Q: What is the difference between an Interface and an Abstract class?
Abstract class | Interface |
1.Abstract Class is a class which must have at least one abstract method. 2. Abstract class they can be default and instance variables also and cannot be instantiated. 3.Abstract class contains the method implementation of the some methods 4.Any class can extend an abstract class..5. A class may extend only one abstract class. 6.Accessibility modifier(Public/Private/internal) is allowed for abstract class.7.Abstract classes are fast. | 1.Interface is a class which is having all abstract methods. 2. By default what ever variables we declare in interface are public static final 3. Interface contains only method declaration, no implementation provided. 4.only an interface can extend another interface 5. A Class may implement several interfaces.6.Interface doesn't allow accessibility modifier7.Interfaces are slow as it requires extra indirection to find corresponding method in in the actual class. |
Q: What is the purpose of garbage collection in Java, and when is it used?
In the Java programming language, dynamic allocation of objects is achieved using the new operator. An object once created uses some memory and the memory remains allocated till there are references for the use of the object. When there are no references for an object, it is assumed to be no longer needed and the memory occupied by the object can be reclaimed.
There is no explicit need to destroy an object as java handles the de-allocation automatically. The technique that accomplishes this is known as Garbage Collection.
1.Garbage collection is a mechanism provided by Java Virtual Machine to reclaim heap space from objects which are eligible for Garbage collection.
2.Garbage collection relieves java programmer from memory management
3.Garbage Collection in Java is carried by a daemon thread called Garbage Collector.
5) Before removing an object from memory Garbage collection thread invokes finalize () method of that object and gives an opportunity to perform any sort of cleanup required.
6) You as Java programmer can not force Garbage collection in Java;
7) There are methods like System.gc () and Runtime.gc () which is used to send request of Garbage collection to JVM but it’s not guaranteed that garbage collection will happen.
Q: Describe synchronization in respect to multithreading.
A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. Synchronization is just managing the resources among Threads. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
Q:Explain different way of using thread?
two ways of creating a thread.
1. Extending the Thread class. Ex: public class ThreadExample extends Thread {}
2. Implementing the Runnable Interface. Ex: public class ThreadExample implements Runnable {}
when you are going for multiple inheritance..the only interface can help.
Q:What are pass by reference and passby value?
A:Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
Q:What is HashMap and Map?
A:Map is Interface and Hashmap is class that implements that.
Q:How HashMAp works in Java?
A:HashMap works on principle of hashing, we have put () and get () method for storing and retrieving object form hashMap.When we pass an both key and value to put() method to store on HashMap, it uses key object hashcode() method to calculate hashcode and they by applying hashing on that hashcode it identifies bucket location for storing value object.
While retrieving it uses key object equals method to find out correct key value pair and return value object associated with that key. HashMap uses linked list in case of collision and object will be stored in next node of linked list.
Also hashMap stores both key+value tuple in every node of linked list.
Q:Difference between HashMap and HashTable?
A:The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
Q:Difference between Vector and ArrayList?
Vector | Arraylist |
1.Vector is synchronized 2.Vector is slow. 3. Vector has defalt size of 10. 4Vector implements Innumerator Interface for traverse. | 1.Arraylist is not synchronized 2.ArrayList is faster than Vector 3.ArrayList has no default size 4.ArrayList implements Iterator Interface for traversing |
Q:Difference between Swing and Awt?
AWT | Swing |
1.AWT are heavy-weight componenets 2.AWT works slower than Swing 3.awt components support Delegate Event Model.4.awt components provide static look and feel. 5.Awt is not platform independent | 1.Swings are light-weight components 2.swing works faster than AWT 3.Swing components support MVC architecture 4.Swing components provide dynamiclook and feel 5. swing is platform independent |
A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
Q:What is the difference between a constructor and a method?
Constructor | Method |
1.A constructor is a member function of a class that is used to create objects of that class 2It has the same name as the class itself 3.it has no return type 4.it is invoked using the new operator. | 1.A method is an ordinary member function of a class. 2.It has its own name, 3.It has a return type (which may be void) 4.It is invoked using the dot operator. |
Q:What is an Iterator?
A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
Q: Difference between LinkedList vs ArrayList in Java:
LinkedList | ArrayList |
1. LinkedList doesn't provide Random or index based access and you need to iterate over linked list to retrieve any element. 2. Insertions and removal are easy and fast in LinkedList because there is no risk of resizing array 3. LinkedList has more memory overhead than ArrayList. in LinkedList each node holds both data and address of next and previous node. | 2. Array is an index based data-structure searching or getting element from Array with index is pretty fast. ArrayList as you need to rearrange all elements. 2. ArrayList Insertions and removal are difficult because there is risk of resizing array 3. ArrayList has less memory than Linked List. because in ArrayList each index only holds actual object (data) |
Q:State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
A:public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : 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 a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
Q:What is an abstract class?
An abstract class is a class that is declared by using the abstract keyword. It may or may not have abstract methods. Abstract classes cannot be instantiated, but they can be extended into sub-classes.A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
Q:What is static in java?
A: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.
Q:what is static variable?
It is a variable which belongs to the class and not to object(instance)
Static variables are initialized only once , at the start of the execution .These variables will be initialized first, before the initialization of any instance variables
A single copy to be shared by all instances of the class
A static variable can be accessed directly by the class name and doesn’t need any object
Syntax : <class-name>.<variable-name>
Q:what is static method?
It is a method which belongs to the class and not to the object(instance)
A static method can access only static data. It can not access non-static data (instance variables)
A static method can call only other static methods and can not call a non-static method from it.
A static method can be accessed directly by the class name and doesn’t need any object
Syntax : <class-name>.<method-name>
A static method cannot refer to "this" or "super" keywords in anyway
Q:What is final?
A:A 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).
Q: What if the main method is declared as private?
A: The program compiles properly but at runtime it will give "Main method not public." message.
Q:What if the static modifier is removed from the signature of the main method?
A: Program compiles. But at runtime throws an error "NoSuchMethodError".
Q:What if I write static public void instead of public static void?
A: Program compiles and runs properly.
Q:What if I do not provide the String array as the argument to the method?
A: program compiles but throws a runtime error "NoSuchMethodError".
Q:What is the first argument of the String array in main method?
A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
Q:If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
A:It is empty. But not null.
Q:How can one prove that the array is not null but empty using one line of code?
A:Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
Q:What environment variables do I need to set on my machine in order to be able to run Java programs?
A:CLASSPATH and PATH are the two variables.
Q:Can an application have multiple classes having main method?
A: 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.
Q:Can I have multiple main methods in the same class?
A: No the program fails to compile. The compiler says that the main method is already defined in the class.
Q:Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.
Q:Can I import same package/class twice? Will the JVM load the package twice at runtime?
A:One can import the same package or same class multiple times. Neither compiler nor JVM complains about it. And the JVM will internally load the class only once no matter how many times you import the same class.
Q:What are Checked and UnChecked Exception?
Checked Exception | UnChecked Exception |
1.Checked Exceptions are checked at compile time 2.Checked Exceptions are not because of program errors, may be some out side sources,like file open exception,IOexceptions 3.A checked exception is any subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. 4.Checked exceptions must be explicitly caught or propagated as described in Basic try-catch-finally Exception Handling. 5.Checked exceptions in Java extend the java.lang.Exception class | 1.Unchecked exception are at runtime.. 2.Unchecked exceptions are because of program errors like array index out of bounds,NUll pointer ,Bad Cast exceptions etc. 3.Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. 4.Unchecked exceptions do not have this requirement. They don't have to be caught or declared thrown. 5.Unchecked exceptions extend the java.lang.RuntimeException. |
Q:What is Overriding?
A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
Q:What are different types of inner classes?
A:Nested top-level classes, Member classes, Local classes, 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.
Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
Q.What are the different scopes for Java variables?
The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance
Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
Q: What is the difference between declaring a variable and defining a variable?
A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
Q:What is the default value of an object reference declared as an instance variable?
A: null unless we define it explicitly.
Q: Can a top level class be private or protected?
A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.
Q: What type of parameter passing does Java support?
A: In Java the arguments are always passed by value .
Q: Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.
Q:Objects are passed by value or by reference?
A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
Q:What is Shallow Copy?
Shallow copy is a bit-wise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
Q:What is Deep Copy?
A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers. shallow copy and deep copy object nedd to implement Cloneable Interface.
Q:What is serialization?
A:Java provides a mechanism, called object serialization where an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object. Primary purpose of java serialization is to write an object into a stream, so that it can be transported through a network and that object can be rebuilt again. Serialization involves saving the current state of an object to a stream, and restoring an equivalent object from that stream.Classes ObjectInputStream and ObjectOutputStream are high-level streams that contain the methods for serializing and deserializing an object.
De-serialization is the process of getting the object back from the file to its original form.
Q: How do I serialize an object to a file?
A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
Q: Which methods of Serializable interface should I implement?
A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process?
A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
Q: What is the common usage of serialization?
A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
Q: What is Externalizable interface?
A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
Q:When you serialize an object, what happens to the object references included in the object?
A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original object.
Q:What one should take care of while serializing the object?
A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
Q:What happens to the static fields of a class during serialization?
A:There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
Q: Does Java provide any construct to find out the size of an object?
A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
Q: Give a simplest way to find out the time a method takes for execution without using any profiling tool?
