Java is a programming language used widely in the distributed environment of internet. Smartphones, edge devices, and Internet of Things (IoT) are the three main areas where Java programming is heavily used. Java applications are designed to run on individual PC, servers, devices, and as a web application in a module. Software developers prefer using Java due to its portability in a network. Also, it is an object-oriented programming language, its code is robust, data stay secure, applets offer flexibility and easy to learn by the aspirants. Java comes with three key platforms, Java SE, Java EE, Java ME.
Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.
Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.
An object consists of methods and classes that depict its state and perform operations. A Java program contains a lot of objects instructing each other their jobs. This concept is part of core Java.
Java encapsulates codes in various classes that define new data types. These new data types are used to create objects.
Java includes the feature of inheritance which is an object-oriented programming concept. Inheritance lets a derived class inherit the methods of a base class.
When a Java program contains more than one method with the same name but with different properties, then it is called method overloading.
Overloading refers to the case of having two methods of the same name but different properties; whereas, overriding occurs when there are two methods of the same name and properties, but one is in the child class and the other is in the parent class.
Yes, we can execute any code, even before the main method. We will be using a static block of code in the class when creating the objects at load time of the class. Any statements within this static block of code will get executed at once while loading the class, even before the creation of objects in the main method.
final is a special keyword in Java that is used as a non-access modifier. A final variable can be used in different contexts such as:
When the final keyword is used with a variable then its value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.
When a method is declared final then it can’t be overridden by the inheriting class.
When a class is declared as final in Java, it can’t be extended by any subclass class but it can extend other class.
In Java, constructor chaining is the process of calling one constructor from another with respect to the current object. Constructor chaining is possible only through legacy where a subclass constructor is responsible for invoking the superclass’ constructor first. There could be any number of classes in the constructor chain. Constructor chaining can be achieved in two ways:
The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:
In Java, Map is an interface of Util package which maps unique keys to values. The Map interface is not a subset of the main Collection interface and thus it behaves little different from the other collection types. Below are a few of the characteristics of Map interface:
Abstraction refers to the quality of dealing with ideas rather than events. It basically deals with hiding the details and showing the essential things to the user. Thus you can say that abstraction in Java is the process of hiding the implementation details from the user and revealing only the functionality to them. Abstraction can be achieved in two ways:
Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.
Two main restrictions are applied to the static methods.
There are the following uses of this keyword.
Usually, we call enumeration as an enum. An enumeration is an interface containing methods for accessing the original data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.
JAXP: It stands for Java API for XML Processing. This provides a common interface for creating and using DOM, SAX, and XSLT APIs in Java regardless of which vendor’s implementation is actually being used.
JAXB: It stands for Java API for XML Binding. This standard defines a system for a script out of Java objects as XML and for creating Java objects from XML structures.
A vector class provides the ability to execute a growable array of objects. A vector proves to be very useful if you don’t know the size of the array in advance or if we need one that can change the size over the lifetime of a program.
Implicit objects, also called pre-defined variables, are created by the JSP engine inside the service method so that it can be accessed directly without being declared explicitly.
Object cloning in Java is the process of creating an exact copy of an object. It basically means the ability to create an object with a similar state as the original object. To achieve this, Java provides a method clone() to make use of this functionality. This method creates a new instance of the class of the current object and then initializes all its fields with the exact same contents of corresponding fields. To object clone(), the marker interface java.lang.Cloneable must be implemented to avoid any runtime exceptions. One thing you must note is Object clone() is a protected method, thus you need to override it.
Copy constructor is a member function that is used to initialize an object using another object of the same class. Though there is no need for copy constructor in Java since all objects are passed by reference. Moreover, Java does not even support automatic pass-by-value.
Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.
There are the following differences between this and super keyword.
The main difference between the final method and abstract method is that the abstract method cannot be final as we need to override them in the subclass to give its definition.
The average unemployment rate is estimated to raise in the job marketplace, however, the requirement of Java developers, Java programmers, and java engineers are at the higher side. Candidates with in-depth knowledge and skill-based experience in java technology are expected to take the full advantage of technological expansion. Besides, java has occupied the topmost place in the arena of programming languages as it provides a range of features to the developers. A java programmer is expected to enjoy minimum salary of 40, 000 dollars per annum. Although the salary of an experienced java developer can escalate to double the figure mentioned before. The salaries are completely dependent upon the location, business, and the company’s requirements.
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