Java Interview Questions and Answers -

1. What are the differences between C++ and Java?

Concept.

C++ is not platform-independent; the principle behind C++ programming is “write once, compile anywhere.”

In contrast, because the byte code generated by the Java compiler is platform-independent, it can run on any machine, Java programs are written once and run everywhere.

Languages Compatibility.

C++ is a programming language that is based on the C programming language. Most other high-level languages are compatible with C++.

Interaction with the library.

It can access the native system libraries directly in C++. As a result, it’s better for programming at the system level.

Java’s native libraries do not provide direct call support. You can use Java Native Interface or access the libraries.

Characteristics

C++ distinguishes itself by having features that are similar to procedural and object-oriented languages. The characteristic that sets Java apart is automatic garbage collection. Java doesn’t support destructors at the moment.

The semantics of the type.

Primitive and object types in C++ have the same kind of semantics. The primitive and object and classes of Java, on the other hand, are not consiste

In the context of Compiler and Interpreter.

Java refers to a compiled and interpreted language. In contrast, C++ is only a compiled language.
In Java, the source code is the compiled output is a platform-independent byte code.
In C++, the source program is compiled into an object code that is further executed to produce an output.

2. What is an Association?

An Association can be defined as a relationship that has no ownership over another. For example, a person can be associated with multiple banks, and a bank can be related to various people, but no one can own the other.

3. What is the default value stored in Local Variables?

Neither the Local Variables nor any primitives and Object references have any default value stored in them.

4. Will the program run if we write static public void main?

Yes, the program will successfully execute if written so. Because, in Java, there is no specific rule for the order of specifiers

5. What are the differences between Heap and Stack Memory in Java?

Stack memory in data structures is the amount of memory allocated to each individual programme. It is a fixed memory space. Heap memory, in contrast, is the portion that was not assigned to the Java code but will be available for use by the Java code when it is required, which is generally during the program's runtime.

6. What are the Memory Allocations available in Java?

Java has five significant types of memory allocations.

1) Class Memory

2) Heap Memory

3) Stack Memory

4) Program Counter-Memory

5) Native Method Stack Memory

7. What is a ClassLoader?

A classloader in Java is a subsystem of Java Virtual Machine, dedicated to loading class files when a program is executed; ClassLoader is the first to load the executable file.

Java has Bootstrap, Extension, and Application classloaders.

8. List the features of the Java Programming language?

Easy:

Java is a language that is considered easy to learn. One fundamental concept of OOP Java has a catch to understand.

Secured Feature:

Java has a secured feature that helps develop a virus-free and tamper-free system for the users.

OOP:

OOP stands for Object-Oriented Programming language. OOP signifies that, in Java, everything is considered an object.

Independent Platform:

Java is not compiled into a platform-specific machine; instead, it is compiled into platform-independent bytecode. This code is interpreted by the Virtual Machine on which the platform runs.

9. What do you get in the Java download file? How do they differ from one another?

We get two major things along with the Java Download file.

JDK - Java Development Kit

JRE - Java Runtime Environment

10. What are the differences between JDK and JRE?

JDK	JRE
Abbreviation for JavaDevelopment Kit	Abbreviation for Java Runtime Environment
JDK is a dedicated kit for solely software development	JRE is a set of software and library designed for executing Java Programs
Unlike JVM, JDK is Platform Dependent	Unlike JVM, JRE is also Platform Dependent
JDK package is a set of tools for debugging and Developing	JRE Package is one that only supports files and libraries for a runtime environment
JDK package will be provided with an installer file	JRE Package does not get an installer but has only a runtime environment

11. Can you implement pointers in a Java Program?

Java Virtual Machine takes care of memory management implicitly. Java's primary motto was to keep programming simple. So, accessing memory directly through pointers is not a recommended action. Hence, pointers are eliminated in Java.

12. Define package in Java.

The package is a collective bundle of classes and interfaces and the necessary libraries and JAR files. The use of packages helps in code reusability.

13. What is a singleton class in Java? And How to implement a singleton class?

A class that can possess only one object at a time is called a singleton class. To implement a singleton class given steps are to be followed:

1) Make sure that the class has only one object

2) Give global access to that object

14. Define Wrapper Classes in Java.

In Java, when you declare primitive datatypes, then Wrapper classes are responsible for converting them into objects(Reference types).

15. What is an object-oriented paradigm?

A Paradigm that is based on the concepts of “Objects.” It contains data and code. Data that is in the form of fields, and regulation, that is in the form of procedures. The exciting feature of this paradigm is that the object’s procedures can access and often modify the data fields themselves.

16. Can Java be said to be the complete object-oriented programming language

No, Java cannot be treated as a complete object-oriented programming language.

17. What is Object Cloning?

An ability to recreate an object entirely similar to an existing object is known as Object Cloning in Java. Java provides a clone() method to clone a current object offering the same functionality as the original object.

18. What is a Marker Interface?

An empty interface in Java is referred to as a Marker interface. Serializable and Cloneable are some famous examples of Marker Interface.

19. Define Copy Constructor in Java

A Copy Constructor in Java is a constructor that initializes an object through another object of the same class.

20. What do you mean by aggregation?

The term aggregation refers to the relationship between two classes best described as a “whole/part” and “has-a” relationship. This kind is the most specialized version of an association relationship. It contains the reference to another class and is said to have ownership of that class.

21. What happens when the main() isn't declared as static?

When the main method is not declared as static, then the program may be compiled correctly but ends up with a severe ambiguity and throws a run time error that reads "NoSuchMethodError."

22. What is the final keyword in Java?

The term final is a predefined word in Java that is used while declaring values to variables. When a value is declared using the final keyword, then the variable's value remains constant throughout the program's execution.

23. What is an Exception?

An Exception handling in Java is considered an unexpected event that can disrupt the program's normal flow. These events can be fixed through the process of Exception Handling.

24. Explain Java String Pool.

A collection of strings in Java's Heap memory is referred to as Java String Pool. In case you try to create a new string object, JVM first checks for the presence of the object in the pool. If available, the same object reference is shared with the variable, else a new object is created.

25. Differentiate between instance and local variables.

For instance, variables are declared inside a class, and the scope of variables in javascript is limited to only a specific object.

A local variable can be anywhere inside a method or a specific block of code. Also, the scope is limited to the code segment where the variable is declared.

26. What are the differences between constructor and method of a class in Java?

Initializing the state of the object is done by constructors. A function Object () { [native code] }, like methods, contains a group of statements (or instructions) that are carried out when an object is created. A method is a group of statements that work together to complete a certain task and return the outcome to the caller. A method has the option of working without returning anything.

27. What is the difference between the program and the process?

A programme is a non-active entity that includes the collection of codes necessary to carry out a specific operation. When a programme is run, an active instance of the programme called a process is launched. A process is begun by a programme once it has been run. The process carries out the program's specified instructions.

28. What part of memory - Stack or Heap - is cleaned in the garbage collection process?

On Heap memory, garbage collection is employed to release the memory used by objects with no references. Every object created in the Heap space has access to the entire application and may be referred to from anywhere.

29. Why is the main method static in Java?

Java's main() function is static by default, allowing the compiler to call it either before or after creating a class object. The main () function is where the compiler begins programme execution in every Java programme. Thus, the main () method needs to be called by the compiler. If the main () method is permitted to be non-static, the JVM must instantiate its class when calling the function.

30. Why is Java a platform independent language?

One of the most well-known and widely used programming languages is Java. It is a programming language that is independent of platforms. Java doesn't demand that the complete programme be rewritten for every possible platform. The Java Virtual Machine and Java Bytecode are used to support platform independence. Any JVM operating system can run this platform-neutral byte code. The application is run after JVM translates the byte code into machine code. Because Java programmes can operate on numerous systems without having to be individually rewritten for each platform, the language is referred to as "Write Once, Run Anywhere" (WORA).

31. How does an exception propagate in the code?

In the event that an exception is not caught, it is initially thrown from the top of the stack and then moves down the call stack to the preceding method. The runtime system looks for a way to handle an exception that a method throws. The ordered list of methods that were called to get to the method where the error occurred is the collection of potential "somethings" that can be used to manage the exception. The call stack is the list of methods, and exception propagation is the search technique.

32. How do exceptions affect the program if it doesn't handle them?

If you don't deal with an exception once it occurs, the programme will end abruptly and the code after the line where the exception occurred won't run.

33. Is it mandatory for a catch block to be followed after a try block?

Each attempt block does not necessarily have to be followed by a catch block. Either a catch block or a final block ought to come after it. Additionally, any exceptions that are expected to be thrown should be mentioned in the method's throws clause.

34. Can you call a constructor of a class inside another constructor?

Yes, a class may include any number of constructors, and each function Object () {[native code] } may call the others using the this() function Object() { [native code] } call function [please do not mix the this() function Object() { [native code] } call function with this keyword]. The constructor's first line should be either this () or this(args). Overloading of constructors is what this is called.

35. Contiguous memory locations are usually used for storing actual values in an array but not in ArrayList. Explain.

Primitive data types like int, float, and others are typically present in an array. In such circumstances, the array immediately saves these elements at contiguous memory regions. While an ArrayList does not contain primitive data types. Instead of the actual object, an ArrayList includes the references to the objects' many locations in memory. The objects are not kept in consecutive memory regions because of this.

36. What happens if there are multiple main methods inside one class in Java?

There is no limit to the number of major approaches you can use. Overloading is the ability to have main methods with different signatures than main (String []), and the JVM will disregard those main methods.

37. Can we make the main() thread a daemon thread?

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38. What happens if the static modifier is not included in the main method signature in Java?

The main function is called by the JVM even before the objects are created, thus even if the code correctly compiles, there will still be an error at runtime.

39. Which among String or String Buffer should be preferred when there are a lot of updates required to be done in the data?

Because StringBuilder is quicker than StringBuffer, it is advised to utilize it wherever possible. However, StringBuffer objects are the best choice if thread safety is required.

40. What are the differences between constructor and method of a class in Java?

41. What are Composition and Aggregation? State the difference.

Aggregation (HAS-A) and composition are its two forms (Belongs-to). In contrast to composition, which has a significant correlation, the aggregation has a very modest association. Aggregation can be thought of as a more confined version of the composition. Since all compositions are aggregates but not all aggregates are compositions, aggregate can be thought of as the superset of composition.

42. Although inheritance is a popular OOPs concept, it is less advantageous than composition. Explain.

A class's testability is improved through composition over inheritance. If a class is comprised of another class, it is simple to create a mock object to simulate the combined class for testing purposes. This privilege is not given by inheritance. Even while Composition and Inheritance both let you reuse code, Inheritance has the drawback of breaking encapsulation. If the function of the subclass depends on the superclass's action, it suddenly becomes vulnerable. Sub-class functionality may be broken without any alteration on the part of the super-class when its behaviour changes.

43. How does the size of ArrayList grow dynamically? And also state how it is implemented internally?

A resizable array implementation in Java is called ArrayList. Dynamically expanding array lists make it possible to add new elements at any time. The underlying data structure of the ArrayList is an array of the Object class. The ArrayList class in Java has three constructors. There are available readObject and writeObject methods specific to it. The Object Array in an ArrayList is temporary. There are implemented and Serialization-capable versions of RandomAccess, Cloneable, and java.io (that are Marker Interface in Java).

44. How many overloaded add() and addAll() methods are available in the List interface? Describe the need and uses.

List is an interface in the Java Collections Framework. The add() and addAll() methods are the main methods at the List interface. The add() method is used to add an element to the list, while the addAll() method is used to add a collection of elements to the list.

The List interface contains two overloaded versions of the add() method:

The first add() method accepts a single argument of type E, the element to be added to the list.

The second add() method accepts a variable number of arguments of type E, which are the elements to be added to the list.

The List interface also contains two overloaded versions of the addAll() method:

The first addAll() method accepts a single argument of type Collection, which is the collection of elements to be added to the list.

The second addAll() method accepts a variable number of arguments of type E, which are the elements to be added to the list.

45. Why is the remove method faster in the linked list than in an array?

Because there is no background scaling of an array, insertion, addition, and removal operations are quicker with a LinkedList. Only references in adjacent items need to update when a new item is added in the middle of the list.

46. Define System.out.println().

System.out.println() in Java outputs the argument that was supplied to it. On the monitor, the println() method displays the findings. An objectname is typically used to call a method.

47. Why is synchronization necessary? Explain with the help of a relevant example.

Multiple threads trying to access the same resources in a multi-threaded software may frequently result in unexpected and incorrect outcomes. Therefore, it must be ensured through some form of synchronization that only one thread can access the resource at any given time. Java offers a method for setting up threads and synchronizing their operations with the aid of synchronized blocks. The synchronized keyword in Java is used to identify synchronized blocks. In Java, a synchronized block is one that is tied to an object. Only one thread can be running at a time inside synchronized blocks since they are all synchronized on the same object. Until the thread inside the synchronized block exits the block, all other threads trying to enter the block are blocked.

48. Is exceeding the memory limit possible in a program despite having a garbage collector?

Yes, even with a garbage collector in place, the programme could still run out of memory. Garbage collection aids in identifying and removing programme objects that are no longer needed in order to release the resources they use. When an object in a programme cannot be reached, trash collection is executed with respect to that object. If there is not enough memory available to create new objects, a garbage collector is used to free up memory for things that have been removed from the scope. When the amount of memory released is insufficient for the creation of new objects, the program's memory limit is exceeded.

49. How is the ‘new' operator different from the ‘newInstance()' operator in java?

Both the new operator and the newInstance() method are used to create objects in Java. If we already know the kind of object to create, we can use the new operator; however, if the type of object to create is supplied to us at runtime, we must use the newInstance() function.

50. How is the creation of a String using new() different from that of a literal?

The new () operator always produces a new object in heap memory when creating a String object. The String pool may return an existing object if we build an object using the String literal syntax, such as "Baeldung," on the other hand.

Top 50 Java Interview Questions and Answers