5.11 Self 9.2

Comparator and comparable

Comparable is a logic that define how to compare, use compareTo​

public class T implements Comparable<T> {
    @Override
    public int compareTo(T obj);

}

Comparator is a method to make object comparable.

public int compare(Object obj1, Object obj2):

​

For example

This is a normal way to sort integer

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
//List is an interface that extends Collection interface  
//You need a concrete class that implements List
public class main {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(45);
        numbers.add(21);
        numbers.add(67);
        numbers.add(23);

        System.out.println("Before sorting: " + numbers);
        Collections.sort(numbers);
        System.out.println("After sorting: " + numbers);    

    }
}

Now, we want to sort these numbers according to the last digit, then we have two ways

1. Using Comparator

1. The basic way

   import java.util.ArrayList;
   import java.util.Collections;
   import java.util.Comparator;
   import java.util.List;
   //List is an interface that extends Collection interface  
   //You need a concrete class that implements List
   public class main {
   
       static class CompareByLastDigit implements Comparator<Integer> {
           @Override
           public int compare(Integer i, Integer j) {
               if (i % 10 < j % 10)
                   return -1;
               else
                   return 1;
           }
       }
   
       public static void main(String[] args) {
   
           Comparator<Integer> comparator = new CompareByLastDigit();
   
           List<Integer> numbers = new ArrayList<>();
   
           numbers.add(41);
           numbers.add(22);
           numbers.add(60);
           numbers.add(23);
   
           System.out.println("Before sorting: " + numbers);
           Collections.sort(numbers, comparator);
           System.out.println("After sorting: " + numbers);    
   
       }
   }
   

2. Using anonymous class

   import java.util.ArrayList;
   import java.util.Collections;
   import java.util.Comparator;
   import java.util.List;
   //List is an interface that extends Collection interface  
   //You need a concrete class that implements List
   public class main {
       public static void main(String[] args) {
   
           Comparator<Integer> comparator = new Comparator<Integer>() {
           public int compare(Integer i, Integer j) {
               if(i % 10 < j % 10)
                   return -1;
               else
                   return 1;
           }
       };
   
           List<Integer> numbers = new ArrayList<>();
   
           numbers.add(41);
           numbers.add(22);
           numbers.add(60);
           numbers.add(23);
   
           System.out.println("Before sorting: " + numbers);
           Collections.sort(numbers, comparator);
           System.out.println("After sorting: " + numbers);    
   
       }
   }
   

3. Using lambda expression

   import java.util.ArrayList;
   import java.util.Collections;
   import java.util.Comparator;
   import java.util.List;
   //List is an interface that extends Collection interface  
   //You need a concrete class that implements List
   public class main {
       public static void main(String[] args) {
   
           Comparator<Integer> comparator = (Integer i, Integer j) -> i % 10 < j % 10 ? -1 : 1;
   
           List<Integer> numbers = new ArrayList<>();
   
           numbers.add(41);
           numbers.add(22);
           numbers.add(60);
           numbers.add(23);
   
           System.out.println("Before sorting: " + numbers);
           Collections.sort(numbers, comparator);
           System.out.println("After sorting: " + numbers);    
   
       }
   }
   

2. Using Comparable

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
//List is an interface that extends Collection interface  
//You need a concrete class that implements List
public class main {

    static class LastDigitNumber implements Comparable<LastDigitNumber> {
        private int value;
        public LastDigitNumber(int value) { 
            this.value = value; 
        }
        @Override
        public int compareTo(LastDigitNumber other) { 
            return Integer.compare(this.value % 10, other.value % 10); 
        }
        @Override
        public String toString() { 
            return String.valueOf(value); 
        }
    }
    public static void main(String[] args) {
        List<LastDigitNumber> numbers = new ArrayList<>();
        numbers.add(new LastDigitNumber(41));
        numbers.add(new LastDigitNumber(22));
        numbers.add(new LastDigitNumber(60));
        numbers.add(new LastDigitNumber(23));
        System.out.println("Before sorting: " + numbers);
        Collections.sort(numbers);
        System.out.println("After sorting: " + numbers);    
    }
}

---

Now, we are given a more complex example

We have some students and want to sort students by age

1. Using comparable

   import java.util.ArrayList;
   import java.util.Collections;
   import java.util.List;
   //List is an interface that extends Collection interface  
   //You need a concrete class that implements List
   public class main {
   
       static class Student implements Comparable<Student> {
           private String name;
           private int age;
           public Student(String name, int age) {
               this.name = name;
               this.age = age;
           }
           @Override
           public int compareTo(Student other) {
               return Integer.compare(this.age, other.age);
           }
           @Override
           public String toString() {
               return name + "(" + age + ")";
           }
       }
       public static void main(String[] args) {
           List<Student> students = new ArrayList<>();
           students.add(new Student("Alice", 21));
           students.add(new Student("Bob", 19));
           students.add(new Student("Charlie", 22));
           students.add(new Student("Dave", 20));
           System.out.println("Before sorting: " + students);
           Collections.sort(students);
           System.out.println("After sorting: " + students);    
       }
   }
   

2. Using Comparator

   import java.util.ArrayList;
   import java.util.Collections;
   import java.util.List;
   import java.util.Comparator;
   //List is an interface that extends Collection interface  
   //You need a concrete class that implements List
   public class main {
   
       static class Student {
           public String name;
           public int age;
           public Student(String name, int age) {
               this.name = name;
               this.age = age;
           }
           @Override
           public String toString() {
               return name + "(" + age + ")";
           }
       }
       public static void main(String[] args) {
           List<Student> students = new ArrayList<>();
           Comparator<Student> comparator = (Student i, Student j) -> i.age < j.age ? -1 : 1;
           students.add(new Student("Alice", 21));
           students.add(new Student("Bob", 19));
           students.add(new Student("Charlie", 22));
           students.add(new Student("Dave", 20));
           System.out.println("Before sorting: " + students);
           Collections.sort(students, comparator);
           System.out.println("After sorting: " + students);    
       }
   }
   

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