Contact.java

public class Contact {
    private String name;
    private int id;
    private String phoneNumber;

    @Override
    public String toString() {
        return "Contact{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", phoneNumber='" + phoneNumber + '\'' +
                '}';
    }

    public String getName() {
        return name;
    }

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

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public Contact(String name, int id, String phoneNumber) {
        this.name = name;
        this.id = id;
        this.phoneNumber = phoneNumber;
    }
}

Main.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

public class Main {


    public static void main(String[] args) {
        // write your code here
        ArrayList<Contact> contactArrayList = new ArrayList<>();
        Contact contact = new Contact("Rami",5,"12312");
        contactArrayList.add(new Contact("Inan",1,"01842101717"));
        contactArrayList.add(new Contact("Sayeed",2,"12312132"));
        contactArrayList.add(new Contact("Ekanto",3,"0184232101717"));
        contactArrayList.add(new Contact("Robin",4,"018122142101717"));

        contactArrayList.add(contact);

        for (int i = 0; i < contactArrayList.size(); i++) {
            System.out.println(contactArrayList.get(i));
        }
        contactArrayList.remove(0);
        System.out.println("----------------------------------\n\n");
        for (int i = 0; i < contactArrayList.size(); i++) {
            System.out.println(contactArrayList.get(i));
        }
        System.out.println(contactArrayList.size());


        System.out.println(contactArrayList.indexOf(contact));

        for (int i = 0; i < contactArrayList.size(); i++) {
            System.out.println(contactArrayList.get(i));
        }

        ArrayList<Contact> pcontactArrayList = new ArrayList<>();
       // Contact contact = new Contact("Rami",5,"12312");
        pcontactArrayList.add(new Contact("sfds",1,"01842101717"));
        pcontactArrayList.add(new Contact("sdfs",2,"12312132"));
        pcontactArrayList.add(new Contact("Eerterekanto",3,"0184232101717"));
        pcontactArrayList.add(new Contact("gfrgf",4,"018122142101717"));

        contactArrayList.addAll(pcontactArrayList);
        System.out.println(Arrays.toString(contactArrayList.toArray()));


    }
}

polymorphism.java

public class Addition {

    /**
     *
     * @param n1
     * @param n2
     *
     */
    static void add(int n1,int n2){
        System.out.println(n1+n2);
    }
    static int add(int n1, int n2,int n3){
        return n1+n2+n3;
    }
    static void add(){
        System.out.println(1+2);
    }
}