6. What is memory leak?
A memory leak is where an unreferenced object that will never be used again still hangs around in memory and doesn't get garbage collected.
7. How to find the size of an object?
The heap size of an object can be found using -
Runtime.totalMemory()-
Runtime.freeMemory() .
8. Why static methods cannot access non static variables or methods?
A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
9. What is difference between stringbuffer and stringbuilder?
The only difference between StringBuffer and StringBuilder is that StringBuilder is unsynchronized whereas StringBuffer is synchronized. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.
Criteria to choose among StringBuffer and StringBuilder
a) If your text can change and will only be accessed from a single thread, use a StringBuilder
b) because StringBuilder is unsynchronized.
If your text can changes, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.
10. Consider a scenario in which the admin want to sure that if some one has written System.exit() at some part of application then before system shutdown all the resources should be released. How is it possible?
This is possible using Runtime.getRuntime().addShutdownHook(Thread hook).
Straight from Java Spec:
This method registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.
Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
11. How does Java allocate stack and heap memory?
Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed.
In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.
12. Explain re-entrant, recursive and idempotent methods/functions?
A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other.
A function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. All recursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods, which are written in such a way that repeated calls to the same method with the same arguments yield same results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a server failure as long as it can reach another server.
A memory leak is where an unreferenced object that will never be used again still hangs around in memory and doesn't get garbage collected.
7. How to find the size of an object?
The heap size of an object can be found using -
Runtime.totalMemory()-
Runtime.freeMemory() .
8. Why static methods cannot access non static variables or methods?
A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
9. What is difference between stringbuffer and stringbuilder?
The only difference between StringBuffer and StringBuilder is that StringBuilder is unsynchronized whereas StringBuffer is synchronized. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.
Criteria to choose among StringBuffer and StringBuilder
a) If your text can change and will only be accessed from a single thread, use a StringBuilder
b) because StringBuilder is unsynchronized.
If your text can changes, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.
10. Consider a scenario in which the admin want to sure that if some one has written System.exit() at some part of application then before system shutdown all the resources should be released. How is it possible?
This is possible using Runtime.getRuntime().addShutdownHook(Thread hook).
Straight from Java Spec:
This method registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
- 1. The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, (or)
2. The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.
A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.
Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
11. How does Java allocate stack and heap memory?
Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed.
In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.
12. Explain re-entrant, recursive and idempotent methods/functions?
A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other.
A function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. All recursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods, which are written in such a way that repeated calls to the same method with the same arguments yield same results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a server failure as long as it can reach another server.
