Difference Between java bean And POJO:

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The Java-Beanness of a POJO is that its public attributes are all accessed via getters and setters that conform to the JavaBeans conventions.

e.g.

private String foo;

public String getFoo(){...}

public void setFoo(String foo){...};

Additionally, true JavaBeans must implement Serializable and have a no-argument constructor.

POJOs don't have these restrictions.

Core Java basics

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Can I have multiple main methods in the same class? »
No the program fails to compile. The compiler says that the main method is already defined in the class.

Can an application have multiple classes having main method? »
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.

How can one prove that the array is not null but empty? »
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.

What if I do not provide the String array as the argument to the method? »Program compiles but throws a runtime error “NoSuchMethodError”.

What if I write static public void instead of public static void? »
Program compiles and runs properly.

What if the static modifier is removed from the signature of the main method? »
Program compiles. But at runtime throws an error “NoSuchMethodError”.

What if the main method is declared as private? »
The program compiles properly but at runtime it will give “Main method not public.” message.A compilation unit is a Java source code file.

What is the purpose of garbage collection? »
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

How are this and super used? »
this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

What interface must an object implement before it can be written to a stream as an object? »
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

What is your platform’s default character encoding? »
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.

What happens when you add a double value to a String? »
The result is a String object.162.

What is the difference between the File and RandomAccessFile classes? »
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.

What classes of exceptions may be thrown by a throw statement? »
A throw statement may throw any expression that may be assigned to the Throwable type.The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

If an object is garbage collected, can it become reachable again? »
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

What are the legal operands of the instanceof operator? »
The left operand is an object reference or null value and the right operand is a class, interface, or array type.

Why are the methods of the Math class static? »
So they can be invoked as if they are a mathematical code library.

How is it possible for two String objects with identical values not to be equal under the == operator? »
The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution? »
A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.Under what conditions is an object’s finalize() method invoked by the garbage collector? »The garbage collector invokes an object’s finalize() method when it detects that the object has become unreachable.

What is the difference between a field variable and a local variable? »
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

What class allows you to read objects directly from a stream? »
The ObjectInputStream class supports the reading of objects from input streams.

What class of exceptions are generated by the Java run-time system? »The Java runtime system generates RuntimeException and Error exceptions.

What happens when you invoke a thread’s interrupt method while it is sleeping or waiting? »
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.

Which non-Unicode letter characters may be used as the first character of an identifier? »
The non-Unicode letter characters $ and _ may appear as the first character of an identifier

If a method is declared as protected, where may the method be accessed? »
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

Which class is the immediate superclass of the Container class? »
ComponentCan an exception be rethrown? »Yes, an exception can be rethrown.

What is the difference between static and non-static variables? »
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.

Name the eight primitive Java types. »
The eight primitive types are byte, char, short, int, long, float, double, and boolean.

What happens when a thread cannot acquire a lock on an object? »
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.

How is rounding performed under integer division? »
The fractional part of the result is truncated. This is known as rounding toward zero

Can an object be garbage collected while it is still reachable? »
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected.

What is an object’s lock and which objects have locks? »
An object’s lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object’s lock. All objects and classes have locks. A class’s lock is acquired on the class’s Class object.

If a variable is declared as private, where may the variable be accessed? »
A private variable may only be accessed within the class in which it is declared.

Can a Byte object be cast to a double value? »No, an object cannot be cast to a primitive value.What value does read() return when it has reached the end of a file? »The read() method returns -1 when it has reached the end of a file.The high-level thread states are ready, running, waiting, and dead.

What method is invoked to cause an object to begin executing as a separate thread? »
The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.The = operator is right associative.

What invokes a thread’s run() method? »
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.

When a thread is created and started, what is its initial state? »
A thread is in the ready state after it has been created and started.

To what value is a variable of the String type automatically initialized? »
The default value of a String type is null.

When a thread blocks on I/O, what state does it enter? »
A thread enters the waiting state when it blocks on I/O.

What value does readLine() return when it has reached the end of a file? »
The readLine() method returns null when it has reached the end of a file.

What is the difference between preemptive scheduling and time slicing? »
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 […]

What is synchronization and why is it important? »
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.

How are Observer and Observable used? »
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.

Core Java FAQ part 3

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1.What is JVM?

