26/03/25
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Q)Explain about Comparator interface.
=>java.util.Comparator is used in sorting of user defined collection objects.

=>compare() method of this interface is used to implement sorting criteria.

=>Comparator fecilitates multiple criteria for ordering the collection objects.

=>user defined objects of the collection need not implement this interface
in order for sorting.

=>java.util.Collections  class has sort method to perform sorting of objects.
       Collection.sort(list,Comarator)

Q)Example program on Comparator.
//ComparatorApplication.java
import java.util.*;
class Employee{
	int empno;
	String name;
	float salary;
	Employee(int empno,String name,float salary){
		this.empno=empno;
		this.name=name;
		this.salary=salary;
	}
	public String toString(){
		return empno+"\t"+name+"\t"+salary;
	}
}
class NameComparator implements Comparator<Employee>{
	public int compare(Employee e1,Employee e2){
		return e1.name.compareTo(e2.name);
	}
}
class NumberComparator implements Comparator<Employee>{
	public int compare(Employee e1,Employee e2){
		return e1.empno-e2.empno;

	}
}
class SalaryComparator implements Comparator<Employee>
{
	public int compare(Employee e1,Employee e2){
		return (int) (e1.salary-e2.salary);

	}
}
class  ComparatorApplication
{
	public static void main(String[] args) 
	{
		ArrayList<Employee> al=new ArrayList<>();
		al.add(new Employee(102,"Rama",70000));
		al.add(new Employee(103,"Rahim",50000));
		al.add(new Employee(101,"Salman",60000));
		System.out.println("Before sorting....");
		for(Employee e:al)
		   System.out.println(e);
		System.out.println("After sorting....");
		Collections.sort(al,new NameComparator());
                for(Employee e:al)
		  System.out.println(e);
	}
}
Q)What is a Set based based collection?
  =>A collection object that implements java.util.Set interface is nothing but a Set based
  collection.
  =>In a set based collection, insertion order is not preserved, duplicates are not allowed
  and at max one null value can be inserted.
  
  =>java.util.HashSet is the most frequently used Set based collection class.

  DIAGRAM

  Note:- A set based collection  abstracts a mathematical set.

Q)What are the most frequently used methods of Set interface?
 1)size()
 2)isEmpty()
 3)contains()
 4)remove()
 5)addAll(collection)
 6)retainAll(collection)
 7)removeAll(collection)
 8)clear()
 Note:- Set interface has not added any new methods.These are from Collection interface
 only.

Q)Example program on set based collection(union,intersection,difference)
import java.util.*;
class SetOperations{
	public static void main(String[] args) {
		HashSet<Integer> one=new HashSet<>();
		Collections.addAll(one,10,20,30,40,50);
		HashSet<Integer> two=new HashSet<>();
		Collections.addAll(two,30,40,50,60,70);
		/*two.retainAll(one);//Intersection
		System.out.println(two);
		one.addAll(two);//Union
		System.out.println(one); */
		one.removeAll(two); //difference of two sets
		System.out.println(one);
	}
}


Q)What is a map based collection?
=>An object that implements java.util.Map interface is nothing but a map based
collection.

=>A map based collection object maps keys to values.

=>A map can't contain duplicate keys.

=>Each key can be mapped to atmost one value.

=>Most frequently used map based collection class is HashMap.


DIAGRAM

=>Most frequently used methods of Map interface 

put(K,V)
get(K)
replace(K,NV)
remove(key)
Set keySet()
Collection values()
clear()

Q)Example program on HashMap.
import java.util.*;
class  HashMapExample{
	public static void main(String[] args){
		HashMap<Integer,String> hm=new HashMap<>();
		hm.put(101,"David");
		hm.put(102,"Rama");
		hm.put(103,"Rahim");//C of CRUD
                System.out.println("Emp name of 103 Id:"+hm.get(103));//R of CRUD
		System.out.println(hm);
		hm.put(103,"Sharif");//U  of CRUD
		System.out.println(hm);
		hm.remove(102);//D of CRUD
		System.out.println(hm);
		Set<Integer> keys=hm.keySet();
		for(Integer key:keys){
			System.out.println(key+":"+hm.get(key));
		}
		Collection<String> values=hm.values();
		System.out.println(values);
		hm.clear();
		System.out.println(hm);
	}
}