Enhanced for loop and Immutability?

There I find a twist in the enhanced for loop and immutable nature. Where it does not change the data in an array of primitives, here I can see a clear variation.

Here is a Person class.

package test;

public class Person {
      private int pId;
      private String name;

      public Person(int pId, String name) {
          this.pId = pId;
          this.name = name;
     }
    public String toString(){
            return "Id : "+pId+" Name : "+name;
    }
    public int getpId() {
           return pId;
    }

    public void setpId(int pId) {
         this.pId = pId;
   }

   public String getName() {
         return name;
   }

   public void setName(String name) {
        this.name = name;
    }
}

And here is a tester class with enhanced for loop.

package test;

import static java.lang.Math.random;
import java.util.ArrayList;
import java.util.List;

public class PersonList {
      public static void main(String[] args) {
           List<Person> pList = new ArrayList<>();
           pList.add( new Person(101,"John"));
           pList.add( new Person(102,"James"));
           pList.add( new Person(103,"Dan"));
           pList.add( new Person(104,"Mathew"));
           pList.add( new Person(105,"Sam"));

          System.out.println("Added List elements : "+pList);
         /* To perfomr element wise operation */
          for(Person p : pList){
                System.out.println(p);
          }
         /* you try to modify the data in pList through enhanced for loop */
                System.out.println("List elements now : ");
                for(Person p : pList){
                     if(p.getName().equals("Sam"))
                          p.setName("Samuel");
                 }

/* OOPs... it has not changed...But let us conform it once again by using enhanced for loop */

          System.out.println("After Updation : view element in the list : ");
          for(Person p : pList){
                 System.out.println(p);
          }
  }
}

When I run this I see something different.

Added List elements : [Id : 101 Name : John, Id : 102 Name : James, Id : 103 Name : Dan, Id : 104 Name : Mathew, Id : 105 Name : Sam]
Id : 101 Name : John
Id : 102 Name : James
Id : 103 Name : Dan
Id : 104 Name : Mathew
Id : 105 Name : Sam
List elements now : 
After Updation : view element in the list : 
Id : 101 Name : John
Id : 102 Name : James
Id : 103 Name : Dan
Id : 104 Name : Mathew
Id : 105 Name : Samuel

Now the question is why the name gets changed from within the enhanced for loop. Now in previous post x reads data from ArrayList, and value change in x does not reflect in ArrayList. But in current example, p holds the reference to an object and using reference, it change the value. Rest is left to your imagination... :) !!!

Enhanced for Loop : Collections and Immutability

Enhanced for loop was added in Java SE 1.5. It can be used with Arrays and collections to iterate item-wise/element-wise. It works like read data of very ancient programming language called BASIC... atleast to me it looks like that... You can define read element of the collection element type and do operations on data. The amazing part is whatever you do to the read element, it does not disturb the original collection, that is data in for loop is immutable...

package test;

import java.util.ArrayList;
import java.util.List;
import static java.lang.Math.random;
/**
*
* @author nkan
*/
public class EnhancedForLoop {
public static void main(String[] args) {
List<Integer> intList = new ArrayList<>();
for (int i=0;i<5;i++){
intList.add((int) (random() * 100));
}

System.out.println("Added List elements : "+intList);
/* To perfomr element wise operation */
for(int x : intList){
x++;
System.out.println("x = "+x);
}
/* you have incremented x, may be you are thinking that ArrayList : intList has be modified */
System.out.println("List elements now : "+intList);
/* OOPs... it has not changed...But let us conform it once again by using enhanced for loop */
System.out.println("After Updation : incremented element list is : ");
for(int x : intList){
System.out.println("x = "+x);
}
}
}

When I executed this program :

Added List elements : [98, 14, 30, 34, 43]
x = 99
x = 15
x = 31
x = 35
x = 44
List elements now : [98, 14, 30, 34, 43]
After Updation : incremented element list is : 
x = 98
x = 14
x = 30
x = 34
x = 43

Created and posted by Nancy @ 12/17/2015 2:25 am

Understanding mysterious static - 2

Oh! that was interesting, if you tried executing the program in the previous post. For a second may be it gave a food for thought to your mind. Why main is not executing first? Why am i getting output as:

