Java

Question 1

Java What is the difference between an interface and an Abstract class?

Answer 1

Interface can only declare constants and methods. However, interface can have default behavior.

Question 2

Java Describe synchronization in respect to multithreading.

Answer 2

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for the one thread to modify a shared variable while another thread is in the process if using or updating same share variable. This usually leads to significant errors.

Question 3

Java Thread usage.

Answer 3

The thread could be implemented by using runnable interface or by inheriting from the thread class. The former is more advance advantageous, ‘cuase when you are going for multiple inheritance..the only interface can help.

Question 4

Java What is a constructor.

Answer 4

A member method using to create the object. It has the same name as the class. invoked using the new operator.

Question 5

What is an Iterator?

Answer 5

Allows to iterate through objects, collections of objects. Use Java.util.Iterator. It is best not to modify anything. Just read it only.

Question 6

What is a public variable?

Answer 6

Public class is visible in

Question 7

What is Java?

Answer 7

A high-level programming language developed by Sun Microsystems. Java was originally called OAK, and was designed for handheld devices and set-top boxes. Oak was unsuccessful so in 1995 Sun changed the name to Java and modified the language to take advantage of the burgeoning World Wide Web.`

Question 8

What are the main features of java?

Answer 8

The main features of java are ■Compiled and Interpreted ■Object oriented ■Robust and secure ■Type safe ■High Performance.

Question 9

What is the Java API?

Answer 9

The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.

Question 10

What is the Java Virtual Machine (JVM)?

Answer 10

The Java Virtual Machine is software that can be ported onto various hardware-based platforms.

Question 11

Java What is variables and then types?

Answer 11

Variables is an identifier that denotes a storage location used to store a data values.unlike constants that remain unchanged during the execution of a program, a variable may takes different values at different times during the execution of the program. ■Instance variables ■Class variables ■Local variable ■Parameters

Question 12

Java What is dot operator?

Answer 12

The dot operator(.) is used to access the instance variables and methods of class objects.It is also used to access classes and sub-packages from a package. Examples : Person1.age ———> Reference to the variable age

Question 13

Java Define strings?

Answer 13

Strings represent a sequence of characters.The easiest way to represent a sequence of characters in java is by using a character array.

Question 14

Java What is serialization?

Answer 14

Serialization is the process of converting a objects into a stream of bytes.

Question 15

Java What are different types of access modifiers?

Answer 15

Access specifiers are keywords that determine the type of access to the member a class. ■Public ■Protected ■Private ■Default

Question 16

Java What is the Collection interface?

Answer 16

The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.

Question 17

Java What must a class do to implement an interface?

Answer 17

The class must provide all of the methods in the interface and identify the interface in its implements clause.

Question 18

Java What is the Collections API?

Answer 18

The Collections API is a set of classes and interfaces that support operations on collections of objects.

Question 19

Java What is an array?

Answer 19

Array is a group of related data items that share a common name.For instance, we can define an array name salary to represent a set of salaries of a group of employees. Examples : salary[10]

Question 20

java What is list iterator?

Answer 20

The List and Set collections provide iterators, which are objects that allow going over all the elements of a collection in sequence. The java.util.Iterator interface provides for one-way traversal and java.util.ListIterator is an iterator for lists that allows the programmer to traverse the list in either direction (i.e. forward and or backward) and modify the list during iteration.

Question 21

Java What is the main difference between a String and a StringBuffer class?

Answer 21

String is immutable : you can’t modify a string object but can replace it by creating a new instance. Creating a new instance is rather expensive. StringBuffer is mutable : use StringBuffer or StringBuilder when you want to modify the contents. StringBuilder was added in Java 5 and it is identical in all respects to StringBuffer except that it is not synchronized,which makes it slightly faster at the cost of not being thread-safe.

Question 22

Java When to use serialization?

Answer 22

A common use of serialization is to use it to send an object over the network or if the state of an object needs to be persisted to a flat file or a database.

Question 23

Java What is the main difference between shallow cloning and deep cloning of objects?

Answer 23

Java supports shallow cloning of objects by default when a class implements the java.lang.Cloneable interface. Deep cloning through serialization is faster to develop and easier to maintain but carries a performance overhead.

Question 24

Java What are wrapper classes?