A Java Virtual Machine is a runtime environment required for execution of a Java application.Every Java application runs inside a runtime instance of some concrete implementation of abstract specifications of JVM.It is JVM which is crux of 'platform independent' nature of the language.

2.What is a JVM consisted of?

Each time a Java Application is executed then an instance of JVM ,responsible for its running,is created.A JVM instance is described in terms of subsystems, memory areas, data types, and instructions.The block diagram given below,depicts a view of Internal Architecture of JVM :

3.What is a class loader and what is its responsibilities?

The Class loader is a subsystem of a JVM which is responsible,predominantly for loading classes and interfaces in the system.Apart from this,a class loader is responsible for the following activities:-Verification of imported types(classes and interfaces)-Allocating memory for class variables and initializing them to default values.Static fields for a class are created and these are set to standard default values but they are not explicitly initialized.The method tables are constructed for the class.-Resolving symbolic references from type to direct references The class loaders can be of two types: a bootstrap or primordial class loader and user defined class loaderEach JVM has a bootstrap class loader which loads trusted classes , including classes from Java API.JVM specs do not tell how to locate these classes and is left to implementation designers.A Java application with user defined class loader objects can customize class loading.These load untrustworthy classes and not an intrinsic part of JVM.They are written in Java,converted to class files and loaded into the JVM and installed like any other objects.If you want to read this in more details then read Chapter 8,"The Linking Model" of Inside Java 2 Virtual Machine by Bill Venners.

4. What is heap and stack?

The heap is the part of memory of JVM where all objects reside.The stack is consisted of stack frames.When a thread invokes a method,the JVM pushes a new frame onto that thread's Java stack.Each stack frame is consisted of operand stack and the local variable array.All arguments,local variables,intermediate computations and return values if any are kept in these stack corresponding to the method invoked.The stack frame on the top of the stack is called the active stack frame,which is the current place of execution.When the method completes, the virtual machine pops and discards the frame for that method.

5. How is your Java program executed inside JVM?

When JVM executes a Java application, a runtime instance of JVM is born.This runtime instance invoke main() method of Java application.The main() method of an application serves as the starting point for that application's initial thread. The initial thread can in turn fire off other threads.This thread has a program counter(PC) and Java stack.Whenever main() method is invoked, a stack frame is pushed onto the stack,this then becomes the active tack frame.The program counter in the new Java stack frame will point to the beginning of the method.If there are more method invocations within main() method then this process of pushing new stack frame onto the stack for each method call is repeated as and when they are invoked.When a method returns, the active frame is popped from the stack and the one below becomes the active stack frame.The PC is set to the instruction after the method call and the method continues.There is only one heap corresponding to an instance of JVM and all objects created are stored here.This heap is shared by all threads created in an application.Inside the Java virtual machine, threads come in two flavors: daemon and non- daemon. A daemon thread is ordinarily a thread used by the virtual machine itself, such as a thread that performs garbage collection. The application, however, can mark any threads it creates as daemon threads.The initial thread of an application--the one that begins at main()--is a non- daemon thread.A Java application continues to execute (the virtual machine instance continues to live) as long as any non-daemon threads are still running. When all non-daemon threads of a Java application terminate, the virtual machine instance will exit. If permitted by the security manager, the application can also cause its own demise by invoking the exit() method of class Runtime or System.When main() returns,it terminates the application's only non-daemon thread, which causes the virtual machine instance to exit.

6. What is Java class file's magic number?

A Magic Number of a class file is a unique identifier for tools to quickly differentiate class files from non class files.The first four bytes of each Java class file has the magic value as 0xCAFEBABE.And the answer to why this number,I do not actually know but there may be very few sensible and acceptable options possible constructed from letters A-F which can surely not be 'CAFEFACE' or 'FADECAFE'....

