1. 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. |
2. What if the main method is declared as private? |
| The program compiles properly but at runtime it will give "Main method not public." message. |
3. 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. |
4. 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. |
5. 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. |
6. 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. |
7. What is HashMap and Map? |
| Map is Interface and Hashmap is class that implements that. |
8. 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. |
9. Difference between Vector and ArrayList? |
| Vector is synchronized whereas arraylist is not. |
10. What is the difference between a constructor and a method? |
| A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. |
11. What is final? |
| A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). |
12. What is static in java? |
| Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass. |
13. What is an Iterator? |
| Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator. |
14. What is the difference between class variable, member variable and automatic(local) variable |
| class variable is a static variable and does not belong to instance of class but rather shared across all the instances
member variable belongs to a particular instance of class and can be called from any method of the class
automatic or local variable is created on entry to a method and has only method scope
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