Shamik Mitra: Your Academic Support System

Answer

The main difference is

1. When you declare an Array you have to provide the size so ita can occupy that amount of space unlikely Vector is dynamic you don't have to provide the size although it's internal structure is like array and it has a loadfacter after reaching the load factor capacity Vector copy it's element to another memory location to dynamically grow.

2. Array can hold primitives as well as Objects where as vector only contains Objects primitives are supported through Wrapper class.

3. Vector is synchronized but array is not.

4. If you not use generics Vector can store any objects so things will be messy at runtime where as Array can only stores those objects who are in same inheritence tree.

Answer

The term Encapsulation means hide class variable for direct external access. You can access then via getter and setter.

What would be the benifit to doing so

 

as if I write

 

Public int i=9;

or private int i=1;

the getI() and setI() method to access them.

 

Think

 

By direct access what I can do is I can brake your code as directly set

yourObject.i=-3;

but through setI you can put a vilade logic If num provide is negetive you can reject the value. This is the benifit bu indirect access

 

So we write POJO mark membervariables as private and provide getter setter so in future we can add validation.

 

 

Answer

Marker interface is such interface which has no abstract methods and variables. JVM traets  them specially.

Serializable,clonable are example of Marker interface

Answer

Please be specific which type of connection are you trying to open.

 

If  it is Databse then

 

you have to load the JDBC drivet then get connection from it

 

like

 

Class.forname(FQDN of Driver class);

Connection con = Drivermanager.getConnection(url,name,password);

sid ,User name of database password of Database are reqired parameter.

Answer

Casting means cast a class to another class. This definition has two  catch points. a. can I cast any type to another type: No You can only cast those classes to one another which are in the same inheritance tree . You can go back and forth but you are not allow to cast between siblings. So you are able to cast child to parent or parent to child not even siblings !!!! so if I have a class hierarchy like Class A : Base class Class B : sub class of A Class C : sub class of A Class D : Subclass of B Then B can allow to  cast  A and D but not C. b. How many  types of Casting are there in Java: There are two types of casting a. Upcasting b. DownCasting Upcasting : when a Subclass is casted to parent that is called Upcast. In java every polymorphic assignment is that type of casting. To do that you do not have to bother about explicit casting. this is logical because inheritance means  inheriting   the properties of parent in to child so you can easily say, property exists in child also  exists in parent. Subcalass = parent properties  + it's own property so , A a=null; B b =new B(); //B extends A a =b;//Upcasted no need to explicit casting. DownCasting When a parent cast to it childs it called downcasting and explicitly casting is needed here. It is logical as Parent does not know which child it needs to be casted more over parent does not knows about Child's own properties. so Parent = subclass - Subclass own property B b =null;   //B extends A A a = new B();//polymorphic assignment. b =(B)a; // need  explicit casting as ref a does not know about Object B. Now add trickiness What will happen B b =null;   //B extends A A a = new A(); b =(B)a; // need  explicit casting. Here when you accessing B's own property it will throw run time exception. as although here Compiler will not complain, but b reference  holds object of A,so when you called class B's own property through reference b. Actual  A object does not have that property and throws runtime exception. So be-careful about Downcasting.  

Answer

Understanding most critical access modifier  protected in java? protected is most critical modifier to understand  . According to the book it is same as default modifier, which can be accessed by other classes in same package but the only difference is it can also be accessed by subclasses from  outside packages through inheritance. Now there are two catch points 1. What do you mean by accessed ? 2. What do you mean by access through inheritance? 1. What do you  mean by accessed ? In java you can access properties of other classes by two ways a. Association (HAS A) 2. inheritance (IS A) Scenario a. packge com.foo; public class Foo extends Bar { Bar bar = new Bar(); // BAR HAS A relationship with FOO(Association) bar.greet; //not compile no visibility through association } package com.bar; public class Bar {  protected String greet = "Hello"; } Scenario b packge com.foo Class Foo extends Bar { System.out.println(greet); //Inherited so it print Hello } package com.bar Class Bar {  protected String greet = "Hello"; } Association means a class has a reference of another class  as you see in Scenario a. Inheritance means  a class inheriting parent class property. For protected the tricky situation arises when  two classes are in different package ,as you see Foo is in com.foo and Bar in com.bar now the question is, can Foo access Bar's greet variable? The Answer is simply no as by rule protected is visible only through inheritance so although class Foo extends Bar but we are trying to access greet by association which is not acceptable in java On other hand in Scenario 2 will be compile fine and show Hello message as Foo inherited Bar's greet property. Now add another criticality suppose class zoo is in the packge com.foo now If I write following code snippet in Zoo Foo foo = new Foo(); foo.greet; will it be compile? Think about it....... The Answer is no as I said earlier by Association (different package ) you can't ever access greet. but in Same package you do.  

Answer

Abstract class needed when you are in design phases and identify some group of classes which are very same in behaviour but minimum difference are so what you do is you can create a Abstract class provide the common behaviours in to it and make a contract mean create   abstract methods for  what varies and deligate the reponsibility to subclass to fulfill the contract.

Answer

ServletContext is kind of Application scope . whre you can put variable which can be shared by all Servelets in that application. so one can think it as an global context map which is shared by all Servlets.

Answer

First of all sleep method is Threads method where as wait is Objects method.

 

When you call sleep current execution thred going to inactive/runnable state for certain amount of time (Provide in the argument) but it does not release the resources. The benifit is that ut provide a chance to execute another thread which is in runnable state.

 

Where as wait is object's method it must be called under synchronized context . By doing so current exeecution thread on that object means the thread which accquires the lock of the objects tell to other threads to wait ,which are waiting for accuire the lock for same object.

Answer

It is require to every member of an class to implement serializable unless it throws serializable exception as When a object going to serialize it erialize whole object graph. Nut if you have such association whicj is not serializable you can mark them transient then it will not a part of serialization process.