7. How JVM performs Thread Synchronization?

JVM associates a lock with an object or a class to achieve mutilthreading. A lock is like a token or privilege that only one thread can "possess" at any one time. When a thread wants to lock a particular object or class, it asks the JVM.JVM responds to thread with a lock maybe very soon, maybe later, or never. When the thread no longer needs the lock, it returns it to the JVM. If another thread has requested the same lock, the JVM passes the lock to that thread.If a thread has a lock,no other thread can access the locked data until the thread that owns the lock releases it.The JVM uses locks in conjunction with monitors.A monitor is basically a guardian in that it watches over a sequence of code, making sure only one thread at a time executes the code.Each monitor is associated with an object reference. It is the responsibility of monitor to watch an arriving thread must obtain a lock on the referenced object.When the thread leaves the block,it releases the lock on the associated object.A single thread is allowed to lock the same object multiple times.JVM maintains a count of the number of times the object has been locked. An unlocked object has a count of zero. When a thread acquires the lock for the first time, the count is incremented to one. Each time the thread acquires a lock on the same object, a count is incremented. Each time the thread releases the lock, the count is decremented. When the count reaches zero, the lock is released and made available to other threads.In Java language terminology, the coordination of multiple threads that must access shared data is called synchronization. The language provides two built-in ways to synchronize access to data: with synchronized statements or synchronized methods.The JVM does not use any special opcodes to invoke or return from synchronized methods. When the JVM resolves the symbolic reference to a method, it determines whether the method is synchronized. If it is, the JVM acquires a lock before invoking the method. For an instance method, the JVM acquires the lock associated with the object upon which the method is being invoked. For a class method, it acquires the lock associated with the class to which the method belongs. After a synchronized method completes, whether it completes by returning or by throwing an exception, the lock is released.Two opcodes, monitorenter and monitorexit are used by JVM for accomplishing this task.When monitorenter is encountered by the Java virtual machine, it acquires the lock for the object referred to by objectref on the stack. If the thread already owns the lock for that object, a count is incremented. Each time monitorexit is executed for the thread on the object, the count is decremented. When the count reaches zero, the monitor is released.

8.How JVM performs Garbage Collection?

Whenever a reference to an object on heap lies dangling or no longer in use then it becomes eligible for being garbage collected by JVM.JVM specifications do not force any specific kind of garbage collection algorithm though there are several algorithms like reference counting,tracing,compacting,copying,generational etc. in place.It is very important that garbage collection should be efficient and non interfering in execution of Java programs.There is a trade off between ease of implementation versus better performance while implementing garbage collection feature for a JVM.

9. How to profile heap usage?

Try using -Xaprof to get a profile of the allocations (objects and sizes) of your application.Also try -agentlib:hprof=heap=all (or other option, try -agentlib:hprof=help for a list).

10. What will you do if VM exits while printing "OutOfMemoryError" and increasing max heap size doesn't help?

The Java HotSpot VM cannot expand its heap size if memory is completely allocated and no swap space is available. This can occur, for example, when several applications are running simultaneously. When this happens, the VM will exit after printing a message similar to the following.Exception java.lang.OutOfMemoryError: requested bytesIf you see this symptom, consider increasing the available swap space by allocating more of your disk for virtual memory and/or by limiting the number of applications you run simultaneously. You may also be able to avoid this problem by setting the command-line flags -Xmx and -Xms to the same value to prevent the VM from trying to expand the heap. Note that simply increasing the value of -Xmx will not help when no swap space is available.

11. Should one pool objects to help Garbage Collector?Should one call System.gc() periodically?

The answer is No!Pooling objects will cause them to live longer than necessary. The garbage collection methods will be much more efficient if you let it do the memory management. The strong advice is taking out object pools.Don't call System.gc(), HotSpot will make the determination of when its appropriate and will generally do a much better job.

12. An application has a lot of threads and is running out of memory, why?

You may be running into a problem with the default stack size for threads. In Java SE 6, the default on Sparc is 512k in the 32-bit VM, and 1024k in the 64-bit VM. On x86 Solaris/Linux it is 320k in the 32-bit VM and 1024k in the 64-bit VM.On Windows, the default thread stack size is read from the binary (java.exe). As of Java SE 6, this value is 320k in the 32-bit VM and 1024k in the 64-bit VM.You can reduce your stack size by running with the -Xss option. For example:java -server -Xss64kNote that on some versions of Windows, the OS may round up thread stack sizes using very coarse granularity. If the requested size is less than the default size by 1K or more, the stack size is rounded up to the default; otherwise, the stack size is rounded up to a multiple of 1 MB.64k is the least amount of stack space allowed per thread.

13. If your program is I/O bound or running in native methods, do these activities engage JVM?

The answer is 'No'.If the program is I/O bound or running in native methods, then the VM is not involved in the consumption of CPU time. The VM technology will engage CPU for running bytecodes. Typical examples of time spent not running bytecode are graphical operations that make heavy use of native methods, and I/O operations such as reading and writing data to network sockets or database files.

14. What is the difference between interpreted code and compiled code?

An interpreter produces a result from a program, while a compiler produces a program written in assembly language and in case of Java from bytecodes.The scripting languages like JavaScript,Python etc. require Interpreter to execute them.So a program written in scripting language will directly be executed with interpreter installed on that computer,if it is absent then this program will not execute.While in case of compiled code,an assembler or a virtual machine in case of Java is required to convert assembly level code or bytecodes into machine level instructions/commands.Generally, interpreted programs are slower than compiled programs, but are easier to debug and revise.

15. Why Java based GUI intensive program has performance issues?

GUI intensive Java application mostly run underlying OS specific native libraries which is time and more CPU cycles consuming.The overall performance of a Java application depends on four factors:The design of the applicationThe speed at which the virtual machine executes the Java bytecodesThe speed at which the libraries that perform basic functional tasks execute (in native code)The speed of the underlying hardware and operating systemThe virtual machine is responsible for byte code execution, storage allocation, thread synchronization, etc. Running with the virtual machine are native code libraries that handle input and output through the operating system, especially graphics operations through the window system. Programs that spend significant portions of their time in those native code libraries will not see their performance on HotSpot improved as much as programs that spend most of their time executing byte codes.

16. What is 64 bit Java ?

A 64-bit version of Java has been available to Solaris SPARC users since the 1.4.0 release of J2SE. A 64-bit capable J2SE is an implementation of the Java SDK (and the JRE along with it) that runs in the 64-bit environment of a 64-bit OS on a 64-bit processor. The primary advantage of running Java in a 64-bit environment is the larger address space.This allows for a much larger Java heap size and an increased maximum number of Java Threads, which is needed for certain kinds of large or long-running applications. The primary complication in doing such a port is that the sizes of some native data types are changed. Not surprisingly the size of pointers is increased to 64 bits. On Solaris and most Unix platforms, the size of the C language long is also increased to 64 bits. Any native code in the 32-bit SDK implementation that relied on the old sizes of these data types is likely to require updating.Within the parts of the SDK written in Java things are simpler, since Java specifies the sizes of its primitive data types precisely. However even some Java code needs updating, such as when a Java int is used to store a value passed to it from a part of the implementation written in C.

17. What is the difference between JVM and JRE?

A Java Runtime Environment (JRE) is a prerequisite for running Java applications on any computer.A JRE contains a Java Virtual Machine(JVM),all standard,core java classes and runtime libraries. It does not contain any development tools such as compiler, debugger, etc. JDK(Java Development Kit) is a whole package required to Java Development which essentially contains JRE+JVM,and tools required to compile and debug,execute Java applications.

Introduction to Java Programming

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1) The Java interpreter is used for the execution of the source code.
a) True
b) False
Ans: a.

2) On successful compilation a file with the class extension is created.
a) True
b) False
Ans: a.

3) The Java source code can be created in a Notepad editor.
a) True
b) False
Ans: a.

4) The Java Program is enclosed in a class definition.
a) True
b) False
Ans: a.

5) What declarations are required for every Java application?
Ans: A class and the main( ) method declarations.

6) What are the two parts in executing a Java program and their purposes?
Ans: Two parts in executing a Java program are:
Java Compiler and Java Interpreter.
The Java Compiler is used for compilation and the Java Interpreter is used for execution of the application.

7) What are the three OOPs principles and define them?
Ans : Encapsulation, Inheritance and Polymorphism are the three OOPs
Principles.

Encapsulation:
Is the Mechanism that binds together code and the data it manipulates, and keeps both safe from outside interference and misuse.

Inheritance:
Is the process by which one object acquires the properties of another object.

Polymorphism:
Is a feature that allows one interface to be used for a general class of actions.

8) What is a compilation unit?
Ans : Java source code file.

9) What output is displayed as the result of executing the following statement?
System.out.println("// Looks like a comment.");
// Looks like a comment
The statement results in a compilation error
Looks like a comment
No output is displayed
Ans : a.

