[英]Finding max/min value using Comparable
我有一个对象类
public class Film implements Comparable<Film>
我正在使用Eclipse,想知道为什么Film
用红色加下划线并显示错误:
The type Film must implement the inherited abstract method Comparable<Film>.compareTo<Film>
现在是我的主要问题:
如何获得用户提交的最大/最小电影长度和标题?
我的对象类Film具有用于电影标题和电影长度的getter和setter方法以及toString方法。 下面这个文章(#3)我创建了我的对象类的两个方法:
public int max(Film maxLength){
int compareLength = ((Film) maxLength).getLength();
return this.length - compareLength;
}
public int min(Film minLength){
int compareLength = ((Film) minLength).getLength();
return compareLength - this.length;
}
我可以使用这些来查找和打印用户提交的胶卷长度的最大值/最小值吗?
如果是这样,怎么办?
如果没有,这样做的正确方法是什么?
测试类如下:
import java.util.Scanner;
public class test {
public static void main (String[] args){
Film[] f = new Film[3];
Scanner input = new Scanner(System.in);
for (int i=0;i<3;i++){
f[i] = new Film();
System.out.println("Enter Film Length:");
f[i].setLength(input.nextInt());
input.nextLine();
System.out.println("Enter Title:");
f[i].setTitle(input.nextLine());
}
input.close();
for (int i = 0; i < 3; i++) {
System.out.println(f[i].toString());
}
}
}
Film
类实现Comparable<Film>
。 这意味着您必须在Film
类中实现一个称为compareTo()
的方法,该方法将为此类的对象提供排序。
@Override
public int compareTo(Film that) {
// Order by film length
return Integer.compare(this.length, that.length);
}
如果只需要按胶片长度对对象进行排序,则可以使用Arrays.sort()
:
Film[] films = new Film[3];
// put the objects into the array
Arrays.sort(films);
然后films[0]
将包含长度最短的电影,而最后一个元素将是长度最长的电影。
如果需要按其他字段(例如电影标题)进行比较,则可以创建一个自定义比较器:
class FilmTitleComparator implements Comparator<Film> {
public int compare(Film a, Film b) {
return Integer.compare(a.getTitle().length(), b.getTitle().length());
}
}
并将其传递给Arrays.sort()
FilmTitleComparator titleComparator = new FilmTitleComparator();
Arrays.sort(films, titleComparator);
然后films[0]
将包含标题最短的电影,而最后一个元素将是标题最长的电影。
为简单起见,我对您的Film
类进行了存根,以显示有关如何实现Comparable
的简单示例
public class Film implements Comparable<Film> {
int maxLength;
int minLength;
String title;
public Film() {
this.maxLength = 0;
this.minLength = 0;
this.title = "";
}
// implement this method to accomplish comparison
public int compareTo(Film f) {
int result = 0; // the result to compute.
if ( this.equals(f) ) {
result = 0; // these objects are actually equal
}
// compare using meaningful data
else if ( f != null) {
// check to see if this film is greater than the specified film
if ( this.getMaxLength() > f.getMaxLength() ) {
// this film is comparatively greater, return > 0
result = 1;
}
else if ( this.getMaxLength() == f.getMaxLength() ) {
// these two films are comparatively equal
result = 0;
}
else {
// this film is comparatively less than the specified film
result = -1;
}
// similarly, you could also check min, but there's really no reason to do that unless your implementation calls for it.
}
else {
throw new IllegalArgumentException("null Film object not allowed here...");
}
return result;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Film film = (Film) o;
if (maxLength != film.maxLength) return false;
if (minLength != film.minLength) return false;
if (!title.equals(film.title)) return false;
return true;
}
@Override
public int hashCode() {
int result = maxLength;
result = 31 * result + minLength;
result = 31 * result + title.hashCode();
return result;
}
public int getMaxLength() {
return maxLength;
}
public void setMaxLength(int maxLength) {
this.maxLength = maxLength;
}
public int getMinLength() {
return minLength;
}
public void setMinLength(int minLength) {
this.minLength = minLength;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
}
要修复测试以实际使用这样的实现(它实际上没有测试任何东西……),您可以执行以下操作:
import java.util.Scanner;
public class test {
public static void main (String[] args){
Film lastFilm = null; // arbitrary reference to film
Film[] f = new Film[3];
Scanner input = new Scanner(System.in);
for (int i=0;i<3;i++){
f[i] = new Film();
System.out.println("Enter Film Length:");
f[i].setLength(input.nextInt());
input.nextLine();
System.out.println("Enter Title:");
f[i].setTitle(input.nextLine());
}
input.close();
for (int i = 0; i < 3; i++) {
if ( lastFilm != null ) {
// compare the films to test. current to last film
if ( f[i].compareTo(lastFilm) > 0 ) {
System.out.println(f[i].getTitle() + " is greater than " + lastFilm.getTitle()");
}
else if ( f[i].compareTo(lastFilm) < 0 ) {
System.out.println(f[i].getTitle() + " is less than " + lastFilm.getTitle()");
}
else {
System.out.println(f[i].getTitle() + " is equal to " + lastFilm.getTitle()");
}
}
System.out.println(f[i].toString());
lastFilm = f[i];
}
}
}
这样的事情可以帮助您入门...祝您好运
另一个解决方案是实现Comparable<Film>
:
@Override
public int compareTo(Film that) {
return this.length - that.length;
}
并使用org.apache.commons.lang3.ObjectUtils#min
或org.apache.commons.lang3.ObjectUtils#max
这样:
Film min = ObjectUtils.min(film1, film2);
Film max = ObjectUtils.max(film1, film2);
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