Java Interview Questions Explained with Core Java

By Rohini Madhavi

Last updated on Oct 26 2020

Java Interview Questions Explained with Core Java

Java Interview Questions and Answers Updated 2020

In case you want to learn more about the important Java Interview questions that might come up in the Java interview. This blog explains the entire top core Java Interview Questions along with the answers that offer you the edge/advantage over the others in the interview. There are over millions and millions of developers that make use of Java to build applications or software for leading technologies. If you want to be one of them you have to pass the Java interview. in order to win the battle, you need proper preparation. These customized questions and answers are really going to do wonders if you embrace them.lets get started It includes devices like DTH boxes and mobile phones. Thus, core Java Interview Questions help an individual to gain better knowledge about Java. Java is used widely across the world. The Java certification is highly recognized across numerous industries globally. 

This blog is explained some of the best Advanced Java Interview Questions can help an applicant to succeed in the interview. As a java expert, one must understand the questions what is JAVA? The important words, know the appropriate terminologies, and adequately prepare for the Java certification.

Below is the set of Advanced Java Interview Questions that are related to core JAVA:

Java Interview Questions (Basic Level)

  1. Define JVM, JRE, and JVM.  

JVM: It is normally referred to as the Java Virtual Machine. This abstract machine usually offers an environment that is based on runtime. In this environment, it is possible to implement java bytecode. There are three kinds of notations abided by it, such as specification, runtime, and execution. 

JRE: It is denoted as a Java runtime environment. It includes a runtime environment within which it is possible to execute the java bytecode. This is an execution of the JVM that is present physically.

JDK: It is mainly referred to as the java development kit. This tool is essential for documenting, programming, compiling, and setting the java programs. Besides, it also includes the important development tools of JRE+.

Mention the reason behind the platform-independent of Java.

It is referred to as platform-independent due to the ability of the byte codes to function on any kind of system despite its original operating system. 

 

  1. Java is not fully object-oriented. Explained

There is a total of eight different kinds of primitive data/resource that includes byte, char, float, Boolean, float, double, long, short, etc. are mainly used in Java. Since these are not objects, it is not considered job-oriented. 

 

  1. Explained: the wrapper classes present in Java.

The primitives of Java are transformed into objects. All the primitive resources or data include a class aimed at it. These are also referred to as wrapper classes due to their role of "wrapping" the primitive information. It transforms these primitive data into an object that belongs to a particular class. 

 

  1. What do you mean by constructors in Java?

It is one of the most frequently asked Advanced Java Interview Questions. It can be answered in the following ways:

The block of code that is normally utilized to prepare an object is known as constructors. It should have a name that is identical to that of the class. Besides, it does not include any return kind or type. After it's developed, it is called by default. 

It contains two different types:

Default Constructor: The constructors that normally does not accept any input is referred to as the default constructor. Similarly, they are also known as the argument constructors that are developed automatically in a situation where the user has not defined any other constructor. According to its main goal, the instance value is performed alongside the default values. Besides, it is frequently utilized for the creation or development of the object. 

Parameterized Constructor: The constructor that can perform the instance variables with the offered values. Besides, the parameterized constructors are called such since they take up the arguments.

 

  1. Define the class singleton along with the steps needed to develop one. 

It is a class where it is possible to only develop a single instance at any provided time, in a single JVM. By creating a private constructor, it is possible to make a singleton class.  This question is reported that it is a repeating one in Java interview questions.

 

  1. Describe the differences existing between the Vector and the array list. 

Vector: It is quite coordinated. It sometimes slows down due to its thread-safe. The size of its array is doubled due to the defaults in Vector. The augmentation of the size is described by the Vector. For the processes of traversing, one can use iterator and enumeration. 

Array List: It is not coordinated well. It is normally very fast but not coordinated. The size of the array rises by 50% if a component/element is added to the array list. The augmentation of size is not described by the Array list. If you are a great learner. Then learning this questions will definitely helps you crack the interview for java in first chance.

