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.
[Received from Venkateswara Manam]
Q: What is synchronization and why is it important?
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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.
[ Received from Venkateswara Manam]
Q: How does Java handle integer overflows and underflows?
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A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
[ Received from Venkateswara Manam]
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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 .
[ Received from Venkateswara Manam]
Q: What is the difference between preemptive scheduling and time slicing?
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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.
[ Received from Venkateswara Manam]
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.
[ Received from Venkateswara Manam]
Q: What is the purpose of finalization?
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A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
[ Received from Venkateswara Manam]
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.
[ Received from Venkateswara Manam]
Q: What is the difference between a while statement and a do statement?
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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.
[ Received from Venkateswara Manam]
Q: What is the difference between static and non-static variables?
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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.
[ Received from Venkateswara Manam]
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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.
[ Received from Venkateswara Manam]
Q: What are synchronized methods and synchronized statements?
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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.
[ Received from Venkateswara Manam]
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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.
[ Received from Shipra Kamra]
Q: Can applets communicate with each other?
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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 ref erence 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.
[ Received from Krishna Kumar ]
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\" );
Step 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();
[ Received from Shri Prakash Kunwar]
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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.
Can an unreachable object become reachable again? A: An unreachable object may become reachable again. This can happen when the object's
finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
[Received from P Rajesh]
Q: What method must be implemented by all threads?
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A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
[ Received from P Rajesh]
Q: What are synchronized methods and synchronized statements?
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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.
[ Received from P Rajesh]
Q: What is Externalizable?
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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)
[ Received from Venkateswara Manam]
Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.
[ Received from P Rajesh]
Q: What are some alternatives to inheritance?
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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).
[ Received from P Rajesh]
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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.
[ Received from P Rajesh]
Q: What is the difference between preemptive scheduling and time slicing?
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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.
[ Received from P Rajesh]
Q: What is the catch or declare rule for method declarations?
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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.
[ Received from P Rajesh]
Is Empty .java file a valid source file? A: Yes, an empty .java file is a perfectly valid source file.
[Received from Sandesh Sadhale]
Q: Can a .java file contain more than one java classes?
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A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
[ Received from Sandesh Sadhale]
Q: Is String a primitive data type in Java?
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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.
[ Received from Sandesh Sadhale]
Q: Is main a keyword in Java?
A: No, main is not a keyword in Java.
[ Received from Sandesh Sadhale]
Q: Is next a keyword in Java?
A: No, next is not a keyword.
[ Received from Sandesh Sadhale]
Q: Is delete a keyword in Java?
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A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.
[ Received from Sandesh Sadhale]
Q: Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System object.
[ Received from Sandesh Sadhale]
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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.
[ Received from Sandesh Sadhale]
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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.
[ Received from Sandesh Sadhale]
Q: What are the different scopes for Java variables?
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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.
[ Received from Sandesh Sadhale]
Q: What is the default value of the local variables?
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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..
[ Received from Sandesh Sadhale]
Q: How many objects are created in the following piece of code?
MyClass c1, c2, c3; c1 = new MyClass (); c3 = new MyClass ();
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A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
[ Received from Sandesh Sadhale]
Q: Can a public class MyClass be defined in a source file named YourClass.java?
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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.
[ Received from Sandesh Sadhale]
Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.
[ Received fromSandesh Sadhale]
Q: What will be the output of the following statement?
System.out.println ("1" + 3); A: It will print 13.
[ Received from Sandesh Sadhale]
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
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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.
[ Received from Sandesh Sadhale]
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