Comparable กับ Comparator

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แนวคิด Comparable

โค้ด: Comparable

public class Main {
  static class Person implements Comparable<Person> {
    String name;
    int age;
    Person(String n, int a) { name = n; age = a; }
    public int compareTo(Person o) {
      return this.age - o.age; // order by age
    }
    public String toString() { return name + "(" + age + ")"; }
  }
  public static void main(String[] args) {
    java.util.List<Person> list = new java.util.ArrayList<>();
    list.add(new Person("Ada",30));
    list.add(new Person("Bob",25));
    java.util.Collections.sort(list);
    System.out.println(list); // [Bob(25), Ada(30)]
  }
}

แนวคิด Comparator

โค้ด: Comparator

public class Main {
  static class Person {
    String name; int age;
    Person(String n,int a){ name=n; age=a; }
    public String toString(){ return name+"("+age+")"; }
  }
  public static void main(String[] args) {
    java.util.List<Person> list = new java.util.ArrayList<>();
    list.add(new Person("Ada",30));
    list.add(new Person("Bob",25));

    java.util.Comparator<Person> byName = (p1,p2) -> p1.name.compareTo(p2.name);
    java.util.Collections.sort(list, byName);
    System.out.println(list); // [Ada(30), Bob(25)]
  }
}

โค้ด: Comparator แบบย้อนกลับ

public class Main {
  public static void main(String[] args) {
    java.util.List<Integer> nums = new java.util.ArrayList<>();
    nums.add(5); nums.add(1); nums.add(3);

    java.util.Comparator<Integer> desc = (a,b) -> b - a;
    java.util.Collections.sort(nums, desc);
    System.out.println(nums); // [5,3,1]
  }
}

แบบฝึกตรวจสอบ Comparable กับ Comparator

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