10) In order for a source code file, containing the public class Test, to successfully compile, which of the following must be true?
It must have a package statement
It must be named Test.java
It must import java.lang
It must declare a public class named Test
Ans : b

11) What are identifiers and what is naming convention?
Ans : Identifiers are used for class names, method names and variable names. An identifier may be any descriptive sequence of upper case & lower case letters,numbers or underscore or dollar sign and must not begin with numbers.

12) What is the return type of program’s main( ) method?
Ans : void

13) What is the argument type of program’s main( ) method?
Ans : string array.

14) Which characters are as first characters of an identifier?
Ans : A – Z, a – z, _ ,$

15) What are different comments?
Ans :
1) // -- single line comment
2)
/* --*/ multiple line comment
3)
/** --*/ documentation

16) What is the difference between constructor method and method?

Ans : Constructor will be automatically invoked when an object is created. Whereas method has to be call explicitly.

17) What is the use of bin and lib in JDK?

Ans : Bin contains all tools such as javac, applet viewer, awt tool etc., whereas Libcontains all packages and variables.

CORE JAVA faqs - part 2

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38. What is a JIT compiler?
JIT compilers are used to convert the java byte codes to native types on the fly.

39. What is the difference between instance method and class method ?
· Instance methods require an instance before they can be invoked; class methods do not.· Instance methods use dynamic (late) binding; class(static methods) methods use static (early) binding.

40. What is the difference between a process and Thread?

A process is a self-contained running program within its own address space while a Thread is a single sequential flow of control within a process.Each process has its own set of variables where as Threads share the same data.Inter-process communication is much slower and restrictive as compared to inter-thread communication is faster.Creating and destroying Thread is much cheaper in terms of performance overhead as compared to launching a new process.

41. What is the difference between pre-emptive multi-tasking and co-operative multi-tasking?

In case of pre-emptive multitasking, the program is interrupted without consulting first where as in case of co-operative multitasking; programs are interrupted only when they are willing to yield control.Windows 95,NT – pre-emptive multi-tasking where Windows 3.1 is co-operative multitasking. Pre-emptive multi-tasking is more effective as compared co-operative multitasking.

42. What is difference between creating a thread by extending Thread class and implementing Runnable?

Explain the four different states of a thread The four different states of a thread are New: When a thread is created with new operator .It is not yet running.Runnable: Once you invoke the start method on the thread it becomes runnable. It is up to the OS to decide when to give time to the thread to run. A runnable thread may or may not be running.Blocked : Thread enters this state under the following condition. Someone calls sleep () on the thread.Thread calls an operation that blocks on input/output.Thread calls wait() method.Thread tries to lock an object that is currently locked by another thread.Dead : The Thread is dead for one of two reasons.It dies a natural death because the run method exits normally.It dies abruptly because an uncaught exception terminates the run method.

43. What happens when there are more than one highest priority threads having same priority?

In that case only one of the thread is picked up. It is up to the thread Scheduler to arbitrate between threads of same priority.There is no guarantee that all the same priority threads will be fairly treated.

44. Why do threads block on I/O ?

Threads block on I/O so that other threads may execute while the I/O operation is performed.

45. What are the methods which can only be called from within a Synchronized method or block ?

Notify (): unblocks one randomly selected thread among the threads that called wait() on this object.NoitifyAll() : unblocks the threads that called wait() on this object.Wait (): causes the thread to wait until it is notified.46. What is the difference between yield and sleep methods of Thread?When a task invokes its yield () method, it returns to the ready state. When a task invokes its sleep () method, it returns to the waiting state.

CORE JAVA faqs - part 1

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1. What is the difference between an Interface and Abstract class ?
• A class may implement several interfaces where as it can extend only one abstract class.

• An interface cannot provide any code at all, much less default code where as an abstract class can provide complete code, default code, and/or just stubs that has to be overridden.

• Static final constants can use the interfaces without qualification in classes that implement the interface where as both static constants and instance variables can use the abstract classes.

• Interface implementation can be added to any existing third part classes where as third party classes must be rewritten to extend from the abstract classes.

• Interfaces are used to often represent peripheral ability of a class, not central identity where as abstract class defines core identity of its descendants.

• If various implementations share is method signature then Interface works best where as If the various implementations are all of a kind and share a common status and behavior, usually an abstract class works best.

