按字母顺序对字符串进行排序

Question

可以编译以下代码，但是sortTitles（）方法不会按预期的字母顺序对电影的标题进行排序。 您将如何修复compareTo（）和sortTitles（）方法？

电影2类

public class Movie2 implements Comparable<Movie2> {
    // instance variables 
    private String title;
    private int year;
    private String studio;

    public Movie2(String title, int year, String studio) {
        // initialise instance variables
        this.title = title;
        this.year = year;
        this.studio = studio;
    }

    public String toString() {
        String listing;
        listing = title + ", " + year + ", " + studio;

        return listing;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public int getYear() {
        return year;
    }

    public void setStudio(String studio) {
        this.studio = studio;
    }

    public String getStudio() {
        return studio;
    }

    public int compareTo(Movie2 obj) {
        if (title < obj.getTitle()) {
            return -1;
        }
        else {
            return 1;
        }
    }
}

TestMovie2类

public class TestMovie2 {
    public static void main(String[] args) {
        Movie2[] myMovies = new Movie2[10];
        Movie2[] sorted = new Movie2[10];

        myMovies[0] = new Movie2("The Muppets Take Manhattan", 2001, "Columbia Tristar");
        myMovies[1] = new Movie2("Mulan Special Edition", 2004, "Disney");
        myMovies[2] = new Movie2("Shrek 2", 2004, "Dreamworks");
        myMovies[3] = new Movie2("The Incredibles", 2004, "Pixar");
        myMovies[4] = new Movie2("Nanny McPhee", 2006, "Universal");
        myMovies[5] = new Movie2("The Curse of the Were-Rabbit", 2006, "Aardman");
        myMovies[6] = new Movie2("Ice Age", 2002, "20th Century Fox");
        myMovies[7] = new Movie2("Lilo & Stitch", 2002, "Disney");
        myMovies[8] = new Movie2("Robots", 2005, "20th Century Fox");
        myMovies[9] = new Movie2("Monsters Inc.", 2001, "Pixar");

        System.out.println("  Movies ");
        System.out.println("______________________________");
        System.out.println();
        printMovies(myMovies);
        System.out.println();
        System.out.println();

        Movie2[] dest = new Movie2[myMovies.length];
        sortTitles(myMovies, dest);

        System.out.println(" Sorted by title - ascending ");
        System.out.println("______________________________");
        System.out.println();
        printMovies(myMovies);
        System.out.println();
        System.out.println();

        sortYears(myMovies, sorted);
        System.out.println(" Sorted by year - descending");
        System.out.println("______________________________");
        System.out.println();
        printMovies(sorted);
        System.out.println();
        System.out.println();
    }

    public static void sortTitles(Movie2[] myMovies, Movie2[] dest) {
        for (int i = 0; i < myMovies.length; i++) {
            Movie2 next = myMovies[i];
            int insertIndex = 0;
            int k = i;
            while (k > 0 && insertIndex == 0) {
                if (myMovies[k].getTitle().compareTo(dest[k - 1].getTitle()) < 1) {
                    insertIndex = k;
                }
                else {
                    dest[k] = dest[k - 1];
                }
                k--;
            }
            dest[insertIndex] = next;
        }
    }

    public static void printMovies(Movie2[] sorted) {
        for (int i = 0; i < sorted.length; i++)
            System.out.println(sorted[i]);
    }

    public static void sortYears(Movie2[] myMovies, Movie2[] sorted) {
        for (int i = 0; i < myMovies.length; i++) {
            Movie2 next = myMovies[i];
            int insertindex = 0;
            int k = i;
            while (k > 0 && insertindex == 0) {
                if (next.getYear() < sorted[k - 1].getYear()) {
                    insertindex = k;
                }
                else {
                    sorted[k] = sorted[k - 1];
                }
                k--;
            }
            sorted[insertindex] = next;
        }
    }
}

Answer 1

使用已经定义的String的compareTo方法，例如：

public int compareTo(Movie2 obj) { 
    return this.getTitle().compareTo(obj.getTitle());
}

在sortTitles方法中，使用

Arrays.sort(myMovies);//you dont need seperate dest array

Answer 2

使用String#compareTo()方法作为

public int compareTo(Movie2 obj)
{
    if (title != null)
        return title.compareTo(obj.getTitle());
    else
        return obj.getTitle() == null ? 0 : -1;
}

Java不支持运算符重载。 因此，您无法将String与关系运算符进行比较。 您应该在那里收到编译时错误。

Answer 3

首先在Movie2修复您的compareTo方法（您可能已经做过，或者甚至无法编译）：

public int compareTo(Movie2 obj)
{
  return title.compareTo(obj.getTitle());
}

然后你sortTitles方法TestMovie2 ：

    public static void sortTitles(Movie2[] myMovies, Movie2[] dest)
    {
        for (int i = 0; i < myMovies.length; i++)
            {
                Movie2 next = myMovies[i];
                int insertIndex = 0;
                int k = i;
                while (k>0 && insertIndex == 0)
                    {
                        if (next.getTitle().compareTo(dest[k-1].getTitle()) > -1)
                            {
                                System.out.println("less than or equal");
                                insertIndex = k;
                            }
                        else 
                            {
                                System.out.println("greater than");
                                dest[k] = dest[k-1]; 
                            }
                        k--;
                    }
                dest[insertIndex] = next;
            } 
    }

这样，您应该得到：

 Sorted by title - ascending 
______________________________

Ice Age, 2002, 20th Century Fox
Lilo & Stitch, 2002, Disney
Monsters Inc., 2001, Pixar
Mulan Special Edition, 2004, Disney
Nanny McPhee, 2006, Universal
Robots, 2005, 20th Century Fox
Shrek 2, 2004, Dreamworks
The Curse of the Were-Rabbit, 2006, Aardman
The Incredibles, 2004, Pixar
The Muppets Take Manhattan, 2001, Columbia Tristar

Answer 4

为什么要使用关系运算符与String对象进行比较？ 另外，您将命名您的方法与现有方法相同。 因此，在您遇到的情况下，您的代码没有调用TestingMovie2类中的方法。 它正在调用默认的java.lang.String方法。 更改名称并使用String类的comapreTo方法。 修改方法为：

public int movieCompareTo(Movie2 obj)
{ 
  if (title.comapreTo(obj.getTitle()) < 0)
    return -1;
  else 
    return 1; 
}

comapreTo方法：（来自JavaDoc）

• 在字典上比较此String对象表示的字符序列与自变量字符串表示的字符序列。
• 如果此String对象在字典上在参数字符串之前，则结果为负整数。
• 如果此String对象在字典上跟随自变量字符串，则结果为正整数。
• 如果字符串相等，则结果为零