Answer 24

primitive data types may be converted into object types by using the wrapper classes contained in the java.lang package. Exampes : int, float, long, char, double

Question 25

Java What is the difference between an instance variable and a static variable?

Answer 25

variable per JVM per class loader.When a class is loaded the class variables are initialized. Instance variables are non-static and there is one occurrence of an instance variable in each class instance.Also known as a member variable or a field.

Question 26

Java Where and how can you use a private constructor?

Answer 26

Private constructor is used if you do not want other classes to instantiate the object and to prevent subclassing.The instantiation is done by a public static method (i.e. a static factory method) within the same class.

Question 27

Java What is type casting?

Answer 27

Type casting means treating a variable of one type as though it is another type. Examples : int m = 5; byte n =i;

Question 28

Java What is a user defined exception?

Answer 28

User defined exceptions may be implemented by defining a new exception class by extending the Exception class.

Question 29

Java What is an instanceof operator?

Answer 29

Instanceof is an object reference operator and returns true if the object on the left-hand side is an instance of the glass given to the right hand side.This operator allows to determine whether the object belongs to a particular class or not.

Question 30

Java What are runtime exceptions?

Answer 30

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

Question 31

Java What is the difference between an interface and an abstract class?

Answer 31

An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

Question 32

Java What is a package?

Answer 32

A package is a namespace that organizes a set of related classes and interfaces.The classes contained in the packages of other programs can be easily reused.Packages, classes can be unique compared with classes in other packages.That is, two classes in two different packages can have the same name.

Question 33

Java Why do threads block on I/O?

Answer 33

Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

Question 34

Java What is the List interface?

Answer 34

The List interface provides support for ordered collections of objects.

Question 35

Java What is the Vector class?

Answer 35

The Vector class provides the capability to implement a growable array of objects.

Question 36

Java What is the base class of all classes?

Answer 36

java.lang.Object

Question 37

Java What is the importance of static variable?

Answer 37

static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.

Question 38

Java What is the difference between a while statement and a do while statement?

Answer 38

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do whilestatement will always execute the body of a loop at least once.

Question 39

Java Describe life cycle of thread?

Answer 39

A thread is similiar to a program that has a single flow of control.A thread is a execution in a program. The life cycle of threads are ■Newborn state ■Runnable state ■Running state ■Blocked state ■Dead state

Question 40

Java What is an Applets?

Answer 40

Applets are small java programs that are primarily used in Internet computing. They can be transported over the internet from one computer to another and run using the Applet Viewer or any web browser that supports java.

Question 41

Java What are wrapped classes?

Answer 41

Wrapped classes are classes that allow primitive types to be accessed as objects.

Question 42

Java What is the difference between an if statement and a switch statement?

Answer 42

The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.s

Question 43

Java What modifiers are allowed for methods in an Interface?

Answer 43

Only public and abstract modifiers are allowed for methods in interfaces.

Question 44

Java What is the difference between method overriding and overloading?

Answer 44

Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments.

Question 45

Java What do you mean by polymorphism?

Answer 45

Polymorphism means the ability to take more than one form. fro example, an operation may exhibit behaviour in different instances. The behaviour depends upon the types of data used in the operatiom.

Question 46

What is the difference between an abstract class and an interface?

Answer 46

Abstract class Interface - Have executable methods and abstract methods.Can only subclass one abstract class Interface - Have no implementation code. All methods are abstract.A class can implement any number of interfaces.

Question 47

Java What is the purpose of finalization?

Answer 47

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Question 48

Java What is the difference between a break statement and a continue statement?

Answer 48

Break statement results in the termination of the statement to which it applies (switch, for, do, or while). A Continue statement is used to end the current loop iteration and return control to the loop statement.

Question 49

Java When is a method said to be overloaded and when is a method said to be overridden?

Answer 49

Overloading deals with multiple methods in the same class with the same name but different method signatures. Overriding deals with two methods, one in the parent class and the other one in the child class and has the same name and signatures.

Question 50

Java How is final different from finally and finalize()?

Answer 50

■Final - constant declaration. ■The finally block always executes when the try block exits, except System.exit(0) call. ■finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.

Question 51

Java What is Byte Code?

Answer 51

All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.

Question 52

Java What is the difference between error and an exception?