 

  1. Give out the main difference between equal () and == in Java.

The method that is mainly used for checking two objects equally well-defined by business logic is referred to as equals (). Also, it is defined in the Java object class. 

Java offers a binary operatory which is utilized for the comparison between objects and primitive. The method that the class of the object offers is the public Boolean equals (Object o). == operation is used by the default execution for the process of comparing two different objects. 

  1. Mention the key differences between Heap and Stack.

Heap: 

  • Every part of the application makes use of heap memory. 
  • The objects that are kept or stored in a heap are accessible across the world. 
  • Memory management is based upon the generation that is related to every object. 
  • From the beginning till the ending of the application implementation, the heap memory is used. 
  • Heap space is used to keep every object that is being developed. 

Stack:

  • Only a single thread of implementation uses the stack memory. 
  • Other threads can't use the stack memory. 
  • LIFO is followed by it to free up memory. 
  • It stays till the thread implementation is completed. 
  • The local primitive and reference variables were included in the stack memory. 
  1. Explain the definition of the package in Java along with the advantages of packages. 

It is another regularly asked Advanced Java Interview Questions. The set of the associated classes and interfaces that are stacked together is referred to as packages. The codes can be modularized by the developers effortlessly with the use of the packages. Besides, it can also be optimized to be used again. The different classes can import the code present inside the packages and be used again. Few advantages of packages are as follows:

  • The clashing of the names can be prevented with the use of the packages.
  • With the use of the packages, it is possible to access the codes easily. 
  • Hidden classes are also included in the packages that are not noticeable/visible to the external classes. Besides, it is only used inside the package itself.
  • Develops an accurate hierarchal structure that makes it simpler and effortless to identify the associated classes. 
  1. Are the pointers not used in Java?

Here comes another regularly asked question for Java interview. Pointers are not used in Java since they are considered risky and enhance the program complexity. The code of Java is known for its simplicity and easiness. Therefore, including the notion of pointers might become challenging and contradicting. Sine, the implicit memory allocation is done by using JVM; therefore, to prevent the straight access/entrée to the memory by the customer, the pointers are not used in Java. 

 

Define the JIT compiler

It is also referred to as just-in-time in Java. This question is a regular one in Java interviews. It is a useful program that is used for producing instructions by transforming the Java bytecodes, which are then sent straight to the computer. In Java, the JIT compiler is permitted by default, and it activates every time the java procedure is invoked. The invoked method's bytecode is then compiled using the JIT compiler into a native machine code. After the method/procedure is compiled, the compiled code is then gathered straight by the JVM instead of the interpretation. It is normally in charge of the optimization of the performance of the java applications during the run time. 

 

What does the access modifiers mean?

The special keywords that are utilized to limit the access of a constructor, data member, class, and procedure in the other class. Modifiers are an unavoidable question when it comes to the java interview questions. There are four kinds of access modifiers, such as:

  • Private
  • Public
  • Default
  • Protected

Explain Java Class

The blueprint that comprises all the information is referred to as a class in Java. There are several variables or fields included in a class and also the methods used to define the object behavior. The class syntax is as follows:

  1. Class Abc {
  2. Member variables // class body
  3. Methods}

Define Java Object and the process used to develop it. 

 The real-life material thing that possesses a behavior and a state is referred to as an object. The three features of an object are as follows:

  • Behavior 
  • Identity
  • State

The "new" keyword is used mainly to build an object. For instance:

ClassName object = new ClassName();

Describe the Object-Oriented programming. 

It is also widely referred to as OOPs. It is a programming approach where the programs are mainly arranged surrounding the objects instead of only functions and logic. The programs that include huge and complex codes that require to be regularly updated, handled, or maintained is perfect for this approach. 

 

State all the important concepts of OOPs.   