A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.
Q:What are wrapper classes?
A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.
Q: Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
Q:What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.
Q:What are runtime exceptions?
A: 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.
Q: What is the difference between error and an exception?
error | exception |
1.Error is unrecoverable 2.Error can not be handled3.Error is the compile time error not runtime and it is not covered it is only fixed by a programmer.4.Error occur then our program stops and do not run | 1.Exception is recoverable 2.Exception can be handled 3.Exception is an run time error when an abnormal condition is occurred it can handle by JVM 4.If exception occur then nothing happening only try block will be suspended |
Q:How to create custom exceptions?
A: Your class should extend class Exception, or some more specific type thereof.
Q:If I want an object of my class to be thrown as an exception object, what should I do?
A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.
Q:If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
Q: How does an exception permeate through the code?
A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.
Q:What are the different ways to handle exceptions?
A: There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.
Q: What is the basic difference between the 2 approaches to exception handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
Q:When should you use which approach?
A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.
Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
Q: If I write return at the end of the try block, will the finally block still execute?
A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
Q: If I write System.exit (0); at the end of the try block, will the finally block still execute?
A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.
Q:How are Observer and Observable used?
A: 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.
Q:What is synchronization and why is it important?
A: With respect to multithreading, synchronization is the capability to control
the access of multiple threads to shared resources. Without 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.
Q:How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
Q:Does garbage collection guarantee that a program will not run out of memory?
A: 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
Q:What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
Q: When a thread is created and started, what is its initial state?
A: A thread is in the ready state after it has been created and started.
Q:What is the purpose of finalization?
A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
Q:What is the difference between a while statement and a do statement?
A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
Q: What is the difference between static and non-static variables?
A: 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.
Q: How are this() and super() used with constructors?
A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q:What is daemon thread and which method is used to create the daemon thread?
A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
Q:Can applets communicate with each other?
A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.
It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.
Q:What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Sep 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query : stmt.exceuteUpdate();
Q: How does a try statement determine which catch clause should be used to handle an exception?
A: 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 exceptionis executed. The remaining catch clauses are ignored.
Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
Q:What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
Q:What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.
Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
Q: What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
Q: Is Empty .java file a valid source file?
A: Yes, an empty .java file is a perfectly valid source file.
Q:Can a .java file contain more than one java classes?
A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
Q: Is String a primitive data type in Java?
A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.
Q:Is main a keyword in Java? Is next a keyword in Java? Is delete a keyword in Java?
A: No, main,next,delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.
Q:Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System object.
Q:What happens if you dont initialize an instance variable of any of the primitive types in Java?
A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.
Q: What will be the initial value of an object reference which is defined as an instance variable?
A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.
Q: What are the different scopes for Java variables?
A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..
Q:How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
Q: Can a public class MyClass be defined in a source file named YourClass.java?
A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.
Q: What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.
Q:What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.
Q:what is Aggregate class?
The aggregate class contains a reference to another class and is said to have ownership of that class. Each class referenced is considered to be part-of the aggregate classOwnership occurs because there can be no cyclic references in an aggregation relationship. If Class A contains a reference to Class B and Class B contains a reference to Class A then no clear ownership can be determined and the relationship is simply one of association.
What is ResultSetMetaData?
ResultSetMetaData methods to determine the characteristics of the ResultSets before you can retrieve data from them.
ResultSetMetaData methods provide the following types of information:
The number of columns in a ResultSet
The qualifier for the underlying table of the ResultSet
Information about a column, such as the data type, length, precision, scale, and nullability
Whether a column is read-only
After you invoke the executeQuery method to generate a ResultSet for a query on a table, follow these basic steps to determine the contents of the ResultSet:
Invoke the getMetaData method on the ResultSet object to create a ResultSetMetaData object.
Invoke the getColumnCount method to determine how many columns are in the ResultSet.
For each column in the ResultSet, execute ResultSetMetaData methods to determine column characteristics.
How to create pdf document in java?
To create a new document, you create a PdfDocument object using a PdfWriter object. When you create the PdfWriter object, you can specify the stream or the file pathname to which the PDF document needs to be saved.
// Create a PdfWriter instance
PdfWriter w = PdfWriter.fileWriter("sample_doc1.pdf");
// Create a PdfDocument instance with the PdfWriter
PdfDocument d = new PdfDocument(w);
// Write some text on page 1
d.writeText("Hello, World!");
// Write document to file
d.write();
// Close all I/O streams associated with the PDF writer
w.dispose();
After you create a new PdfDocument object, the writeText() method will allow you write text on a page, which is created by default in the document.
When you begin writing text in this manner, text will be rendered on the top-left corner of the document. Subsequent calls to the method will change this location to where the previous writeText() method had let off.
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