Question

我尝试按重量对这些Pet对象进行排序时出现错误。我确定它很简单，但我不明白为什么这种比较不起作用。

错误：返回类型与java.util.Comparator.compare（Pet，Pet）不兼容

ArrayListNoDups类

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <version>2.3</version>
    <executions>
      <execution>
        <phase>package</phase>
        <goals>
          <goal>shade</goal>
        </goals>
        <configuration>
          <transformers>
            <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
          </transformers>
        </configuration>
      </execution>
    </executions>
</plugin>

宠物类

import java.util.*;
import java.util.Collections;
import java.util.Comparator;
import java.lang.Object;

public class ArrayListNoDups {
    public static void main(String[] args) {
        ArrayList<Pet> list = new ArrayList<Pet>();
        String name;
        Integer age;
        Double weight;
        Scanner keyboard = new Scanner(System.in);
        System.out.println("If you wish to stop adding Pets to the list, please put a 0 for all 3 fields.");

        do {
            System.out.println("Enter a String for Pet name: ");
            name = keyboard.next();
            System.out.println("Enter an int for Pet age: ");
            age = keyboard.nextInt();
            System.out.println("Enter a double for Pet weight: ");
            weight = keyboard.nextDouble();

            if (name.length() > 0 && age > 0 && weight > 0)
                list.add(new Pet(name, age, weight));
        } while (name.length() > 0 && age > 0 && weight > 0);

        System.out.println("Your list sorted by WEIGHT ========================= ");
        Collections.sort(list, Pet.SortByWeight);
        for (Pet p2 : list)
            p2.writeOutput();

    }
}

Answer 1

compare接口的

Comparator方法返回int，而不是double。

变化：

public double compare(Pet pet1, Pet pet2)

为：

public int compare(Pet pet1, Pet pet2)

比较器可能如下所示：

public static Comparator<Pet> SortByWeight = new Comparator<Pet>() {
    public int compare(Pet pet1, Pet pet2) {
        return (int)(pet1.getWeight() - pet2.getWeight());
    }
};

Answer 2

问题是compare需要返回一个整数以符合Comparator接口。

您可以在比较中转换为int，或者更好，只需使用Double.compare()。

旁注：在这些情况下使用@Override注释（实现通用接口）将使编译器帮助您更早地看到这些问题。

Answer 3

试试这个：

public int compare(Pet pet1, Pet pet2) {
            return new Double(pet1.getWeight()).compareTo(new Double(pet2.getWeight()));
        }

ArrayList使用Comparator对对象进行排序

3 个答案: