Ques: 1. What do you understand by Private, Protected and Public?
Ans: Private, Protected and Public are the accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.
Ques: 2. What is a java package? How is it used?
Ans: A Java package is a naming context for classes and interfaces.
- A package is used to create a separate name space for groups of classes and interfaces.
- Packages are used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
Enterprise Archives (EAR): An EAR file contains all the components that make up a J2EE application.
Java Archives (JAR): A JAR file encapsulates one or more Java classes, a manifest, and a descriptor. JAR files are the lowest level of archive. JAR files are used in J2EE for packaging EJBs and client-side Java Applications.
Web Archives (WAR): WAR files are like JAR files, except that they are specifically for web applications made from Servlets, JSPs, and supporting classes.
Ques: 4. How Java Source Code files are named?
Ans: A source code file may contain at most one public class or interface. A Java source code file takes the name of a public class or interface that is defined within the file. Source code files use the .java extension.
If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces.
Ques: 5. What do you understand by Numeric Promotion?
Ans: In numerical promotion, byte, char, and short values are converted to int values. It is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. if required, the int values are also converted to long values. The long and float values can be converted to double values.
Ques: 6. What is the difference between Time Slicing and Preemptive Scheduling?
Ans: Under time slicing in java, a task executes for a predefined slice of time and then reenters the pool of ready tasks.
But under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states, or a higher priority task comes into existence.
The scheduler then determines which task should execute next, based on priority and other factors.
Ques: 7. What do you mean by a Task's Priority? How is it used in Scheduling?
Ans: A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
Ques: 8. What technologies are included in J2EE?
Ans: The various technologies in J2EE are:
- Enterprise JavaBeansTM (EJBsTM).
- JavaServer PagesTM (JSPsTM).
- Java Servlets.
- The Java Naming and Directory InterfaceTM (JNDITM).
- The Java Transaction API (JTA).
- The JDBCTM data access API.
Ans: These methods are used to provide an efficient way for threads to wait for a shared resource. When a thread in java executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods.
Ques: 10. What happens when you invoke a Thread's interrupt method while it is Sleeping or Waiting?
Ans: When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.
Ques: 11. What are the various restrictions placed on method overriding?
- The overriding method may not limit the access of the method it overrides.
- Overridden methods must have the same name, argument list, and return type.
- The overriding method may not throw any exceptions that may not be thrown by the overridden method.
Ques: 12. What do you understand by Java Swing?
Ans: Swing is basically a type of Toolkit which is GUI toolkit for Java. It is one part of the Java Foundation Classes (JFC). Swing includes graphical user interface (GUI) widgets such as text boxes, buttons, split-panes, and tables.
Swing supports pluggable look and feel, not by using the native platform's facilities, but by roughly emulating them. This means you can get any supported look and feel on any platform.
Swing widgets provide more sophisticated GUI components than the earlier Abstract Window Toolkit. Since they are written in pure Java, they run the same on all platforms. The advantage is uniform behavior on all platforms. The disadvantage of lightweight components is slower execution.
Ques: 13. What are the differences between Swing and AWT?
Ans: There are many differences between Swing and AWT:
- AWT is heavy-weight components, but Swing is light-weight components.
- AWT is OS dependent because it uses native components, But Swing components are OS independent.
- Can change the look and feel in Swing which is not possible in AWT.
- Swing takes less memory compared to AWT.
- For drawing AWT uses screen rendering where Swing uses double buffering.
Ans: In java, there are two types of scaling: Horizontal Scaling and Vertical Scaling.
Horizontal Scaling: When Clones of an application server are defined on multiple physical m/c, it is called Horizontal Scaling. The objective is to use more than one less powerful m/c more efficiently.
Vertical Scaling: When multiple server clones of an application server are defined on the same physical m/c, it is called Vertical Scaling. The objective is to use the processing power of that m/c more efficiently.
Ques: 15. What are the types of Class Loaders in java?
Ans: In java, there are three types of class loader as bootstarp class loader, extension class loader and system class loader.
Bootstrap Class Loader: Bootstrap class loader loads java’s core classes like java.lang, java.util etc. These are classes that are part of java runtime environment. Bootstrap class loader is native implementation and so they may differ across different JVMs.
