Core Java Interview Questions

Q1.How could a Java program redirect error messages to a file while sending other messages to the system console?
A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

Q2.What's the difference between an interface and an abstract class?
A. 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.

Q3. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized methods.

Q4. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces? Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

Q5. How can you force garbage collection?
You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

Q6. How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example: Object a; Customer b; b = (Customer) a; When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

Q7. What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

Q8. Can you write a Java class that could be used both as an applet as well as an application?
A. Yes. Add a main() method to the applet.

Q9. What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

Q10. Can you call one constructor from another if a class has multiple constructors
A. Yes. Use this() syntax.
Q11. Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package; and is located in the file c:\dev\com\xyz\hr\ In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows: c:\>java

Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?
A.There's no difference, Sun Microsystems just re-branded this version.

Q14. What's the difference between a queue and a stack?
A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

Q15. Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

Q16. What comes to mind when you hear about a young generation in Java?
A. Garbage collection.

Q17. What comes to mind when someone mentions a shallow copy in Java?
A. Object cloning.

Q18. If you're overriding the method equals() of an object, which other method you might also consider?
A. hashCode()

Q19. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?
A. ArrayList

Q20. What's the main difference between a Vector and ArrayList
A. Java Vector class is internally synchronized and ArrayList is not.

Q21. When should the method invokeLater()be used?
A. This method is used to ensure that Swing components are updated through the event-dispatching thread.

Q22. How would you make a copy of an entire Java object with its state?
A. Have this class implement Cloneable interface and call its method clone().

Q23. What would you use to compare two String variables - the operator == or the method equals()?
A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

Q24. How can you minimize the need of garbage collection and make the memory use more effective?
A. Use object pooling and weak object references.

Q25. How can a subclass call a method or a constructor defined in a superclass?
A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

Q26. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?
If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.

Q27. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
A. You do not need to specify any access level, and Java will use a default package access level.

Q28. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
A. A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

Q29. Can an inner class, declared inside of a method, access local variables of this method?
A. It's possible only if these variables are final.

Q30. What can go wrong if you replace && with & in the following code:String a=null; if (a!=null && a.length()>10) {...} A. A single ampersand here would lead to a NullPointerException.

