Collections

This is my simulation if Array class.

package main;

public class MyArrayList {

    private Object[] elementData = new Object[10];
    int size = 0;

    public void add(Object element) {
        ensureCapacity();
        elementData[size++] = element;
    }

    public void ensureCapacity() {
        if (size == elementData.length) {
            Object[] arr2 = new Object[elementData.length / 2 + elementData.length];
            for (int i = 0; i < elementData.length; i++) {
                arr2[i] = elementData[i];
            }
            elementData = arr2;
        }
    }

    public Object get(int i) {
        return elementData[i];
    }

    public int size() {
        return size;
    }
}

ArrayList works kind of similar to this class.

Simulation of LinkedList:

package main;

public class User {

    private User user;

    public void setUser(User user) {
        this.user = user;
    }

    public User getUser() {
        return this.user;
    }
}
1. ArrayList is better in select than LinkedList.
2.LinkdList is better in delete than ArrayList.
3.Vector is synchronized and thread safe.

Comparable interface:
package main;

public class User implements Comparable<User> {

    public int age;

    @Override
    public int compareTo(User o) {
        int result = 0;
        if (o.age > this.age) {
            return -1;
        }
        if (o.age < this.age) {
            return 1;
        }
        return result;
    }
}
Comparator interface:
package main;

import java.util.Comparator;

public class Main {

    public static void main(String[] args) throws Exception {

        Comparator<Integer> compar = new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                int i1 = (int) o1;
                int i2 = (int) o2;
                int result = 0;
                if (i1 < i2) {
                    return -1;
                }
                if (i1 > i2) {
                    return 1;
                }
                return result;
            }
        };

        System.out.println(compar.compare(5, 5));
    }
}
package main;

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        Comparator<User> compar = new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                User u1 = (User) o1;
                User u2 = (User) o2;
                int result = 0;
                if (u1.age < u2.age) {
                    return -1;
                }
                if (u1.age > u2.age) {
                    return 1;
                }
                return result;
            }
        };

        List<User> list = Arrays.asList(new User(3), new User(10), new User(5), new User(7), new User(1));
        Collections.sort(list, compar);

        for(User u : list) {
            System.out.println(u.age);
        }
    }
}