Extensions Class Loader: JAVA_HOME/jre/lib/ext contains jar packages that are extensions of standard core java classes. Extensions class loader loads classes from this ext folder. Using the system environment property java.ext.dirs you can add ‘ext’ folders and jar files to be loaded using extensions class loader.
System Class Loader: Java classes that are available in the java classpath are loaded using System class loader.
Ques: 16. What are differences between the boolean & operator and the && operator?
Ans: If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated.
If the first operand returns a value of true, then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
Ques: 17. What do you mean by Casting?
Ans: In java, there are two types of casting:
1). casting between primitive numeric types: Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values.
2). casting between object references: Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
Ques: 18. How does a Try statement determine which Catch clause should be used to handle an exception?
Ans: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.
Ques: 19. What will happens if Remove( ) is never invoked on a session bean?
Ans: In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool.
In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container.
Ques: 20. What Is Java Reflection Api?
Ans: Reflection is one of the most powerful api which help to work with classes, methods and variables dynamically. Basically, it inspects the class attributes at runtime. Also, we can say it provides a metadata about the class.
Ques: 21. What is the Collection API in java?
Ans: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
Ques: 22. What is the difference between Session and Entity Beans?
Ans: An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the user disconnects (ends his session). Generally, the session beans implement business methods that call entity beans .
Ques: 23. What do you understand by Abstract Schema in java?
Ans: In java, you can specify the name of the Abstract schema name in the deployment descriptor. The queries written in EJB QL for the finder methods references this name. It is a part of an entity bean’s deployment descriptor which defines the bean’s persistent fields and their relationship. Abstract schema is specified for entity beans with container managed persistence. The information provided in this Abstract Schema is used by the container for persistence management and relationship management.
Ques: 24. What is the use of the Finally block? Is Finally block in Java guaranteed to be called? When will it is not called?
Ans: The code in finally block will execute even if an exception is occurred. Finally, is the block of code that executes always. Finally block is NOT called in following conditions:
- If the JVM exits while the try or catch code is being executed, then the finally block may not execute. This may happen due to System.exit() call.
- if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.
- If an exception is thrown in finally block and not handled, then remaining code in finally block may not be executed.
Ques: 25. What do you mean by Local Interface. How values will be passed to them?
Ans: An EJB can use local client view only if it is really guaranteed that other enterprise beans or clients will only address the bean within a single JVM. With local client view, you can do pass-by-reference, which means your bean, as well as the client, will work directly with one copy of the data. Any changes made by the bean will be seen by the client and vice versa. Pass-by-reference eliminates time/system expenses for copying data variables, which provides a performance advantage.
Ques: 26. What is the main difference between Serializable and Externalizable Interfaces?
Ans: Both interfaces are used for implementing serialization. But, the basic difference is Serializable interface does not have any method (it’s a marker interface) and Externalizable interface having 2 methods such as readExternal() and writeExternal(). Serializable interface is the super interface for Externalizable interface.
Ques: 27. What is the relation between Local Interfaces and Container-managed relationships?
Ans: To be the target of a container-managed relationship, an entity bean with container-managed persistence must provide a local interface. Entity beans that have container-managed relationships with other entity beans, must be accessed in the same local scope as those related beans, and therefore typically provide a local client view.
Ques: 28. What are the differences between Creating String as New() and Literal?
Ans: 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”);
It 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.
Ques: 29. What is the main difference between Final, Finally And Finalize?
Final is used to apply restrictions on class, method and variable. Final class can’t be inherited, final method can’t be overridden, and final variable value can’t be changed.
Finally is used to place important code, it will be executed whether exception is handled or not.
Finalize is used to perform clean up processing just before object is garbage collected.
Ques: 30. How can an Object's Finalize() Method be invoked while it is reachable?
Ans: An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.
Ques: 31. What do you mean by an Object’s Lock? Which Object Have Locks?
Ans: A thread may execute a synchronized method of an object only after it has acquired the object’s lock. An object’s lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. All objects and classes have locks. A class’s lock is acquired on the class’s Class object.
Ques: 32. What are the uses of Observer and Observable?
Ans: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
Ques: 33. How ConcurrentHashMap Works?
