Question

我正在尝试按其ID排序对象列表。这是我的代码：

List<Employee> employee = getEmployeeList();
Collections.sort(employee, new Comparator<Employee>(){
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
    }
});

但排序后没有任何反应，它仍显示原始员工名单。难道我做错了什么？我到处搜索，但他们都只是这样做，它适用于他们。这是一组硬编码的员工ID：

public class Employee{
    private String employeeId = "";
    private String employeeName = "";
    private String contactNbr = "";

    //getters and setters
}

List<Employee> empList = new ArrayList<Employee>();
Employee emp1 = new Employee();
emp1.setEmployeeId("A1B1");
empList.add(emp1);

Employee emp2 = new Employee();
emp2.setEmployeeId("A2B1");
empList.add(emp2);

Employee emp3 = new Employee();
emp3.setEmployeeId("A3B1");
empList.add(emp3);

Collections.sort(empList, new Comparator<Employee>(){
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
    }
});

Answer 1

检查employeeId compareTo方法的结果。

如果它总是返回0，则排序将认为所有元素都相等。集合执行稳定的排序，因此如果所有元素都相等，它将保持顺序不变。

Answer 2

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

public class MainTest {

    public static void main(String[] arg)
    {
        List<Employee> empList = new ArrayList<Employee>();
        Employee emp1 = new Employee();
        emp1.setEmployeeId("A2B1");
        empList.add(emp1);

        Employee emp2 = new Employee();
        emp2.setEmployeeId("A1B1");
        empList.add(emp2);

        Employee emp3 = new Employee();
        emp3.setEmployeeId("A3B1");
        empList.add(emp3);

        Collections.sort(empList, new Comparator<Employee>(){

            public int compare(Employee employee1, Employee employee2) {
                return employee1.getEmployeeId().compareTo(employee2.getEmployeeId());
            }
        });


        Iterator i = empList.iterator();
        while(i.hasNext()){
            System.out.println(((Employee)i.next()).getEmployeeId());
        }


    }
}
class Employee{
    private String employeeId = "";
    private String employeeName = "";
    private String contactNbr = "";
    public String getContactNbr() {
        return contactNbr;
    }
    public void setContactNbr(String contactNbr) {
        this.contactNbr = contactNbr;
    }
    public String getEmployeeId() {
        return employeeId;
    }
    public void setEmployeeId(String employeeId) {
        this.employeeId = employeeId;
    }
    public String getEmployeeName() {
        return employeeName;
    }
    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    //getters and setters
}

这是正确打印输出： A1B1 A2B1 A3B1

Answer 3

您要按排序顺序添加列表，这就是您认为列表未获得更新的原因。就您的来源而言，没有任何问题，您正在做正确的事情。问题是你的意见。您按照排序顺序提供以下输入：

emp1.setEmployeeId("A1B1");
empList.add(emp1);

emp1.setEmployeeId("A2B1");
empList.add(emp2);

emp1.setEmployeeId("A3B1");
empList.add(emp3);

您的输出将如下

Before sorting: [A1B1, A2B1, A3B1]
After sorting:  [A1B1, A2B1, A3B1]

尝试按如下所示修改来源并检查

emp1.setEmployeeId("A3B1");
empList.add(emp3);

emp1.setEmployeeId("A2B1");
empList.add(emp2);

emp1.setEmployeeId("A1B1");
empList.add(emp1);

您将得到如下结果

Before sorting: [A3B1, A2B1, A1B1]
After sorting:  [A1B1, A2B1, A3B1]

Collections.sort不对我的对象列表进行排序

3 个答案: