How to create an immutable class in java a simple example - biju vas

package com.cfed.tutorials;
/**
 *  creating an immutable class - class which state does not change
 * 
 *  helps for caching, thread safe etc
 * 
 *  class is final
 * 
 *  all mutable fields are changed to final
 * 
 *  no setters
 * 
 */
import java.util.HashMap;
import java.util.Iterator;

public final class MedicalRepresentives {

private final int tin;

private final String companyName;

private final HashMap<String,String> employees;

public int getId() {
return tin;
}


public String getName() {
return companyName;
}

/**
* Accessor function for mutable objects
*/
public HashMap<String, String> getTestMap() {
//return testMap;
return (HashMap<String, String>) employees.clone();
}


public MedicalRepresentives(int i, String n, HashMap<String,String> hm){

this.tin=i;
this.companyName=n;
HashMap<String,String> tempMap=new HashMap<String,String>();
String key;
Iterator<String> it = hm.keySet().iterator();
while(it.hasNext()){
key=it.next();
tempMap.put(key, hm.get(key));
}
this.employees=tempMap;
}


/**
* Constructor performing Shallow Copy
* @param i
* @param n
* @param hm
*/
/**
public FinalClassExample(int i, String n, HashMap<String,String> hm){
System.out.println("Performing Shallow Copy for Object initialization");
this.id=i;
this.name=n;
this.testMap=hm;
}
*/

/**
* To test the consequences of Shallow Copy and how to avoid it with Deep Copy for creating immutable classes
* @param args
*/
public static void main(String[] args) {
HashMap<String, String> h1 = new HashMap<String,String>();
h1.put("1", "Biju Vas");
h1.put("2", "Smithesh");
h1.put("3", "Sreekumar");

String s = "Guru Medicals , Vadakara";

int i=19999999;

MedicalRepresentives ce = new MedicalRepresentives(i,s,h1);

//Lets see whether its copy by field or reference
System.out.println(s==ce.getName());
System.out.println(h1 == ce.getTestMap());
//print the ce values
System.out.println(" Tin number:"+ce.getId());
System.out.println("organisation name :"+ce.getName());
System.out.println("employees :"+ce.getTestMap());
//change the local variable values
i=20;
s="New organisation";
h1.put("4", "Manohar");
//print the values again
System.out.println("tin after local variable change:"+ce.getId());
System.out.println("organistion name after local variable change:"+ce.getName());
System.out.println("employess after local variable change:"+ce.getTestMap());

HashMap<String, String> hmTest = ce.getTestMap();
hmTest.put("4", "new");

System.out.println("ce testMap after changing variable from accessor methods:"+ce.getTestMap());

}

}


output


true
false
 Tin number:19999999
organisation name :Guru Medicals , Vadakara
employees :{1=Biju Vas, 2=Smithesh, 3=Sreekumar}
tin after local variable change:19999999
organistion name after local variable change:Guru Medicals , Vadakara
employess after local variable change:{1=Biju Vas, 2=Smithesh, 3=Sreekumar}
ce testMap after changing variable from accessor methods:{1=Biju Vas, 2=Smithesh, 3=Sreekumar}