Difference between Vector and ArrayList?

Vector is synchronized whereas arraylist is not.

Difference between HashMap and HashTable?

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.

What is HashMap and Map?

Map is Interface and Hashmap is class that implements that.

What are pass by reference and passby value?

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.

Explain different way of using thread?

The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

Describe synchronization in respect to multithreading.

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.

What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

What is reflection ?

Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.

What is the basic difference between string and stringbuffer object ?

String is an immutable object. StringBuffer is a mutable object.

What is mutable object and immutable object ?

If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)

What is a daemon thread ?

These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.

What is garbage collection ? What is the process that is responsible for doing that in java ?

Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process.

What is the purpose of Void class ?

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.

What if the main method is declared as private ?

The program compiles properly but at runtime it will give "Main method not public." message.

What is meant by pass by reference and pass by value in Java ?

Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.

What if the main method is declared as private ?

The program compiles properly but at runtime it will give "Main method not public." message.

What if I do not provide the String array as the argument to the method ?

Program compiles. But at runtime throws an error "NoSuchMethodError".

What does it mean that a class or member is final ?

o final - declare constant
o finally - handles exception
o finalize - helps in garbage collection

Variables defined in an interface are implicitly final. A final class can't be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.