Top Java Interview Questions you need to know

java interview questions

These questions are designed to be a quick revision guide for all the concepts in Java before interviews. As Java is widely used in developing various software solutions and applications, it is important to go through these top java interview questions to help you ace a Java interview.

Stated below are the most frequently asked questions related to Java:

Q.What is your basic understanding of Java?

Ans. Java is a programming language developed by Sun Microsystem and released in 1995. It is compatible with various operating systems such as Mac-OS, Windows and various other versions of UNIX.

Q. Define and Explain JDK.

Ans. Java Development Kit (JDK) is a tool used for functions such as compiling, documenting and packaging Java programs. It is made up of JRE+ development tools.

Q. Define and Explain JRE.

Ans. Java Runtime Environment (JRE) is basically a runtime environment used to execute Java bytecode. It is one of the three notations of JVM- Implementation.

Q. Define and Explain JVM.

Ans. Java Virtual Machine (JVM), where functions such as specification, implementations and runtime instances can be executed.

Q. Name a few features of Java?

Ans. The features include Object-Oriented, Platform Independent and, Robust, Interpreted and Multi-threaded in nature.

Q. What makes Java an Independent platform?

Ans. Java language does not depend upon any hardware and software. The compiler compiles the code, the respective code is then converted to platform-independent bytecode, which can run on multiple systems. JRE should be installed on the machine.

Q. What are local variable and instance variables?

Ans. Local variables are present within a block, function, or constructor and can only be accessed within the above stated. They can only be utilized within the block scope. The other class methods can not acknowledge the local variable declared inside a method.

The methods in the class can access all the instance variables. The properties of an object are being described by these variables, and are declared outside the methods and inside the class.

Q.  What makes Java dynamic and a high performer?

Ans. The ability to adapt to an evolving environment makes java dynamic in nature. Java has its own JIT compiler and run-time environment, designed to execute large amounts of run-time information, which is then processed to verify and resolve accesses to objects on run-time.

Just-In-Time compiler allows high performance. Just-In-Time compiler contains instructions that are interpreted into instructions that can be sent directly to the processor, the program turns Java bytecode.

Q. Give any two examples of Java IDEs.

Ans. Netbeans and Eclipse are the two examples of Java IDEs.

Q. What is Data Encapsulation?

Ans. The concept of placing the data attributes and their behaviours in a single unit is known as Data Encapsulation. Mostly used in object-oriented programming. Software can be developed by making each object independent of other objects by stating its own methods, attributes, and functionalities. Making it easy to follow modularity. Data can be protected, by securing the private properties of an object.

Q. Give a few examples of miscellaneous Java keywords.

Ans. Import, super, finally, assert, these are the few examples of miscellaneous Java keywords.

Q. What do you understand by the concept of constructor overloading?

Ans. When multiple constructors are created in the class consisting of the same name with a difference in the constructor parameters, the process is known as constructor overloading. The compiler distinguishes between the different types of constructors Depending upon the number of parameters and their corresponding types.

Q. Define an object. 

Ans. Object provides an internal state where the methods can be operated. They are executed in a runtime environment, where the state of the respective object is stored in the field and the characteristics and behaviour is shown via methods.

Q. Define final, finally and finalize keywords. 

Ans. Final: The final keyword is used to define restriction for classes, variables or methods, to limit the reach of the users.

Finally: Execution of the codes takes place in the respective block, irrespective of handling of exceptions.

Finalize: The finalize method is called to implement the clean-up activity.

Q. Define Wrapper class in Java.

Ans. There are primitive data types present in Java, they have a respective class assigned to them. These classes are used to convert the java primitives into the reference types (objects). They “wrap” the primitive data type into an object of that class, hence the name wrapper classes.

Q. Define Singleton class. 

Ans. The main function is to control object creation, they limit the number to one. If the situation changes, they also allow the flexibility to create more objects.

Q. Explain the function of constructors in Java.

Ans. Initialization of an object is done with the help of constructers. They are a block of codes having the same name as that of the class. When an object is created, they are automatically called and have no return type.

The two types of constructors are:

Default Constructor: They take no inputs. They are no argument constructors. When no constructor is defined by the user, these constructors are created by default. They initialize the instance variables with the default values. Mainly used for object creation.

Parameterized Constructor: The ability to initialize the instance variables with the provided values. These constructors take the arguments defined by the user.

Q. Define Exception.

Ans. Any error or problem can hinder the process of execution of the application. To point out the exact problem instead of searching and re-writing the whole code, exceptions are generated. Handlers identify the occurring problem and generate the respective exception. These handlers are allocated along the thread’s method invocation stack.

Q. What are super keywords used for?

Ans. The hidden fields, overridden methods and attributes of the parent class can be accessed with the help of super keyword

The conditions in which this keyword can be executed:

The case when the member names of the class and its child subclasses are the same, the user can use the method to access data members of the parent class.

The user can call the default and parameterized constructor of the parent class inside the child class by executing the method.

The case when the child classes have overridden the parent class, the user can access the parent class by using the method.

Q. State the main objective of garbage collection. 

Ans. The main objective of this process is to free up the memory space occupied by the unnecessary and unreachable objects during the Java program execution by deleting those unreachable objects, but availability of sufficient memory for the program execution is not guaranteed by the garbage collection method.

Q. Define and explain Array list and vector in Java.

Ans. Array list in java is used as an iterator for traversing an array list. As they are not synchronized, they are executed at short time intervals. When an element is added to the array list the size of the array increases by 50%. Array list does not have the ability to define the increment size.

Enumeration and iterator for traversing can be performed with the execution of vectors in Java. As they are synchronized, they take time to execute. When an element is added to the array list, the size of the array is doubled on default. Vector list has the ability to define the increment size.

Q. Explain Stack memory and Heap memory in Java.

Ans. Stack memory can be used by only one thread of execution and can’t be accessed by other threads. They make use of LIFO manners to execute free-memory actions. Stack memory lives only for the duration of execution of the respective thread. They only store local primitive data and perform the process of referring the variables to the object in the heap space.

All the parts of the application make use of the Heap memory. The objects stored in the heap are available for global access. Each object has its own respective memory management. Heap memory exists from the start to the end of the application execution. The objects created are always secured in heap space.

Q. Define and explain Package in Java?

Ans. To make the process of execution easy, related classes and interfaces are stacked together, these stacked-bundles are known as Packages in Java. As these bundles contain similar or related classes and interfaces, developers are able to easily modularize the code and reuse it. Other classes can easily import the code in the respective packages and reuse it. Name clashes are avoided as they are stored in organised hierarchical structure, hence it becomes easy to locate and control the various related classes. Also, hidden classes can be stored and executed within these packages.

These are some of the java interview questions that are frequently asked in a Java interview. The interview may also ask you to write a few demo programs to grade your practical application of the theory. Well, that’s all from our side, study these java interview questions, revise the topics related to these questions and practise a few demo codes, and you are ready to ace the interview. All the best!


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