Q&A to java english interview

Refer to stuff as follow:

java core:
http://www.journaldev.com/1162/java-multi-threading-concurrency-interview-questions-with-answers

java multithread:
http://www.journaldev.com/1162/java-multi-threading-concurrency-interview-questions-with-answers

Question:What does happen if the main method is declared as private?

Answer:

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

Question:What is the advantage of OOP?
Answer:
Major advantages of OOP are:

simplicity: software objects model real world objects, so the complexity is reduced and the program structure is very clear;

modularity: each object forms a separate entity whose internal workings are decoupled from
other parts of the system;

modifiability: it is easy to make minor changes in the data representation or the procedures in an OO program. Changes inside a class do not affect any other part of a program, since the only public interface that the external world has to a class is through the use of methods;

extensibility: adding new features or responding to changing operating environments can be solved by introducing a few new objects and modifying some existing ones;

maintainability: objects can be maintained separately, making locating and fixing problems
easier;

re-usability: objects can be reused in different programs.



Question: Describe what happens when an object is created in Java

Answer:
Several things happen in a particular order to ensure the object is constructed properly:

• Memory is allocated from heap to hold all instance variables and implementation-specific
data of the object and its superclasses.  Implementation-specific data includes pointers to
class and method data.
•The instance variables of the objects are initialized to their default values.
•The constructor for the most derived class is invoked.  The first thing a constructor does is call the constructor for its superclasses.  This process continues until the constructor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java.

Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed.  Then the body of the constructor is executed.  Thus, the constructor for the base class completes first and constructor for the most derived class completes last.  The methods of the class and its parent hierarchy are made available.  Lastly,the address of the object is returned.

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

Answer:

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.

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

Answer:
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.


Question:Describe synchronization in respect to multithreading.

Answer:

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.


Question:Explain different way of using thread?

Answer:

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.


Question:What are pass by reference and passby value?

Answer:

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.


Question:What is the difference between HashMap and HashTable?

Answer:

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.

Question:What is the difference between Vector and ArrayList?

Answer:

Vector is synchronized whereas arraylist is not.

Question:What is the difference between a constructor and a method?

Answer:
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.

Question:What is an Iterator?

Answer:

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.

Question:State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
Answer:

public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

Question:What is an abstract class?
Answer:

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Question:What is static in java?
Answer:

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.

Question:What is final?
Answer:

A final class can't be extended i.e. 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).