How to make normal java class as immutable

public final class ImmutableClass

{

 

    /**

    * Integer class is immutable as it does not provide any setter to change its content

    * */

    private final Integer immutableField1;

    /**

    * String class is immutable as it also does not provide setter to change its content

    * */

    private final String immutableField2;

    /**

    * Date class is mutable as it provide setters to change various date/time parts

    * */

    private final Date mutableField;

 

    //Default private constructor will ensure no unplanned construction of class

    private ImmutableClass(Integer fld1, String fld2, Date date)

    {

        this.immutableField1 = fld1;

        this.immutableField2 = fld2;

        this.mutableField = new Date(date.getTime());

    }

 

    //Factory method to store object creation logic in single place

    public static ImmutableClass createNewInstance(Integer fld1, String fld2, Date date)

    {

        return new ImmutableClass(fld1, fld2, date);

    }

 }

Query to find Nth heighest salary from Employee Table

SELECT SAL FROM EMPLOYEE emp1 WHERE (

      SELECT COUNT(DISTINCT SAL) FROM EMPLOYEE  emp2

              WHERE emp2.SAL >=  emp1.SAL) = N

Here if N is 3 you can find 3rd highest salary ..

Code for user defined objects as a key in HashMap

import java.util.HashMap;

import java.util.Set;

 

public class MyObjectKeySearch {

 

public static void main(String a[]){

         

        HashMap<Price, String> hm = new HashMap<Price, String>();

        hm.put(new Price("Banana", 20), "Banana");

        hm.put(new Price("Apple", 40), "Apple");

        hm.put(new Price("Orange", 30), "Orange");

        printMap(hm);

        Price key = new Price("Banana", 20);

        System.out.println("Does key available? "+hm.containsKey(key));

    }

     

    public static void printMap(HashMap<Price, String> map){

         

        Set<Price> keys = map.keySet();

        for(Price p:keys){

            System.out.println(p+"==>"+map.get(p));

        }

    }

}

 

class Price{

     

    private String item;

    private int price;

     

    public Price(String itm, int pr){

        this.item = itm;

        this.price = pr;

    }

     

    public int hashCode(){

        System.out.println("In hashcode");

        int hashcode = 0;

        hashcode = price*20;

        hashcode += item.hashCode();

        return hashcode;

    }

     

    public boolean equals(Object obj){

        System.out.println("In equals");

        if (obj instanceof Price) {

            Price pp = (Price) obj;

            return (pp.item.equals(this.item) && pp.price == this.price);

        } else {

            return false;

        }

    }

     

    public String getItem() {

        return item;

    }

    public void setItem(String item) {

        this.item = item;

    }

    public int getPrice() {

        return price;

    }

    public void setPrice(int price) {

        this.price = price;

    }

     

    public String toString(){

        return "item: "+item+"  price: "+price;

    }

}