help with this q

Hello could someone help me with this question please?
thanks
cat
What's an immutable object? What are some examples of immutable objects?
Why is immutability important in Java? How would you create an immutable
class?

immutable object are the objects which cannot be changed.
Example of immutable object is java.lang.String class
immutability is important in java for security reasons.
make the class variables as final to make the class immutable

Thanks for your help. Would it be ok to ask you a couple more questions?
Thanks again
cat

sure cat...
not assigned any projects till now so you can go ahead and can ask questions....
if i know you will definitely get a reply..

Thanks Subhit
This is my first question.
What is design by contract? What are the different ways to implement it
in a method using the latest Java language spec.?
what is the adaptor design pattern.

What is the difference between composition and inheritance.
What is a weak reference?

Thanks again subhit for your time
cat

design by contract is a theory, according to which a software project or a software system is viewed as a set of communicating components whose interaction with each other is based on precisely defined specifications i.e. some contracts.
It is a systematic approach to build a bug free software project...
Adaptor Pattern is one of the design pattern. I never used that so i dont know... If you want you can check out this link http://www.patterndepot.com/put/8/JavaPatterns.htm
composition means putting and class inside a class... i.e. the way in which we model objects which contains other objects is with composition.
and inheritance is extending the functionality of already existing class by extending your class with it... making a parent and child relationship..
a weak reference is one that does not prevent the referenced object from being garbage collected...

Hello again
I have a couple more questions if you dount mind.
I did check out that link you gave me but it get given me an error i will try again then i will search .
What does this question mean?
Can you create and compile class which do not invoke Object's
constructor? Can you run it?
and my last question is
What is the difference between abstract and interface? When to use
which one?
Thanks for your time
Cat

I didnt got exactly what the question means...
object of any class you wanna create, if that class dont have its own constructor then the java runtime calls the java.lang.object class's constructor, that means java.lang.Object class is the parent of all the classes whether you extend your class from it or not.... its extended by default...
If you dont want your class to be instantiated then you can create one private constructor
private constructors is being used in Singleton Design Pattern
when you dont know the full functionality of a behaviour of an object and dont want to instantiate a class, you give common functionality and leave the rest for the child class to provide the implementation.... thats where you make the parent class an abstract and the method which you want your child class to implement.
but this reduces ones ability for multiple inheritance for which one used interfaces...

Hello again
one more and that is the lot
How would you store a primitive type in a Collection such as a List?
Thanks
Cat

- put it in a wrapper object
or
- create an array of length 1 of the primitive type and add the primitive array
ArrayList ar = new ArrayList(); int num = 5; //first( and recommended ) way to add num to the list: ar.add( new Integer ( i ) ); //first way to add num to the list int[] num_arr = new int[1]; num_arr[0] = num; list.add( num_arr );
maybe you can get bonus marks for the array solution, but really, using the object wrapper class is the best solution.
Jamie
[ April 15, 2003: Message edited by: Jamie Robertson ]