Comparator vs Comparable

使用Comparator

class Student{
    int id;
    String name;

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

public class Demo {
    public static void main(String[] args) {

        List<Student> studentList = new ArrayList<>();

        studentList.add(new Student(2, "Chris"));
        studentList.add(new Student(1, "Genos"));
        studentList.add(new Student(4, "Mike"));
        studentList.add(new Student(3, "Jack"));
        
         System.out.println(studentList); 
        // [Student{id=2, name='Chris'}, Student{id=1, name='Genos'}, Student{id=4, name='Mike'}, Student{id=3, name='Jack'}]

        Comparator<Student> comparator = new Comparator<>(){

            @Override
            public int compare(Student s1, Student s2) {
                if(s1.id > s2.id) return 1;
                else if(s1.id < s2.id) return -1;
                return 0;
            }
        };
       
        Collections.sort(studentList, comparator);

        System.out.println(studentList);
        // [Student{id=1, name='Genos'}, Student{id=2, name='Chris'}, Student{id=3, name='Jack'}, Student{id=4, name='Mike'}]
    }
}

排序前，將list印出來看的話，會按照我們所add進list的順序而顯示 (第29行)

排序之後，印出來的結果將會按照我們定義的Comparator的compare方法而排序，我要求以id由小到大排序

使用Comparable

class Student implements Comparable<Student> {
    int id;
    String name;

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }

    @Override
    public int compareTo(Student otherStudent) {
        if(this.id > otherStudent.id) return 1;
        else if(this.id < otherStudent.id) return -1;
        return 0;
    }
}

public class Demo {
    public static void main(String[] args) {

        List<Student> studentList = new ArrayList<>();

        studentList.add(new Student(2, "Chris"));
        studentList.add(new Student(1, "Genos"));
        studentList.add(new Student(4, "Mike"));
        studentList.add(new Student(3, "Jack"));
        
        System.out.println(studentList);
        // [Student{id=2, name='Chris'}, Student{id=1, name='Genos'}, Student{id=4, name='Mike'}, Student{id=3, name='Jack'}]

        Collections.sort(studentList);

        System.out.println(studentList);
        // [Student{id=1, name='Genos'}, Student{id=2, name='Chris'}, Student{id=3, name='Jack'}, Student{id=4, name='Mike'}]
    }
}

與上一個方法的差別在於

Student class implements了Comparable，並且重寫了compareTo方法

注意implements Comparable<Student> 有指定Student型態

compareTo方法的內容和前一個方法的compare方法一樣

只是compareTo方法的參數只會傳入一個，也就是Student型態的物件

也就是要被比較的Student物件

因此第39行的Collections.sort(studentList); 部分，就不需要特地指定要使用哪一個Comparator作排序規則參考

因為我們的Student class裡面已經先定義好該如何比較了