Java Interview Questions iii. 4 Java Collections. 8. What are the basic interfaces of Java Collections Framework? 8. Why. Are you a fresher who wants to crack the Java Interview Questions? Here we offer you some of the major Core Java Interview Questions for. Basic Java Interview Questions and Answers for Freshers or Experienced Pdf. This interview section questions contains a brief introduction to the Java.
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There is the list of core java interview questions. If there is any core java interview question that has been asked to you, kindly post it in the ask question. CORE JAVA INTERVIEW. QUESTIONS AND ANSWERS. Table of Contents. 1) what are static blocks and static initalizers in Java?. Dear readers, these Java Interview Questions have been designed especially to any particular question during your interview, normally questions start with some basic concept of If you are fresher then interviewer does not expect you .
What is a numeric promotion? Answer: Numeric promotions of a numeric operator are used for the conversion of the operands into a common type. In order to perform calculation 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. The int values are converted to long values, and the long and float values are converted to double values.
What's the base class in Java from which all classes are derived? Ans: java. Can main method in Java can return any data? Ans: In java, main method can't return any data and hence, it's always declared with a void return type. What are Java Packages? What's the significance of packages? Ans: In Java, package is a collection of classes and interfaces which are bundled together as they are related to each other.
Use of packages helps developers to modularize the code and group the code for proper re-use. Once code has been packaged in Packages, it can be imported in other classes and used. Can we declare a class as Abstract without having any abstract method?
Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn't have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error. What's the difference between an Abstract Class and Interface in Java? Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers public, private etc with or without concrete implementation.
Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn't require implementation of all the methods of its super class.
A class can implement multiple interfaces but it can extend only one abstract class. What are the performance implications of Interfaces over abstract classes?
Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces. Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface. Does Importing a package imports its sub-packages as well in Java?
Ans: In java, when a package is imported, its sub-packages aren't imported and developer needs to import them separately if required. For example, if a developer imports a package university. To load the classes from its sub-package say department , developer has to import it explicitly as follows: Import university. Can we declare the main method of our class as private?
Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won't get any compilation error however, it will not get executed and will give a runtime error. How can we pass argument to a function by reference instead of pass by value? Ans: In java, we can pass argument to a function only by value and not by reference.
How an object is serialized in java? Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in byte stream.
When we should use serialization? Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object's state is saved and converted into byte stream.
The byte stream is transferred over the network and the object is re-created at destination.
Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.
Is there any way to skip Finally block of exception even if some exception occurs in the exception block? Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block: System.
When the constructor of a class is invoked? Ans: The constructor of a class is invoked every time an object is created with new keyword. For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times. Can a class have multiple constructors? Ans: Yes, a class can have multiple constructors with different parameters.
Which constructor gets used for object creation depends on the arguments passed while creating the objects. Can we override static methods of a class? Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation not at runtime.
Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code. In the below example, what will be the output? Is String a data type in java?
Ans: String is not a primitive data type in java. When a string is created in java, it's actually an object of Java. String class that gets created. After creation of this string object, all built-in methods of String class can be used on the string object.
In the below example, how many String Objects are created?
String class are created. Why Strings in Java are called as Immutable? Ans: In java, string objects are called immutable as once value has been assigned to a string, it can't be changed and if changed, a new object is created. In below example, reference str refers to a string object having value "Value one".
What's the difference between an array and Vector? Ans: An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types. What is multi-threading? Ans: Multi threading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share same process stack and running in parallel.
It helps in performance improvement of any program. Why Runnable Interface is used in Java? Ans: Runnable interface is used in java for implementing multi threaded applications. Runnable interface is implemented by a class to support multi threading. What are the two ways of implementing multi-threading in Java? Ans: Multi threaded applications can be developed in Java by using any of the following two methodologies: 1.
By using Java. Runnable Interface. Classes implement this interface to enable multi threading. There is a Run method in this interface which is implemented. By writing a class that extend Java. Thread class. When a lot of changes are required in data, which one should be a preference to be used? String or StringBuffer? Ans: Since StringBuffers are dynamic in nature and we can change the values of StringBuffer objects unlike String which is immutable, it's always a good choice to use StringBuffer when data is being changed too much.
If we use String in such a case, for every data change a new String object will be created which will be an extra overhead. What's the purpose of using Break in each case of Switch Statement?
Ans: Break is used after each case except the last one in a switch so that code breaks after the valid case and doesn't flow in the proceeding cases too. If break isn't used after each case, all cases after the valid case also get executed resulting in wrong results. How garbage collection is done in Java? Ans: In java, when an object is not referenced any more, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.
How we can execute any code even before main method? Ans: If we want to execute any statements before even creation of objects at load time of class, we can use a static block of code in the class. Any statements inside this static block of code will get executed once at the time of loading the class even before creation of objects in the main method. Can a class be a super class and a sub-class at the same time? Private: — the methods and fields can be only accessed from the same class to which they belong.
JVM Java Virtual Machine : Java virtual machine is actually an abstract machine which provides a runtime environment in which the java bytecode gets executed. A JVM performs some main tasks such as- loading, verifying and executing the code. It also provides a runtime environment.
It is implemented by JVM. It consists of certain libraries which are used by JVM at runtime. What is early binding and late binding in Java? The process of the connecting function call with function body is called binding. When this binding is done at compile time, it is called early binding. Early binding is static binding it is done when the type of an object is determined at compile time. On the hand, late binding is done at runtime.
Late binding is dynamic binding because it is done at runtime by the compiler itself. How dead lock situation occurs in java and how you can identify it? Deadlock is a situation that occurs when the same resource is required by two processes.
This situation occurs in multithreading when one thread holds a resource which is required by the second thread and second thread holds a resource which is required by the first thread. Why object class is super class for every class in java? After creating the object JVM internally calls the toString method to generate the indirect address of the object.
To string method is present inside object class, which is defined as non-static. So that object class makes it as a superclass for every class in java. Outline the major features of Java. The major features of Java are listed below: — Object-oriented: — Java language is based on object-oriented programming.
In this, the class and the methods describe the state and behavior of an object. The programming is done by relating a problem with the real world object. Portable: — the conversion of java program into Java bytecodes take place. This helps in running the program in any environment or platform without any dependency. The memory leaks are prohibited with the help of automatic garbage collection in java.
Interpreted: — the java code is converted into bytecodes by the java compiler which is then interpreted and executed by the Java interpreter or a just-in-time compiler JIT. A custom exception is also known as a user-defined exception are derived classes of Java Exception classes.
In order to create your Custom exception in Java following point must be taken care of. All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
If you want to write a runtime exception, you need to extend the RuntimeException class. If age is less than 18 years than it throws an InvalidAgeException.