• Interface is slow where as abstract class is fast in performance.

• If you add a new method to an interface, you must track down all implementations of that interface in the universe and provide them with a concrete implementation of that method
where as in case of an abstract class if you add a new method to an abstract class, you have the option of providing a default implementation of it. Then all existing code will continue to work without change.

• You can put shared code into an abstract class, where you cannot into an interface.

2. How do you write user-defined exceptions in Java?

• To write user-defined exception in java, you exception class should extend Exception base class have call the super class methods in your own methods.

3. What is multiple inheritance? Is it supported in java?

• This means one object having multiple parents, which is not truly supported in java but supported in C++. But overall design of java suggests that we can implement multiple inheritances in java using interfaces.

4. What is difference between an inner class and static inner class?

• A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

5. What is an inner class and what is its advantage?

• Inner class is class, which is defined inside a class as private class and always bears a reference to the outer class.

o Inner classes can be nested within the body of a method.

• Scope of inner class is the entire enclosing class in which the inner class is nested.

• Inner classes can access attributes and methods in nesting class.

• Each inner class is compiled into a separate. class file labeled:NestingClass$InnerClass.

• Inner classes can contain methods that return handles to inner class instances.

• Major advantage of inner classes is the ability to create adaptor classes that implement an interface.

o Make all inner classes private to ensure hidden implementation.

o Rather than handle classes returning inner classes, they return interfaces.

• Inner classes frequently used with event handling in applets.

6. What is a transient variable?

o The variable, which cannot be serialized, is known as transient variable.

7. What are wrapped classes?

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

8. What is an immutable object and what are its advantages?

• An immutable object is an object and any object it references that does not change after construction. The object is, therefore, immutable for its lifetime.

• Immutable classes are commonly used to represent strings, colors, and numeric values.
Advantage:

• They guarantee that their state cannot change after construction, they are inherently thread-safe.

9. What are basic rules, which govern creation of immutable classes?

• Declare the class final.

• Declare all data private.

• Provide only getter methods and no setter methods.

• Set all instance data in the constructor.

• Clone mutable objects for which a reference to them is returned.

• Clone mutable objects for which a reference to them is received.

• Implement a deep clone if the default shallow clone is not correct for a properly behaved immutable object.

10. What is a Java package and how is it used?

• 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 also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

11.What restrictions are placed on method overriding?

• Overridden methods must have the same name, argument list, and return type.

• The overriding method may not limit the access of the method it overrides.

• The overriding method may not throw any exceptions that may not be thrown by the overridden method.

12. What is use of this and super key words?

• ‘this’ is used for referring to current instance of the object where as ‘super’ is used to refer to variables and methods of its super class.

• Incase of constructors, this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.


13. What is the difference between a static and non-static variable?

• 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.

14. What is polymorphism and what are different types of polymorphism?

• One name, different form and there are three forms.

1. Method overloading

2. Method overriding through Interfaces

3. Method overriding through java interfaces.

15. What is the essence of run-time polymorphism behavior?

With runtime polymorphism based on method overriding, the decision as to which version of a method will be executed is based on the actual type of object whose reference is stored in the reference variable, and not on the type of the reference variable on which the method is invoked.
The decisions as to which version of the method to invoke cannot be made at compile time. That decision must be deferred and made at runtime. This is sometimes referred to as late binding.

16. What is a singleton class and how to implement it? What are the advantages of Singleton class?

Singleton classes are created to have only one instance.
A singleton may be used to represent some unique system.
It is generally implemented by keeping the constructor private.
It should have static members and methods.
The main advantage of singleton class is memory management( Garbage collection)

17. Why java prohibits use of ‘synchronized’ inside a constructor ?

Because other threads cannot get a reference to the object until construction of the object completes. This has got a performance overhead.

18. What do you mean by Garbage collection and what are its advantages and disadvantages?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed.

An object is eligible for garbage collection when its reference is set to null. Method variables or local variables are eligible for Garbage collection when they go out of scope.

Garbage Collection cannot be forced explicitly. JVM may do garbage collection if it is running short of memory.The call System.gc() does NOT force the garbage collection but only suggests that the JVM may make an effort to do garbage collection.

It frees up the heap space and takes care of heap fragmentation.

