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Java Interview Questions - Adhithya Srinivasan Edition
#1


Q: What do you know about Java?

A: Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX.
Q: What are the supported platforms by Java Programming Language?

A: Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc.
Q: List any five features of Java?

A: Some features include Object Oriented, Platform Independent, Robust, Interpreted, Multi-threaded
Q: Why is Java Architectural Neutral?

A: It’s compiler generates an architecture-neutral object file format, which makes the compiled code to be executable on many processors, with the presence of Java runtime system.
Q: How Java enabled High Performance?

A: Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode, which is a program that contains instructions that must be interpreted into instructions that can be sent directly to the processor.
Q: Why Java is considered dynamic?

A: It is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.
Q: What is Java Virtual Machine and how it is considered in context of Java’s platform independent feature?

A: When Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.
Q: List two Java IDE’s?

A: Netbeans, Eclipse, etc.
Q: List some Java keywords(unlike C, C++ keywords)?

A: Some Java keywords are import, super, finally, etc.
Q: What do you mean by Object?

A: Object is a runtime entity and it’s state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication.
Q: Define class?

A: A class is a blue print from which individual objects are created. A class can contain fields and methods to describe the behavior of an object.
Q: What kind of variables a class can consist of?

A: A class consist of Local variable, instance variables and class variables.
Q: What is a Local Variable

A: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and it will be destroyed when the method has completed.
Q: What is a Instance Variable

A: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded.
Q: What is a Class Variable

A: These are variables declared with in a class, outside any method, with the static keyword.
Q: What is Singleton class?

A: Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.
Q: What do you mean by Constructor?

A: Constructor gets invoked when a new object is created. Every class has a constructor. If we do not explicitly write a constructor for a class the java compiler builds a default constructor for that class.
Q: List the three steps for creating an Object for a class?

A: An Object is first declared, then instantiated and then it is initialized.
Q: What is the default value of byte datatype in Java?

A: Default value of byte datatype is 0.
Q: What is the default value of float and double datatype in Java?

A: Default value of float and double datatype in different as compared to C/C++. For float its 0.0f and for double it’s 0.0d
Q: When a byte datatype is used?

A: This data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.
Q: What is a static variable?

A: Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
Q: What do you mean by Access Modifier?

A: Java provides access modifiers to set access levels for classes, variables, methods and constructors. A member has package or default accessibility when no accessibility modifier is specified.
Q: What is protected access modifier?

A: Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.
Q: What do you mean by synchronized Non Access Modifier?

A: Java provides these modifiers for providing functionalities other than Access Modifiers, synchronized used to indicate that a method can be accessed by only one thread at a time.
Q: According to Java Operator precedence, which operator is considered to be with highest precedence?

A: Postfix operators i.e () [] . is at the highest precedence.
Q: Variables used in a switch statement can be used with which datatypes?

A: Variables used in a switch statement can only be a byte, short, int, or char.
Q: When parseInt() method can be used?

A: This method is used to get the primitive data type of a certain String.
Q: Why is String class considered immutable?

A: The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.
Q: Why is StringBuffer called mutable?

A: The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make alot of modifications to Strings of characters then StringBuffer should be used.
Q: What is the difference between StringBuffer and StringBuilder class?

A: Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.
Q: Which package is used for pattern matching with regular expressions?

A: java.util.regex package is used for this purpose.
Q: java.util.regex consists of which classes?

A: java.util.regex consists of three classes: Pattern class, Matcher class and PatternSyntaxException class.
Q: What is finalize() method?

A: It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.
Q: What is an Exception?

A: An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread's method invocation stack.
Q: What do you mean by Checked Exceptions?

A: It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
Q: Explain Runtime Exceptions?

A: It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
Q: Which are the two subclasses under Exception class?

A: The Exception class has two main subclasses : IOException class and RuntimeException Class.
Q: When throws keyword is used?

A: If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method's signature.
Q: When throw keyword is used?

A: An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.
Q: How finally used under Exception Handling?

A: The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
Q: What things should be kept in mind while creating your own exceptions in Java?

A: While creating your own exception:

   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.

   You want to write a runtime exception, you need to extend the RuntimeException class.

Q: Define Inheritance?

A: It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.
Q: When super keyword is used?

A: If the method overrides one of its superclass's methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field



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#2

Q: What is Polymorphism?

A: Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
Q: What is Abstraction?

A: It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.
Q: What is Abstract class

A: These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.
Q: When Abstract methods are used?

A: If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.
Q: What is Encapsulation?

A: It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.
Q: What is the primary benefit of Encapsulation?

A: The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.
Q: What is an Interface?

A: An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface



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#3


Q: Give some features of Interface?

A: It includes:

   Interface cannot be instantiated

   An interface does not contain any constructors.

   All of the methods in an interface are abstract.

Q: Define Packages in Java?

A: A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.
Q: Why Packages are used?

A: Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations, etc., easier.
Q: What do you mean by Multithreaded program?

A: A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
Q: What are the two ways in which Thread can be created?

A: Thread can be created by: implementing Runnable interface, extending the Thread class.
Q: What is an applet?

A: An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
Q: An applet extend which class?

A: An applet extends java.applet.Applet class.
Q: Explain garbage collection in Java?

A: It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.



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