers is important for you to give your best and get hired in your dream company.
These are core Java interview questions for experienced and entry-level candidates that are often asked on repeat, so prepare them and give your best shot in an upcoming interview. Also, along the journey, never underestimate the Java role as its future scope is quite bright.
In case you are looking for complete preparation of Core Java Career, get along and explore our Java Training course online, led by real-programmers with real-time industry projects, along with complete resume feedback and impactful preparation of Core java interview questions and answers for experienced & freshers.
Let us know in the comments below if you have any queries or questions related to these core Java interview questions for experienced & beginners!
Q1. What will I learn in JanBask's Core Java Certification course?
Ans: You will master core Java concepts like OOPs, Exception Handling, Multithreading, Collections etc. The course covers Java 8 features like Lambda Expressions, Method References and Functional Interfaces. You will learn Core Java application development using Spring Boot, Hibernate and popular Java frameworks. The course culminates with Core Java certification preparation.
Q2. What are the benefits of doing this Core Java course?
Ans: Doing this course helps you learn Core Java in a structured format from industry experts. You will be able to develop complex Core Java applications for modern software needs. The course helps prepare for Core Java certification which validates your skills and boosts your career growth.
Q3. Why should I enroll for JanBask's Core Java certification course?
Ans: JanBask is a reputed online training provider known for quality content and expert trainers. The Java course is comprehensive, covers latest Core Java features and focuses on practical application development. JanBask offers mentor guidance and interview preparation support which maximizes your chances of getting hired as a Core Java developer.
Q4. What is the course duration and study time required?
Ans: It is an 8 weeks long course. You need to allocate around 10-15 hours per week for studying course materials, doing coding exercises, assignments and preparing for assessments. The self-paced online model offers flexibility to balance learning with other commitments.
Q5. What background do I need for doing this Core Java course?
Ans: Basic programming experience in any language is required. Familiarity with concepts like variables, data structures, loops, functions etc. is expected. Prior object oriented programming experience is beneficial but not mandatory.
Q6. Do I need to appear for any certification exam?
Ans: Yes, the course includes preparation for Oracle Certified Professional Java SE 11 Developer (OCPJP) exam. JanBask will provide all the required training, mock tests and study resources to appear and clear this credential exam which validates your Java skills.
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