JVM Stack and Heap.

Stack

Each Java virtual machine thread has a private Java virtual machine stack, created at the same time as the thread. A Java virtual machine stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Because the Java virtual machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java virtual machine stack does not need to be contiguous.

The Java virtual machine specification permits Java virtual machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java virtual machine stacks are of a fixed size, the size of each Java virtual machine stack may be chosen independently when that stack is created. A Java virtual machine implementation may provide the programmer or the user control over the initial size of Java virtual machine stacks, as well as, in the case of dynamically expanding or contracting Java virtual machine stacks, control over the maximum and minimum sizes.

The following exceptional conditions are associated with Java virtual machine stacks:

If the computation in a thread requires a larger Java virtual machine stack than is permitted, the Java virtual machine throws a StackOverflowError.
If Java virtual machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java virtual machine stack for a new thread, the Java virtual machine throws an OutOfMemoryError

Heap

The Java virtual machine has a heap that is shared among all Java virtual machine threads. The heap is the runtime data area from which memory for all class instances and arrays is allocated.

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java virtual machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

The heap mainly store objects create using or class level variables.

The following exceptional condition is associated with the heap:

If a computation requires more heap than can be made available by the automatic storage management system, the Java virtual machine throws an OutOfMemoryError

The JVM size can be increased or decreased as needed:

The old generation’s default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms
<initial size> -Xmx <maximum size> program
For example:
java -Xms64m -Xmx128m program

What is difference between ClassNotFoundException and NoClassDefFoundError?

Many people get confused with these 2 errors. So here is a brief explanation.

A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader in the CLASSPATH. It could also mean that the class in question is trying to be loaded from another class which was loaded in a parent classloader and hence the class from the child classloader is not visible.

Consider if NoClassDefFoundError occurs which is something like

java.lang.NoClassDefFoundError

src/com/TestClass

does not mean that the TestClass class is not in the CLASSPATH. It means that the class TestClass was found by the ClassLoader however when trying to load the class, it ran into an error reading the class definition. This typically happens when the class in question has static blocks or members which use a Class that’s not found by the ClassLoader. So to find the culprit, view the source of the class in question (TestClass in this case) and look for code using static blocks or static members.

When to use ArrayList ,LinkedList,Vector?

Consider a list collection. Now lets consider some scenario:

1) the list needs to be modify such that the elements are added only at the end of list, and if required only the last element of the list to be deleted, use ArrayList

2) the list needs to be modify such that the elements can be added at any position of the list, and if required the element can be deleted from any position, use LinkedList

3) the list needs to be threadsafe, use Vector.

4) the list is read only and needs to be iterated frequently, use ArrayList

why inner class can access only final variable?

Local classes can most definitely reference instance variables. The reason they cannot reference non final local variables is because the local class instance can remain in memory after the method returns. When the method returns the local variables go out of scope, so a copy of them is needed. If the variables weren’t final then the copy of the variable in the method could change, while the copy in the local class didn’t, so they’d be out of synch.

Anonymous inner classes require final variables because of the way they are implemented in Java. An anonymous inner class (AIC) uses local variables by creating a private instance field which holds a copy of the value of the local variable. The inner class isn’t actually using the local variable, but a copy. It should be fairly obvious at this point that a “Bad Thing”™ can happen if either the original value or the copied value changes; there will be some unexpected data synchronization problems. In order to prevent this kind of problem, Java requires you to mark local variables that will be used by the AIC as final (i.e., unchangeable). This guarantees that the inner class’ copies of local variables will always match the actual values.

Polymorphism

What is polymorphism?

Polymorphism is one of the highly used concept of OOPS . It gives us the ultimate flexibility in extensibility. Polymorphism is a term that describes a situation where one name may refer to different methods. In java there are two type of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).

When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time. Overriding occurs when a class method has the same name and signature as a method in parent class. Overloading occurs when several methods have same names with

· Different method signature and different number/type of parameters.

· Same method signature but different number of parameters.

· Same method signature and same number of parameters but of different type

int add(int a,int b)

float add(float a,int b)

float add(int a ,float b

void add(float a)

int add(int a)

void add(int a) //error conflict with the method int add(int a)

Overloading is determined at the compile time.