Java Interview Questions For Freshers
- 1. JAVA INTERVIEW QUESTIONS FOR FRESHERS Why Java is platform independent? Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system. Why Java is not 100%Object-oriented? Java is not 100% Object-oriented becauseit makes use of eight primitive data types such as boolean, byte, char, int, float, double, long, short which are not objects. What are constructors in Java? In Java, constructorrefers to a block of codewhich is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created. There are two types of constructors: 1. Default Constructor: In Java, a default constructoris the one which does not take any inputs. In other words, default constructors are the no argument constructors which will be created by default in case you no other constructoris defined by the user. Its main purposeis to initialize the instance variables with the default values. Also, it is majorly used for object creation. 2. ParameterizedConstructor: The parameterized constructorin Java, is the constructorwhich is capable of initializing the instance variables with the provided values. In other words, the constructors which take the arguments are called parameterized constructors. Why pointers are not used in Java? Java doesn’tuse pointers because they are unsafe and increases the complexity of the program. Since, Java is known for its simplicity of code, adding the conceptof pointers will be contradicting. Moreover, since JVM is responsible for implicit memory allocation, thus in order to avoid direct access to memory by the user, pointers are discouraged in Java.
- 2. What is the difference between an array and an array list? Array ArrayList Cannot contain values of different data types Can contain values of different data types. Size must be defined at the time of declaration Size can be dynamically changed Need to specify the index in order to add data No need to specify the index Arrays are not type parameterized Arraylists are type Arrays can contain primitive data types as well as objects Arraylists can contain only objects, no primitive data types are allowed What is a Map in Java? 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: 1. Map doesn’tcontain duplicate keys. 2. Each key can map at max one value. What is a classloaderin Java? The Java ClassLoader is a subsetof 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: 1. Bootstrap ClassLoader 2. Extension ClassLoader 3. System/Application ClassLoader
- 3. What are access modifiers in Java? In Java, access modifiers are special keywords which are used to restrict the access of a class, constructor, data member and method in another class. Java supports four types of access modifiers: 1. Default 2. Private 3. Protected 4. Public Define a Java Class. A class in Java is a blueprint which includes all your data. A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class. 1 2 3 class Abc { member variables // class body methods} What is an objectin Java and how is it created? An object is a real-world entity that has a state and behavior. An object has three characteristics: 1. State 2. Behavior 3. Identity An object is created using the ‘new’ keyword. For example: ClassName obj= new ClassName();
- 4. OOPS Java Interview Questions What is run time polymorphism and compile time polymorphism? Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface. In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. Let’s take a look at the example below to understand it better. What is abstractionin Java? 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 ofhiding the implementation details from the user and revealing only the functionality to them. Abstraction can be achieved in two ways: 1. Abstract Classes (0-100% of abstraction can be achieved) 2. Interfaces (100% of abstraction can be achieved) What do you mean by an interface in Java? An interface in Java is a blueprint of a class or you can say it is a collection of abstract methods and static constants. In an interface, each method is public and abstract but it does not contain any constructor. Thus, interface basically is a group of related methods with empty bodies. Example: public interface Animal { public void eat(); public void sleep(); public void run(); }
- 5. What is inheritance in Java? Inheritance in Java is the conceptwhere the properties of one class can be inherited by the other. It helps to reuse the codeand establish a relationship between different classes. Inheritance is performed between two types of classes: 1. Parent class (Super or Base class) 2. Child class (Subclass or Derived class) A class which inherits the properties is known as Child Class whereas a class whose properties are inherited is known as Parent class. What are the different types of inheritance in Java? Java supports four types of inheritance which are: 1. Single Inheritance: In single inheritance, one class inherits the properties of another i.e there will be only one parent as well as one child class. 2. Multilevel Inheritance: When a class is derived from a class which is also derived from another class, i.e. a class having more than one parent class but at different levels, such type of inheritance is called Multilevel Inheritance. 3. HierarchicalInheritance: When a class has more than one child classes (subclasses)or in other words, more than one child classes have the same parent class, then such kind of inheritance is known as hierarchical. 4. Hybrid Inheritance: Hybrid inheritance is a combination of two or more types of inheritance. What is method overloading and method overriding? Method Overloading : In Method Overloading, Methods of the same class shares the same name but each method must have a different number of parameters or parameters having different types and order. Method Overloading is to “add”or “extend” more to the method’s behavior. It is a compile-time polymorphism. The methods must have a different signature. It may or may not need inheritance in Method Overloading.
