Tuesday, August 5, 2014

[Java] Creating Immutable Class

what is immutable class. A class whose object can not be modified.

So we have to make sure we have stop all the mechanism which allows user to change the stage.

* Member Variable - Make them final so no one can change state.
* Class as Final - So derived class can not override the behavior.
* No Setter method - So its can not be changed later in case you missed to  initialize in constructor.

public Final class DataReceiver
final string port_id;
public DataReceiver( String port)
this.port_id = port;

public String getPortID()
return port_number;

Above class will be used as immutable now. :)

[PUZZLE] Measure 50% water in Cylindrical glass

We have filled a cylindrical glass with water. We have to measure that  glass have exactly 50% of water. We don't have any measuring instrument. 


Two ways can be there.

First tilt the glass to the point water reaches to brim of glass. This same point other side water should be exactly at upper bottom. If this happens them you have water half measured.
If water is lower than upper bottom then its less than 50% and if more than upper bottom then more than 50% water is there.

Other way is to mark the current level of water on glass by placing a finger there. 
Cover glass head with you palm or disc and turn over the glass. If water still touches marked location only then its exact half.


A merchant has bough some 3000 banana from market and want to bring them to his home which is 1000 km far from Market. He have a elephant which can carry maximum of 1000/- banana at time. That could have been easy but this elephant is very shrewd and he charges 1 banana for every one kilometer he travel.

Now we have to maximise number of banana which should reach to home.


At present we are at 0th km with 3000 banana and elephant.
Lets see if elephant have to shift all the 3000 banana and elephant by 1 km.

For 1 km shift:

Take first 1000 banana and drop at 1st km. 2 banana consumed. One banana each for to and fro.
Second  1000 banana and drop at 1st km. 2 banana consumed.
Third 1000 banana and reach at 1st km. 1 banana consumed.

So all in all total 5 banana will be consumed if we shift a kilometer.

But that's not all, our total banana number are also reducing so we may have to reduce the number of turns too. And this will happen once we have reduced the banana by 1000 number. That means to reduce the count of banana by 1000 for first move we have to shift base to 200 km. See 5 banana for 1 km ratio i.e. 1000 banana / 5 = 200 km.

So we are now at 200th KMs with 2000 banana and elephant.

Post 200 km, for every 1 km shift in base;

Take first 1000 banana and drop at 1st km. 2 banana consumed. One banana each for to and fro.
Third 1000 banana and reach at 1st km. 1 banana consumed.

So all in all total 3 banana will be consumed if we shift a kilometer post 200 kms. 

Now the second phase will be to reduce the 1000 banana. So we can move to 1000/3 = 333.3 km maximum in this phase. 

So we will be moved to (200 + 333.3) 533.3  km base with 1000 banana.

Now onward we need only 1 banana for every km shift as elephant can carry all the 1000 banana and need not to come back. 
So out of 1000 banana, elephant will consume (1000 - 533.3) more banana to reach home.  

And total saved banana will be (1000 -(1000 -533.3)) = 533.3 banana. 

[PUZZLE] Solve the following series

1, 20, 33, 400, 505, 660, 777, 8000, 9009, ........

Seems to be little strange ..huh. Most of times when i see this kind of series and i dont get a clue in couple of minutes, i become so mad with me. Fortunately today i could solve this in minutes.

Here is explanation, sequence is Natural Number's binary representation * Number.

Number  Binary  Binary * Number
1 1 1
2 10 20
3 11 33
4 100 400
5 101 505
6 110 660
7 111 777
8 1000 8000
9 1001 9009

Monday, August 4, 2014

[JAVA] Evil Null

Elvis operator could have been a good use here. But unfortunately have been decline till now.

Its used to say "?." Operate only if not null.

Class A
 public B getB()
   return new B();

public void test( A a)
    if ( a != null  )
        return a.getB();

Above method would be replaced with

public void test ( A a)
     return a?.getB();

Unfortunately its still some time we can see some think like that. So till that we have to live with two choices:

1. Check for null.
2. Use the Null Object Pattern.

So we all know that how to check for null objects and believe me i had real long chain of checking null.

Second way can be useful but if this chain is pretty long its can be tiresome to have null object for all the hierarchy

Null Object means there will be definition of class A (mostly derived from same interface.) and will have dummy methods which will nullify any call to these methods.

JAVA CLASSLOADER- Types, usages.

JVM loads library and classes dynamically only. Its on demand only.
A Class will be loaded only when needed and only once.

There can be system supported class loader and user supplied class loader.
When JVM starts it loads three type of class loader
1. Bootstrap loader - When system boots. Loads from jre_home/lib/
2. External class loader - Loads from jre_home/lib/ext.
3. System Class loader - Loads classes from system property CLASSPATH.

Besides this user can provide their own class loader which is pretty easy to implement in Java.
User supplied loader will work in conjunction with other loader i.e. system loader too.

Some of the examples are:

1. Load library at runtime from http resources. Example scripting classes, bean classes.
2. Can load encrypted class files with new class loader.
3. Modify the byte code.

Application Container loads classes from deployed WAR or EAR files using a tree of class loaders.