如何根据给定的条件使用 Comparator.comparing() 对类进行排序

Question

I got following class which is not compilable code. I want to send criteria as a parameter to sort the employees array. How can I do it by using Comparator.comparing() in Java 8 functional style? For example, I want to sort it by name or by age, etc.

public class Employee {
        String name;
        int age;
        double salary;
        long mobile;    
}


public class EmpolyeeSorter {
    Employee[] employees = new Employee[]{
            new Employee("John", 25, 3000.0, 9922001),
            new Employee("Ace", 22, 2000.0, 5924001),
            new Employee("Keith", 35, 4000.0, 3924401)};


    public static void sortEmpoyeeByCriteria(Function<? super T, ? extends U> byCriteria) {
        Comparator<Employee> employeeComparator
                = Comparator.comparing(byCriteria);

    }
}

Answer 1

You are missing a bit the definition of the of your method:

public static <T, U extends Comparable<? super U>> void sortEmpoyeeByCriteria(Function<? super T, ? extends U> byCriteria) {
        Comparator<T> c = Comparator.comparing(byCriteria);
}

You should also pass the List to that method that you want to sort in this case.

But then why not use Collections.sort in this case and pass the List and the Comparator in the form (for example):

Collections.sort(yourList, Comparator.comparing(Employee::getXXX))

EDIT You could declare it like this:

public static <T, U extends Comparable<? super U>> void sortEmpoyeeByCriteria(
            List<T> list,
            Function<? super T, ? extends U> byCriteria) {
        Comparator<? super T> c = Comparator.comparing(byCriteria);
        list.sort(c);
}

Answer 2

You can use a reference method

public static void main(String[] args) {
    Employee[] employees = new Employee[] { new Employee("John", 25, 3000.0, 9922001),
            new Employee("Ace", 22, 2000.0, 5924001), new Employee("Keith", 35, 4000.0, 3924401) };


    Comparator<Employee> comparingAge = Comparator.comparing(Employee::getAge);
    Comparator<Employee> comparingName = Comparator.comparing(Employee::getName);
    Comparator<Employee> comparingSalary = Comparator.comparing(Employee::getSalary);

    Arrays.sort(employees, comparingAge);
    System.out.println(Arrays.toString(employees));

    Arrays.sort(employees, comparingName);
    System.out.println(Arrays.toString(employees));

    Arrays.sort(employees, comparingSalary);
    System.out.println(Arrays.toString(employees));
}

---

you can do too:

    sortMe(employees, comparingAge);
    sortMe(employees, comparingName);
    sortMe(employees, comparingSalary);

where sortme is :

private static void sortMe(Employee[] employees, Comparator<Employee> comparingCriteria) {
    Arrays.sort(employees, comparingCriteria);
}

Answer 3

Tried lambda approach in Comparator.comparing()

class Employee {

  String name;
  int age;
  double salary;
  long mobile;

public Employee(String name, int age, double salary, long mobile) {
    this.name = name;
    this.age = age;
    this.salary = salary;
    this.mobile = mobile;
}

public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public int getAge() {
    return age;
}
public void setAge(int age) {
    this.age = age;
}
public double getSalary() {
    return salary;
}
public void setSalary(double salary) {
    this.salary = salary;
}
public long getMobile() {
    return mobile;
}
public void setMobile(long mobile) {
    this.mobile = mobile;
}

@Override
public String toString() {
    return "Employee [name=" + name + ", age=" + age + ", salary=" + salary + ", mobile=" + mobile + "]";
 }
}


public class MainClass
 {
    public static List<Employee> getEmployee()
    {
        List<Employee> ob = new ArrayList<>();
        ob.add(new Employee("John", 25, 3000.0, 9922001));
        ob.add(new Employee("Ace", 22, 2000.0, 5924001));
        ob.add(new Employee("Keith", 35, 4000.0, 3924401));
        return ob;
    }

    public static void main(String a[]) {
        List<Employee> ob = getEmployee();

        System.out.println("Sort By Name");
        ob.sort(Comparator.comparing(e -> e.getName()));
        System.out.println(ob);

        System.out.println("Sort By Age");
        ob.sort(Comparator.comparing(e -> e.getAge()));
        System.out.println(ob);         
    }
}

Output:

Sort By Name
[Employee [name=Ace, age=22, salary=2000.0, mobile=5924001], Employee [name=John, age=25, salary=3000.0, mobile=9922001], Employee [name=Keith, age=35, salary=4000.0, mobile=3924401]]
Sort By Age
[Employee [name=Ace, age=22, salary=2000.0, mobile=5924001], Employee [name=John, age=25, salary=3000.0, mobile=9922001], Employee [name=Keith, age=35, salary=4000.0, mobile=3924401]]

Hope this help!!