Java Interview Questions

What is the diffrence between an Abstract class and Interface ?
What is user defined exception ?
What do you know about the garbate collector ?
What is the difference between C++ & Java ?
Explain RMI Architecture?
How do you communicate in between Applets & Servlets ?
What is the use of Servlets ?
What is JDBC? How do you connect to the Database ?
In an HTML form I have a Button which makes us to open another page in 15 seconds. How will do you that ?
What is the difference between Process and Threads ?
What is the difference between RMI & Corba ?
What are the services in RMI ?
How will you initialize an Applet ?
What is the order of method invocation in an Applet ?
When is update method called ?
How will you pass values from HTML page to the Servlet ?
Have you ever used HashTable and Dictionary ?
How will you communicate between two Applets ?
What are statements in JAVA ?
What is JAR file ?
What is JNI ?
What is the base class for all swing components ?
What is JFC ?
What is Difference between AWT and Swing ?
Considering notepad/IE or any other thing as process, What will happen if you start notepad or IE 3 times? Where 3 processesare started or 3 threads are started ?
How does thread synchronization occurs inside a monitor ?
How will you call an Applet using a Java Script function ?
Is there any tag in HTML to upload and download files ?
Why do you Canvas ?
How can you push data from an Applet to Servlet ?
What are 4 drivers available in JDBC ?
How you can know about drivers and database information ?
If you are truncated using JDBC, How can you know ..that how much data is truncated ? And What situation , each of the 4 drivers used ?
How will you perform transaction using JDBC ?
In RMI, server object first loaded into the memory and then the stub reference is sent to the client ? or whether a stub reference is directly sent to the client ?
Suppose server object is not loaded into the memory, and the client request for it , what will happen?
What is serialization ?
Can you load the server object dynamically? If so, what are the major 3 steps involved in it ?
What is difference RMI registry and OSAgent ?
To a server method, the client wants to send a value 20, with this value exceeds to 20,. a message should be sent to the client ?
What will you do for achieving for this ?
What are the benefits of Swing over AWT ?
Where the CardLayout is used ?
What is the Layout for ToolBar ?
What is the difference between Grid and GridbagLayout ?
How will you add panel to a Frame ?
What is the corresponding Layout for Card in Swing ?
What is light weight component ?
Can you run the product development on all operating systems ?
What is the webserver used for running the Servlets ?
What is Servlet API used for conneting database ?
What is bean ? Where it can be used ?
What is difference in between Java Class and Bean ?
Can we send object using Sockets ?
What is the RMI and Socket ?
How to communicate 2 threads each other ?
What are the files generated after using IDL to Java Compilet ?
What is the protocol used by server and client ?
Can I modify an object in CORBA ?
What is the functionality stubs and skeletons ?
What is the mapping mechanism used by Java to identify IDL language ?
Diff between Application and Applet ?
What is serializable Interface ?
What is the difference between CGI and Servlet ?
What is the use of Interface ?
Why Java is not fully objective oriented ?
Why does not support multiple Inheritance ?
What it the root class for all Java classes ?
What is polymorphism ?
Suppose If we have variable ' I ' in run method, If I can create one or more thread each thread will occupy a separate copy or same variable will be shared ?
In servlets, we are having a web page that is invoking servlets username and password ? which is cheks in the database ?
Suppose the second page also If we want to verify the same information whethe it will connect to the database or it will be used previous information?
What are virtual functions ?
Write down how will you create a binary Tree ?
What are the traverses in Binary Tree ?
Write a program for recursive Traverse ?
What are session variable in Servlets ?
What is client server computing ?
What is Constructor and Virtual function? Can we call Virtual funciton in a constructor ?
Why we use OOPS concepts? What is its advantage ?
What is the middleware ? What is the functionality of Webserver ?
Why Java is not 100 % pure OOPS ? ( EcomServer )
When we will use an Interface and Abstract class ?
What is an RMI?
How will you pass parameters in RMI ? Why u serialize?
What is the exact difference in between Unicast and Multicast object ? Where we will use ?
What is the main functionality of the Remote Reference Layer ?
How do you download stubs from a Remote place ?
What is the difference in between C++ and Java ? can u explain in detail ?
I want to store more than 10 objects in a remote server ? Which methodology will follow ?
What is the main functionality of the Prepared Statement ?
What is meant by static query and dynamic query ?
What are the Normalization Rules ? Define the Normalization ?
What is meant by Servelet? What are the parameters of the service method ?
What is meant by Session ? Tell me something about HTTPSession Class ?
How do you invoke a Servelt? What is the difference in between doPost and doGet methods ?
What is the difference in between the HTTPServlet and Generic Servlet ? Expalin their methods ? Tell me their parameter names also ?
Have you used threads in Servelet ?
Write a program on RMI and JDBC using StoredProcedure ?
How do you sing an Applet ?
In a Container there are 5 components. I want to display the all the components names, how will you do that one ?
Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA ?
Tell me the latest versions in JAVA related areas ?
What is meant by class loader ? How many types are there? When will we use them ?
How do you load an Image in a Servlet ?
What is meant by flickering ?
What is meant by distributed Application ? Why we are using that in our applications ?
What is the functionality of the stub ?
Have you used any version control ?
What is the latest version of JDBC ? What are the new features are added in that ?
Explain 2 tier and 3 -tier Architecture ?
What is the role of the webserver ?
How have you done validation of the fileds in your project ?
What is the main difficulties that you are faced in your project ?
What is meant by cookies ? Explain ? Problem faced in your earlier project
How OOPS concept is achieved in Java
Features for using Java
How does Java 2.0 differ from Java 1.0
Public static void main – Explain
What are command line arguments
Explain about the three-tier model
Difference between String & StringBuffer
Wrapper class. Is String a Wrapper Class
What are the restriction for static method
Purpose of the file class
Default modifier in Interface
Difference between Interface & Abstract class
Can abstract be declared as Final
Can we declare variables inside a method as Final Variables
What is the package concept and use of package
How can a dead thread be started
Difference between Applet & Application
Life cycle of the Applet
Can Applet have constructors
Differeence between canvas class & graphics class
Explain about Superclass & subclass
Difference between TCP & UDP
What is AppletStub
Explain Stream Tokenizer
What is the difference between two types of threads
Checked & Unchecked exception
Use of throws exception
What is finally in exception handling
Vector class
What will happen to the Exception object after exception handling
Two types of multi-tasking
Two ways to create the thread
I/O Filter
How can you retrieve warnings in JDBC
Can applet in different page communicate with each other
Four driver Manager
Features of JDBC 20
Explain about stored procedures
Servlet Life cycle
Why do you go for servlet rather than CGI
How to generate skeleton & Stub classes
Explain lazy activation
Firewalls in RMI

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