Ans: Basically, ConcurrentHashMap locks each of the box (by default 16) which can be locked independently and thread safe for operation. The basic design of ConcurrentHashMap is to handling threading. And it does not expose the internal lock process.
Ques: 34. What are Transient Variables in java?
Ans: In Java, Transient variables can’t be serialized. For example, if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can’t be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Ques: 35. What do you understand by JFC?
Ans: JFC stands for Java Foundation Classes. The Java Foundation Classes (JFC) are a set of Java class libraries provided as part of Java 2 Platform, Standard Edition (J2SE) to support building graphics user interface (GUI) and graphics functionality for client applications that will run on popular platforms such as Microsoft Windows, Linux, and Mac OSX.
Ques: 36. What is the main difference between Find and Select methods In EJB?
Ans: A select method can return a persistent field (or a collection thereof) of a related entity bean. A finder method can return only a local or remote interface (or a collection of interfaces).
A select method is defined in the entity bean class. For bean-managed persistence, a finder method is defined in the entity bean class, but for container-managed persistence it is not.
Because it is not exposed in any of the local or remote interfaces, a select method cannot be invoked by a client. It can be invoked only by the methods implemented within the entity bean class. A select method is usually invoked by either a business or a home method.
Ques: 37. If some new data has entered in the database, how can a servlet refresh automatically?
Ans: It depends on the scenario, how you can handle them. You need to handle this in dao layer, when doing insert operation, you can call a utility method which will load the context ServletContextListener. Because, servlets are basically used for handling request and give the response.
Ques: 38. What do you mean by In-memory replication?
Ans: The process by which the contents in the memory of one physical m/c are replicated in all the m/c in the cluster is called in-memory replication.
Ques: 39. What do you understand by synchronization? Why is it so Important?
Ans: In respect of multithreading, synchronization is the capability to control the access of multiple threads to shared resources. In the absence of synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
Ques: 40. How many Bits are used to represent Unicode, ASCII, UTF-16 and UTF-8 characters?
Ans: Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. Unicode requires 16 bits and ASCII require 7 bits. UTF-8 represents characters using 8, 16, and 18 bits patterns. UTF-16 uses 16-bit and larger bit patterns.
Ques: 41. What is a Clone in java?
Ans: In java, the copies of a server group are called Clones. But unlike a Server Group Clones are associated with a node and are real server process running in that node.
Ques: 42. How does the Garbage Collection guarantee that a program will not run out of memory?
Ans: In java, garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
Ques: 43. What do you mean by AWT?
Ans: AWT is stands for Abstract Window Toolkit. AWT enables programmers to develop Java applications with GUI components, such as windows, and buttons. The Java Virtual Machine (JVM) is responsible for translating the AWT calls into the appropriate calls to the host operating system.
Ques: 44. What Advantage do Java's Layout Managers provide over traditional Windowing Systems?
Ans: Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.
Ques: 45. Is Decorator an EJB design pattern?
Ans: Decorator design pattern is not an EJB design pattern. It is the one which exhibits very low-level runtime polymorphism, for the specific and single object (Instance of the class). But not for at least for a class. It is the stuff to add specific functionality to a single & pointed object and leaves others like it unmodified. It is having close similarities like AspectJ stuff, but not with EJB stuff.
Ques: 46. What is the main purpose of the Enableevents() method?
Ans: The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event.
The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
Ques: 47. What is the main difference between Paint() and PaintComponent()?
Ans: The main point is that the paint() method invokes three methods in the following order:
Ques: 48. What do you understand by double buffering ?
Ans: Double buffering is the process of using two buffers rather than one to temporarily hold data being moved to and from an I/O device. Double buffering increases data transfer speed because one buffer can be filled while the other is being emptied.
Ques: 49. What is the main difference between the File and RandomAccessFile Classes?
Ans: The File class encapsulates the files and directories of the local file system.
The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
Ques: 50. Can you Name some of the Layoutmanagers In Java?
Ans: Some of the LayoutManagers in java are:
- Flow Layout Manager
- Grid Layout Manager
- Box Layout Manager
- Border Layout Manager
- Card Layout Manager
- GridBag Layout Manager