Pen.java

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

public class Pen implements Comparable<Pen> {

  private String serialNumber;
  private int length;

  public Pen(String serialNumber, int length) {
    this.serialNumber = serialNumber;
    this.length = length;
  }

  @Override
  public int compareTo(Pen target) {
    if (length > target.length) {
      return 1;
    } else if (length < target.length) {
      return -1;
    } else {
      return 0;
    }
  }

  @Override
  public boolean equals(Object target) {
    if (target == this) return true;
    if (!(target instanceof Pen)) return false;
    Pen p = (Pen) target;
    return serialNumber.equals(p.serialNumber);
  }

  @Override
  public String toString() {
    return String.format("SN: %s, length: %d", serialNumber, length);
  }

  public static void main(String[] argv) {
    ArrayList<Pen> pensInMyPocket = new ArrayList<>(5);
    pensInMyPocket.add(new Pen("a", 10));
    pensInMyPocket.add(new Pen("a", 4));
    pensInMyPocket.add(new Pen("b", 16));
    pensInMyPocket.add(new Pen("c", 7));
    pensInMyPocket.add(new Pen("b", 1));

    System.out.println("Before sort by length:");
    for (Pen pen : pensInMyPocket) {
      System.out.println(String.format("\t%s", pen));
    }

    Collections.sort(pensInMyPocket);
    System.out.println("After sort by length:");
    for (Pen pen : pensInMyPocket) {
      System.out.println(String.format("\t%s", pen));
    }
  }
}