It also helps to ensure program integrity as programmers are unable to crash the JVM by incorrectly freeing the memory accidentally or purposefully.

The only disadvantage is garbage-collected heap is a performance overhead.

19.What do you mean by conservative garbage collector?

The garbage collectors that cannot distinguish between genuine object references and look-alikes.

20. Can an object's finalize () method be invoked while it is reachable?

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.

21. How many times may an object's finalize() method be invoked by the garbage collector and when it is invoked ?

An object's finalize() method may only be invoked once by the garbage collector when the object is unreachable.

22. Does garbage collection guarantee that a program will not run out of memory?

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.

23. How does the distributed garbage collection work in java?

• The RMI subsystem implements a reference counting-based distributed garbage collection (DGC) algorithm to provide automatic memory management facilities for remote server objects.

• Basically, DGC works by having the remote server keep track of all external client references to it at any given time. When a client obtains a remote reference, it is addded to the remote object's referenced set. The DGC then marks the remote object as dirty and increases its reference count by one. When a client drops a reference, the DGC decreases its reference count by one, and marks the object as clean.

When the reference count reaches zero, the remote object is free of any live client references. It is then placed on the weak reference list and subject to periodic garbage collection.

24.What is the priority of Garbage collection thread in java ?

This is a low-priority thread in java.

25. How many applications can be run by one run-time instance of JVM ?

One run-time instance of java application can run only one instance.

26.When the JVM terminates ?

The Java Virtual machine terminates when all the non-daemon threads of java application terminates.
If permitted by the security manager it can also call System.runtime.ext () for terminating the JVM.

27. Is it possible to know from the content of a class file whether it is a java class file or not?

The first four bytes of every java class file is a magic number “0xCAFEBABE”. So it is very easy to identify whether it is a java class file or not.

28.Name the process thru which JVM makes the types available to the running program?

The Java Virtual Machine makes types available to the running program through a process of loading, linking, and initialization.

29. Define loading, linking and initialization of java class ?

Loading : Loading is the process of bringing a binary form for a type into the Java Virtual Machine.

Linking : Linking is the process of incorporating the binary type data into the runtime state of the virtual machine. Linking is divided into three sub-steps: verification, preparation, and resolution.

Initialization: During initialization the class variables are their proper initial values.

30. What do you mean by first active use of a class in JVM?

• The invocation of a constructor on a new instance of the class.

• The creation of an array that has the class as its an element type.

• The invocation of a method declared by the class (not inherited from a super class) .

• The use or assignment of a field declared by the class (not inherited from a superclass or super interface), except for fields that are both static and final, and are initialized by a compile-time constant expression.

31. What are steps involved in loading of a class file into JVM?

• Produce a stream of binary data that represents the type.

• Parse the stream of binary data into internal data structures in the method area.

• Create an instance of class java.lang.Class that represents the type.

32.Is it possible to detect a malformed class file during early loading by the class loader?

No it is not possible. If a class loader encounters a missing or malformed class file during early loading, it must wait to report that error until the class is first active use by the program.

33.What are the different stages of linking after the class is loaded?

Verification, prepare and resolve are the three phases of linking.

34. What is verification stage and what are steps involved during verification?

It ensures that the types obey the semantics of java language and doesn’t violate the integrity of JVM.

The entire process of detecting any kind of problem with loaded types is placed under the category of verification.

Verification of symbolic references and converting them to direct references.

• checking that final classes are not sub classed

• checking that final methods are not overridden

• if the type being checked is a non-abstract class, checking that all the methods declared in any
interfaces implemented by the class are indeed implemented by the class

• Making sure no incompatible method declarations (such as two methods that have the same name, the same number, order, and types of parameters, but different return types) appear between the type and its supertypes.

35. What is done during Preparation Stage?

The Java Virtual Machine allocates memory for the class variables and sets them to default initial values. The class variables are not initialized to their proper initial values until the initialization phase.

Java Virtual Machine implementations may also allocate memory for data structures that are intended to improve the performance of the running program.

36. What happens during Resolution Phase?

Resolution is the process of locating classes, interfaces, fields, and methods referenced symbolically from a types constant pool, and replacing those symbolic references with direct references.

37. How the initialization of class is is different from initialization of interface?

Initialization of class requires initialization of its super classes but initialization of interfaces doesn’t require initialization of it super interfaces.