The concepts that OOPs is related are as follows:

  • Inheritance: this process occurs when one class obtains the possessions or assets of the other class. 
  • Encapsulation: The process of wrapping up the code and data is combined as one unit is referred to as Encapsulation. 
  • Abstraction: The procedure of hiding the execution information from the customers or users and only offering the functionality. 
  • Polymorphism: The facility of a variable, object, or function to take several forms is referred to as polymorphism. 

State the difference between a local and instance variable.

A local variable is mainly used within a constructor, block, or method and only local scope are included in it. Therefore, this variable can only be used inside the scope of a block. One of the main benefits of attaining this variable is that it is not noticeable by the other methods included in a class.  

Whereas, the variable that is restricted to the object is referred to as an instance variable. These are normally acknowledged inside a class, although outside a method. Each object included in the class builds its variable copy during the process of using it. Therefore, adding any type of change will not be visible or reflected in any other class instances and will be restricted to only that specific instance. 

Differentiate between the constructors and methods present within Java?

Methods: 

  • The behavior of the object is signified using a method. 
  • It normally has a return type to it. 
  • It requires to be explicitly invoked. 
  • No default method is provided by the compiler.
  • The name of the method might or might not be identical to the class name. 

Constructors

  • The state of an object is initialized with the use of constructors. 
  • It does not include any return type. 
  • It can be implicitly invoked. 
  • The compiler provides a default constructor in case the class does not include one.  
  • The name of the constructor should always be identical to that of the class.

State the final keyword in Java. 

The special keyword that is used as a non-access modifier in Java is referred to as the final keyword. There are several contexts in which the final variable can be used:

  • Final variable: The value of the final keyword can be changed for a single time when using a variable with it. If the final variable has no value allocated to it, then the value can be allocated to it using the class constructor. 
  • Final method: A method can't be overridden by inheriting class if it is declared final. 
  • Final class: A class can't be prolonged by any subclass if it is declared as final, but it can be used to prolong the other classes. 

Distinguish between the continue and break statements.

Continue:

  • Only the loop statements can be used with it. 
  • The loop doesn't get terminated, but it can result in the loop to move on to the next iteration. 
  • Inside the loop, together with a switch, can lead to the next loop iteration to implement. 

Break: 

  • It can be used in both loop and switch statements. 
  • It can result in the termination of the loop or switch statements once it is implemented. 
  • The deepest closing switch or loop is terminated directly. 

Explain the infinite loop present in Java. Give an example.  

When a functional exit isn't fulfilled the instructional sequence that loops constantly is referred to as an infinite loop. This can occur due to the issue present in programming or can also be due to a cautious action based on the behavior of an application. After the application exists, the infinite loop gets terminated on its own or automatically. 

State the difference between super() and this() in Java.

Super(): 

  • The parent class's present instance is represented by it. 
  • The parent class/base's default constructor is called using super(). 
  • The methods of the base class are accessed using it. 
  • Pointing to the superclass instance is done by using it. 
  • It should always be the beginning or first line of a block. 

This(): 

  • The current instance of a class is represented by it.  
  • The default constructor of the same class is called using this().
  • The current class's method is accessed by using this().
  • Pointing to the current class instance is done by using this().
  • It should be included in a block's first line.  

 

Define the Java String Pool.

The set of strings that are kept in the heap memory is referred to as the Java String pool. Each time a new or additional object is built the string pool is responsible for examining whether the object is available in the pool or not. In case it is available, then the same reference is given back to the variable. Or else additional or new objects will be developed in the strong pool and the reference will be given back. 

What is the difference between static and non-static methods?

Static Method

  • Before the name of the method, the static keyword must be used. 
  • Using the class, it is called. (className.methodName)
  • Accessing any non-static instance variable is impossible.  

Non-static method

  • The static keyword is not required before the name of the method. 
  • It is called using any type of general method. 
  • Any static method can be accessed by it along with any static variable without building an instance of the class. 

 

Define the constrainer chaining. 