Answer 52

Exception means When a method encounters an abnormal condition (an exception condition) that it can’t handle itself, it may throw an exception. ssError mens system doesn’t handle.For example:Overflow,Out of memory.

Question 53

What if the main method is declared as private?

Answer 53

When a method is declared as private, the program compiles properly but it will give runtime error Main method not “public„.

Question 54

Java What type of parameter passing does Java support?

Answer 54

In Java the arguments are always passed by value.

Question 55

Java What is singleton?

Answer 55

It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class.

Question 56

Java What is Locale?

Answer 56

A Locale object represents a specific geographical, political, or cultural region.

Question 57

Java What is the difference between constructors and normal methods?

Answer 57

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Question 58

Java What is the difference between static and non-static variables?

Answer 58

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Question 59

Java What is the difference between java and c++?

Answer 59

■Java is a true object - oriented language while c++ is basically c with object-oriented extension. ■C++ supports multiple inheritence but Java provides interfaces in case of multiple inheritence. ■Java does not support operator overloading. ■Java does not have template classes as in c++. ■java does not use pointers.

Question 60

Java What a private modifier?

Answer 60

Private variables or methods may be used only by an instance of the same class that declares the variables or method. A private feature may only be accessed by the class that owns the feature.

Question 61

Java What is the protected variable or method?

Answer 61

Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in the a differenct package from the class that owns the protected feature.

Question 62

When to use interface in Java.

Answer 62

Interface is best choice for Type declaration or defining contract between multiple parties. If multiple programmer are working in different module of project they still use each others API by defining interface and not waiting for actual implementation to be ready. This brings us lot of flexibility and speed in terms of coding and development. Use of Interface also ensures best practices like “programming for interfaces than implementation” and results in more flexible and maintainable code. Though interface in Java is not the only one who provides higher level abstraction, you can also use abstract class but choosing between Interface in Java and abstract class is a skill

Question 63

Java What is the difference between creating String as new() and literal?

Answer 63

When we create string with new() Operator, it’s created in heap and not added into string pool while String created using literal are created in String pool itself which exists in PermGen area of heap. String s = new String(“Test”); does not put the object in String pool , we need to call String.intern() method which is used to put them into String pool explicitly. its only when you create String object as String literal e.g. String s = “Test” Java automatically put that into String pool.

Question 64

Java How does substring () inside String works?

Answer 64

Another good Java interview question, I think answer is not sufficient but here it is “Substring creates new object out of source string by taking a portion of original string”. see my post How SubString works in Java for detailed answer of this Java question.

Question 65

Java Which two method you need to implement for key Object in HashMap ?

Answer 65

In order to use any object as Key in HashMap, it must implements equals and hashcode method in Java. Read How HashMap works in Java for detailed explanation on how equals and hashcode method is used to put and get object from HashMap. You can also see my post 5 tips to correctly override equals in Java to learn more about equals.

Question 66

Java Where does these two method comes in picture during get operation?

Answer 66

This core Java interview question is follow-up of previous Java question and candidate should know that once you mention hashCode, people are most likely ask How its used in HashMap. See How HashMap works in Java for detailed explanation.

Question 67

Java How do you handle error condition while writing stored procedure or accessing stored procedure from java?

Answer 67

This is one of the tough Java interview question and its open for all, my friend didn’t know the answer so he didn’t mind telling me. my take is that stored procedure should return error code if some operation fails but if stored procedure itself fail than catching SQLException is only choice.

Question 68

Java What is difference between Executor.submit() and Executer.execute() method ?

Answer 68

This Java interview question is from my list of Top 15 Java multi-threading question answers, Its getting popular day by day because of huge demand of Java developer with good concurrency skill. Answer of this Java interview question is that former returns an object of Future which can be used to find result from worker thread)

Question 69

Java What is the difference between factory and abstract factory pattern?

Answer 69

Abstract Factory provides one more level of abstraction. Consider different factories each extended from an Abstract Factory and responsible for creation of different hierarchies of objects based on the type of factory. E.g. AbstractFactory extended by AutomobileFactory, UserFactory, RoleFactory etc. Each individual factory would be responsible for creation of objects in that genre.

Question 70

What is garbage collection? What is the process that is responsible for doing that in java?

