Math Exercise 4 Task 5a Java Version

Isbn.java

/**
* ISBN Class
* Represents an ISBN can check its correctness and
* complete an partial ISBN
* @author Tim Kächele
* @copyright Copyright (c) 2014 Tim Kächele
* @license MIT License
*/

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

public class Isbn {

  private ArrayList<Integer> list;

  /**
  * Initialize the class
  *
  * @param integer an ISBN
  */
  public Isbn(int integer) {
    this.list = integerToList(integer);
  }

  /**
  * Validates the correct length of the ISBN
  *
  * @return boolean with the correct length of the ISBN
  */
  public boolean isComplete(){
    return this.list.size() == 10;
  }

  /**
  * Validates the correctness of an ISBN
  * returns false if the ISBN is inomplete
  *
  * @return boolean with the correctness of the ISBN
  */
  public boolean isCorrect(){
    if (!this.isComplete()) {
      return false;
    } else {
      return (this.sum() % 11) == 0;
    }
  }

  /**
  * Checks if the ISBN is incomplete and completes
  * the ISBN if necessary.
  */
  public void complete(){
    if(this.list.size() == 9) {
      this.list.add(this.calcLastDigit());
    }
  }

  /**
  * Calculates the last digit of an ISBN
  *
  * @return value of the last digit
  */
  private int calcLastDigit(){
    return this.sum() % 11;
  }

  /**
  * Generates a string with the ISBN
  * Replaces the last digit with X if the digit is 10
  *
  * @return ISBN string
  */
  public String toString(){
    String string = "";
    int integer;
    for (int i = 0; i < this.list.size() ; i++) {
      integer = this.list.get(i);
      if(integer == 10) {
          string += "X";
      } else {
          string += integer;
      }
    }
    return string;
  }

  /**
  * Converts an integer to a list
  *
  * @param integer ISBN integer
  * @return ArrayList with the integers of the ISBN
  */
  private static ArrayList integerToList(int integer) {
    ArrayList<Integer> list;
    list = new ArrayList<Integer>();
    while(integer != 0) {
      list.add(integer % 10);
      integer = integer / 10;
    }
    Collections.reverse(list);
    return list;
  }

  /**
  * calucaltes the sum of all integers in array multiplied with
  * their position.
  *
  * @return sum of all integers
  */
  private int sum(){
    int sum = 0;
    for (int i = 0; this.list.size() > i; i++) {
      sum += (i+1) * this.list.get(i);
    }
    return sum;
  }
}

Main.java

/**
* Main class
* Example usage of the ISBN class
*
* @author Tim Kächele
* @copyright Copyright (c) 2014 Tim Kächele
* @license MIT License
*/
public class Main {
  public static void main(String[] args) {
    Isbn isbn = new Isbn(900200902);
    isbn.complete();
    System.out.println(isbn);
  }
}