The procedure of calling one constructor from another or additional regarding the present object is referred to as constructor chaining. If the subclass constructor is accountable for invoking the superclass beforehand, then the constructor chaining occurs. There can be numerous numbers classes present in the constructor chain. There are two ways in which it can be achieved. For example:

  • Using this() inside the class
  • Using super() from the base class

 

Define classloader in Java

The subset that belongs to JVM is referred to as the classloader. Loading the class files is completed by the classloader. Every implemented java program is loaded by the classloader first. There are three types of in-built classloaders: 

  • Bootstrap ClassLoader
  • System ClassLoader
  • Extension ClassLoader

Give a reason behind the immutable nature of the Java Strings. 

The string objects usually have an immutable nature which indicates that the string object is built and no alteration can be made in its state. And when one attempts at updating the value of that object rather than updating, the values of that specific object, a new string object is then created by Java. Since the string objects are normally cached int, he strings pool, the string objects of Java are immutable. There are several clients between which the string literals are shared. Besides, the action of one customer might disturb the others. It improves the caching, synchronization, performance, and security of the application. 

State the difference between an array list and array. 

ArrayList:

  • The values of the dissimilar data types are included in it. 
  • Changes can be added to the size. 
  • The index does not need any specification. 
  • They are considered as types. 
  • It can only include objects and does not allow the primitive data types. 

Array: 

  • It does not include the values of dissimilar data types.
  • At the time of declaration, the size must be clearly defined. 
  • The index must be specified for adding information.
  • They are not parameterized as type. 
  • It includes objects along with primitive data. 

Explain Map in Java. 

The util package interface that maps the rare keys to value is referred to as a map. It is not included in the subset of the main collection interface. Therefore, it has a bit of different behavior as compared to the other collection types. Some of the characteristics of Map interface:

  • There are no duplicate keys included in the Map. 
  • Only one value can be mapped using every key. 

Explain the collection class in Java and state its interfaces and methods. 

The framework that is utilized for storing and operating a set/group of objects is referred to as a collection class. It can be utilized to accomplish numerous types of activities like sorting, searching, operation, deletion, insertion, and so on. This framework contains the following:

  • Classes
  • Interfaces
  • Methods

Object-Oriented Programming System Advanced Java Interview Questions

Define Polymorphism 

The ability to allocate a dissimilar usage or meaning to a thing in various contexts – especially to enable a unit like a function, an object, or variable to acquire additional forms. Two kinds of polymorphism are present such as:

  • Run time polymorphism
  • Compile-time polymorphism

The method overloading is referred to as compile-time polymorphism, and the inheritance and interface are used to perform the run time polymorphism. 

Define abstraction in Java. 

The quality of dealing with the ideas instead of the events are referred to as abstraction. It is responsible for hiding the information and displaying the important things to the customers. Therefore, the method of hiding the execution information from the customer and only reveals the functionality to the customers. There are two ways in which it can be achieved:

  • Interfaces
  • Abstract classes

 

Define the interface in Java. 

The collection of static constants and methods or the class blueprint is known as the interface. All the methods included in an interface are abstract and public, although it does not include any constructor. Therefore, the set of related methods with empty bodies is referred to as an interface. 

 

Conclusion

In case, you are preparing to clear the java certification, and you can check these best Advanced Java  Interview Questions. These questions can help you to increase your performance in the interview greatly. 

Besides, all of the above Advanced Java Interview Questions are hand-picked and give you the chance to improve your basic knowledge about Java. You can pass an interview if you read these questions beforehand. All the best for your Java Interview ahead.

About the Author

Sprintzeal   Rohini Madhavi

Digital Content Writer with great experience in SaaS Marketing content creation

Recommended Courses

Recommended Resources

Growing Demand for Certified Professionals and the Importance of Certification

Growing Demand for Certified Professionals and the Importance of Certification

Article


Top Career benefits of Lean Six Sigma Green Belt

Top Career benefits of Lean Six Sigma Green Belt

Article


Project Initiation Phase - Importance and Roles Involved

Project Initiation Phase - Importance and Roles Involved

Article


TRENDING NOW