- 6. Method Overriding: In Method Overriding, the subclass has the same method with the same name and exactly the same number and type of parameters and same return type as a superclass. Method Overriding is to “Change” existing behavior of the method. It is a run time polymorphism. The methods must have the same signature. It always requires inheritance in Method Overriding. What is multiple inheritance? Is it supported by Java? If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes. The problem with multiple inheritance is that if multiple parent classes have the same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class. Therefore, Java doesn’tsupportmultiple inheritance. The problem is commonly referred to as Diamond Problem. In case you are facing any challenges with these java interview questions, please comment on your problems in the section below. What is encapsulationin Java? Encapsulation is a mechanism where you bind your data(variables) and code(methods)together as a single unit. Here, the data is hidden from the outer world and can be accessed only via current class methods. This helps in protecting the data from any unnecessary modification. We can achieve encapsulation in Java by: Declaring the variables of a class as private. Providing public setter and getter methods to modify and view the values of the variables.
- 7. Servlets – Java Interview Questions What is RequestDispatcher? RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response. There are two methods defined in this interface: 1.void forward() 2.void include() What is the life-cycle of a servlet? There are 5 stages in the lifecycle of a servlet: 1. Servlet is loaded 2. Servlet is instantiated 3. Servlet is initialized 4. Service the request 5. Servlet is destroyed . What are the different methods of sessionmanagementin servlets? Some of the common ways of session management in servlets are: 1. User Authentication 2. HTML Hidden Field 3. Cookies 4. URL Rewriting 5. Session Management API
- 8. JDBC – Java Interview Questions What is JDBC Driver? JDBC Driver is a software componentthat enables java application to interact with the database. There are 4 types of JDBC drivers: 1. JDBC-ODBC bridge driver 2. Native-API driver (partially java driver) 3. Network Protocoldriver (fully java driver) 4. Thin driver (fully java driver) What are the JDBC API components? The java.sql package contains interfaces and classes for JDBC API. Interfaces: Connection Statement PreparedStatement ResultSet ResultSetMetaData DatabaseMetaData CallableStatement etc. Classes: DriverManager Blob Clob Types SQLException etc.
- 9. What is the role of JDBC DriverManagerclass? The DriverManager class manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection. What is JDBC Connectioninterface? The Connection interface maintains a sessionwith the database. It can be used for transaction management. It provides factory methods that returns the instance of Statement, PreparedStatement, CallableStatement and DatabaseMetaData. What are the steps to connectto a database in java? Registering the driver class Creating connection Creating statement Executing queries Closing connection What is JDBC DatabaseMetaDatainterface? The DatabaseMetaData interface returns the information of the database such as username, driver name, driver version, number of tables, number of views etc. Spring Framework – Java Interview Questions List some of the important annotations in annotation-basedSpring configuration. The important annotations are: @Required @Autowired @Qualifier @Resource @PostConstruct @PreDestroy
- 10. What is autowiring in Spring? What are the autowiring modes? Autowiring enables the programmer to inject the bean automatically. We don’t need to write explicit injection logic. The autowiring modes are given below: No, by name, by type, constructor Name the types of transactionmanagement that Spring supports. Programmatic transaction management Declarative transaction management JSP – Java Interview Questions What are the different tags provided in JSTL? There are 5 type of JSTL tags. 1. core tags 2. sqltags 3. xml tags 4. internationalization tags 5. functions tags How to disable sessionin JSP? 1. <%@ page session=“false” %> Exception and Thread Java Interview Questions How can you handle Java exceptions? There are five keywords used to handle exceptions in Java: 1. try 2. catch 3. finally 4. throw 5. throws
- 11. What are the important methods of Java Exception Class? String getMessage() String getLocalizedMessage() Synchronized Throwable getCause() String toString() void printStackTrace() What is OutOfMemoryError in Java? OutOfMemoryError is the subclass of java.lang.Error which generally occurs when our JVM runs out of memory. What are the two ways to create a thread? In Java, threads can be created in the following two ways:- By implementing the Runnable interface. By extending the Thread