Answer 70

Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process

Question 71

What kind of thread is the Garbage collector thread?

Answer 71

It is a daemon thread.

Question 72

What is a daemon thread?

Answer 72

These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.

Question 73

What is reflection?

Answer 73

Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.

Question 74

What are the approaches that you will follow for making a program very efficient?

Answer 74

By avoiding too much of static methods avoiding the excessive and unnecessary use of synchronized methods Selection of related classes based on the application (meaning synchronized classes for multiuser and non-synchronized classes for single user) Usage of appropriate design patterns Using cache methodologies for remote invocations Avoiding creation of variables within a loop and lot more.

Question 75

When you think about optimization, what is the best way to findout the time/memory consuming process?

Answer 75

Using profiler

Question 76

What is the difference between JDK and JRE?

Answer 76

Java Development Kit (JDK) is the most widely used Java Software Development Kit. Java Runtime Environment (JRE) is an implementation of the Java Virtual Machine which executes Java programs

Question 77

What is the difference between equals() and “==” ?

Answer 77

Answer: Equals is intended to check logical equality and == checks if both references point to same object. (Thanks Sandeep) a == b; // Compares references, not values. a.equals(b); // Compares values for equality. Source

Question 78

When will you use Comparator and Comparable interfaces?

Question 79

What is the difference between checked and unchecked exceptions?

Answer 79

In general, unchecked exceptions represent defects in the program (bugs), which are normally Runtime exceptions. Furthermore, checked exceptions represent invalid conditions in areas outside the immediate control of the program.

Question 80

What is the difference between final, finally and finalize?

Answer 80

“final” is the keyword to declare a constant AND prevents a class from producing subclasses. (Thanks Tom Ellis) “finally” is a block of code that always executes when the try block is finished, unless System.exit() was called. finalize() is an method that is invoked before an object is discarded by the garbage collector

Question 81

Can you write critical section code for singleton?

Answer 81

This core Java question is followup of previous question and expecting candidate to write Java singleton using double checked locking. Remember to use volatile variable to make Singleton thread-safe.

Question 82

What is MVC Architecture?

Answer 82

The main aim of the MVC (Model View Control) architecture is to separate the
business logic and application data from the presentation
data to the user.

Here are the reasons why we should use the MVC design pattern.

1. They are resuable : When the problems recurs, there is
   no need to invent a new solution, we just have to follow the
   pattern and adapt it as necessary.
2. They are expressive: By using the MVC design pattern
   our application becomes more expressive.

1). Model: The model object knows about all the data that
need to be displayed. It is model who is aware about all the
operations that can be applied to transform that object. It
only represents the data of an application. The model
represents enterprise data and the business rules that
govern access to and updates of this data. Model is not
aware about the presentation data and how that data will be
displayed to the browser.

2). View : The view represents the presentation of the
application. The view object refers to the model. It uses
the query methods of the model to obtain the contents and
renders it. The view is not dependent on the application
logic. It remains same if there is any modification in the
business logic. In other words, we can say that it is the
responsibility of the of the view’s to maintain the
consistency in its presentation when the model changes.

3). Controller: Whenever the user sends a request for
something then it always go through the controller. The
controller is responsible for intercepting the requests from
view and passes it to the model for the appropriate action.
After the action has been taken on the data, the controller
is responsible for directing the appropriate view to the
user. In GUIs, the views and the controllers often work
very closely together.

Question 83

What is an abstract class?

Answer 83

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Question 84

What is static in java?

Answer 84

Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object.

A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method with a nonstatic method. In other words, you can’t change a static method into an instance method in a subclass.

Question 85

What is final?

Answer 85

final class can’t be extended ie., final class may not be subclassed. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant).

Question 86

What environment variables do I need to set on my machine in order to be able to run Java programs?

Answer 86

CLASSPATH and PATH are the two variables.

Question 87

Can an application have multiple classes having main method?

Answer 87

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

Question 88

Do I need to import java.lang package any time? Why ?

Answer 88

No. It is by default loaded internally by the JVM.

Question 89

What are different types of inner classes?

Answer 89

1. Nested top-level classes
2. Member classes
3. Local classes
4. Anonymous classes

**Nested top-level classes**- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.
 Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.

**Local classes** - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
 more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.