In static block...InterestingStatic1
In main...InterestingStatic1

May be something is happening before main is executing. I must know it. Everything is mystery until understood. So let me take you to another interesting and mysterious scenario :

Here is what you do. Nothing much. Add InterestingStatic1Test class to the project you created yesterday. Let the class InterestingStatic1 be there in the project without any changes.

package teststatic;
public class InterestingStatic1Test {
    static{
        System.out.println("In static block...InterestingStatic1Test");
    }
    public void printSomething(){
        System.out.println("Just printing ... Welcome to Mysteries of Java");
    }
    public static void main(String[] args) {
        System.out.println("In main...InterestingStatic1Test");
        InterestingStatic1Test justObject = new InterestingStatic1Test();
        justObject.printSomething();
        InterestingStatic1 newObject = new InterestingStatic1();
    }
}

Yes, you are right. I am going to ask you to guess the output first and then run the program. Does it match your guess? Why the output is what it is.  wait... there is more to come ... See you...

©K.A.N. Nancy

Understanding mysterious static - 1

package teststatic;

public class InterestingStatic1 {

    static{

        System.out.println("In static block...InterestingStatic1");

    }

    public static void main(String[] args) {

        System.out.println("In main...InterestingStatic1");

    }

}

What do you think should be the output. Normal understanding is main is executed first by JVM. Run this code. Do you find something unexpected?

More interesting code … tomorrow… let the mystery of static be resolved slowly, so that you enjoy…

©K.A.N. Nancy

static in Java - 1

When a static block in a Java class is executed?

     a) When the class is compiled

     b) When the first method in class is executed

     c) When the object is created, but before the constructor is called

     d) When the object is created, and after the constructor is called

     e) When the class is loaded

©K.A.N. Nancy

Adding 2 numbers using Java

Let us have fun to add two numbers in variety of ways :

public class FunWithAdd {
   private int num1;
   private int num2;
  public FunWithAdd(int num1, int num2){
       this.num1=num1;
       this.num2=num2;
   }
   public int getNum1(){
       return num1;
   }
    public int getNum2(){
       return num2;
   }
    public void setNum1(int num1){
        this.num1=num1;
    }
    public void setNum2(int num2){
        this.num2=num2;
    }
   int sum(){
       return num1+num2;
   }
   int sum(int n1, int n2){
       return n1+n2;
   }
   int sumWithItr (int n1, int n2){
      int s=n1;
     
      for(int i=1;i<=n2;i++){
          s++;
      }
      return s;
   }
   int sumRecursive(int n1, int n2){
        if (n2 ==0){
            return n1;
        }
        return sumRecursive(++n1,--n2);
      }
   }

Create a class to test variety of sum methods.

public class Add2Nums {
    public static void main(String[] args) {
        /* Summing Locals */
        int num1=25;
        int num2=12;
        System.out.println("sum : "+(num1+num2));
        /* Sum using Object of FunWithAdd*/
        FunWithAdd numsObject= new FunWithAdd(30,60);
        System.out.println("Adding the numbers in Object : "+numsObject.sum());
        System.out.println("Adding the numbers in Object by sending data from outside :"+numsObject.sum(19,20));
        numsObject.setNum1(7);
        numsObject.setNum2(9);
        System.out.println("sum using sumWithItr method : "  +numsObject.sumWithItr(numsObject.getNum1(),numsObject.getNum2()));
         numsObject.setNum1(43);
        numsObject.setNum2(2);
        System.out.println("sum using sumRecursive: "+numsObject.sumRecursive(numsObject.getNum1(),numsObject.getNum2()));
    }
}

Output :
sum : 37
Adding the numbers in Object : 90
Adding the numbers in Object by sending data from outside :39
sum using sumWithItr method : 16
sum using sumRecursive: 45

Change the values and have fun… :)
©Nancy : created at 2:07 am IST

Java EE 6 Collocated Model

EJB 3.1 provides subset of EJB. This subset is known as EJBLite. EJBLite is available as a part of Java EE6 Web Container. You can create local EJBs with and without interface view. 

EJB 3.1 The benefit is when Web component and EJB component need to access the object for business processing, no marshaling is required. If an EJB makes changes in the object state , web component can view the changes. 

The local EJBs can be packaged in a WAR file. This is called as Collocated model of Java EE 6.