Question

我正在尝试Java Collections.sort方法并得到一个我不明白的错误（请参阅代码中）。 Java说，“类型集合中的方法排序（List，Comparator）不适用于参数（List，StudentComparator）”。我不知道我在这里做错了什么。

我有一个名为Students的课程，它看起来像这样。

//import all necessary

public class Students implements Comparable<Students>{
public List<String> list;
public String firstname;
public String lastname;


public Students(){
    list = new ArrayList<String>();
}
public void addStudent(String firstname, String lastname){
    this.firstname = firstname;
    this.lastname = lastname;
    list.add(firstname + " " + lastname);
    Collections.sort(list); //This is fine though.
    Collections.sort(list, new StudentComparator()); //This gives me the error. I want to use this for custom comparison and in this case, by student's last name.

}

public String getFirstname() {
    return firstname;
}

public String getLastname() {
    return lastname;
}

@Override
public int compareTo(Students student) {
    return this.compareTo(student);
}


}

我还有另一个实现Comparator的类。

public class StudentComparator implements Comparator<Students>{

@Override
public int compare(Students student1, Students student2) {
    return student1.getLastname().compareTo(student2.getLastname());
}


}

使用Anonymous Comparator接口并在Students类中实现它没有问题，但是当我创建一个实现Comparator接口并尝试对列表进行排序的新类时，它会给我错误。

Answer 1

您尝试使用List<String>对Comparator<Students>进行排序，显然无法正常工作。也许您打算使用List<Students>代替？或Comparator<String>？

另外，你有这个：

@Override
public int compareTo(Students student) {
    return this.compareTo(student);
}

只会无限期地自我调用。我不完全确定你想在那里做什么;或许按照StudentComparator中的姓氏排序？

@Override
public int compareTo(Students student) {
    return getLastname().compareTo(student.getLastname());
}

Answer 2

我强烈建议您修改Students bean并直接添加Comparable界面。从语义上讲，当您需要多个排序选项时，Comparator最有效。如果您不需要同一数据类型的多个选项，或者如果您想提供默认排序，那么您应该实现Comparable接口。它的代码较少，您无需进行准备工作即可实现。

mkyong有一个关于Comparator和Comparable如何工作的很好的例子：http://www.mkyong.com/java/java-object-sorting-example-comparable-and-comparator/

Answer 3

所以，我清理了代码。注意我如何在构造函数中设置每个学生的属性？这是设置对象属性的非常好的做法。

我们需要import java.lang.Comparable;

public int compareTo(Students student){ 
    return this.getLastName().compareTo(student.lastName);
}

我们需要import java.util.Comparator; 为了

@Override
public int compare(Students student1, Students student2){
    return student1.getLastName().compareTo(student2.getLastName());
}
import java.util.Collections;
import java.util.List;
import java.util.ArrayList;
import java.lang.Comparable;


public class Students implements Comparable<Students>{

public List<Students> list = new ArrayList<Students>();
public String firstName;
public String lastName;

public Students(String firstName, String lastName){
    this.firstName = firstName;
    this.lastName = lastName;
}

public void addStudent(String firstName, String lastName){
    list.add(new Students(firstName,lastName));
    Collections.sort(list); // this is fine though
    Collections.sort(list, new StudentComparator()); // this gives me an error
}

public String getFirstName(){
    return firstName;
}

public String getLastName(){
    return lastName;
}

@Override 
/* Compare a given student lastName to the current Object
 *  if greater than the current object, return -1
 *  if equals to the current object, return 0
 *  if less than the current object, return 1
 *  Here is Comparable below:
 */
public int compareTo(Students student){ 
    return this.getLastName().compareTo(student.lastName);
}
}

我们需要import java.util.Comparator;

public class StudentComparator implements Comparator<Students>{

@Override
    public int compare(Students student1, Students student2){
        return student1.getLastName().compareTo(student2.getLastName());
    }
}

希望能够解释它。

Java Collection.sort出错

3 个答案: