Question

I have a class that looks like this

class Student {
   public String str;   
   public int    marks;
}

Say 3 objects for

Student S1 (str: sub1, marks:10) , S2 (str: sub2, marks:5) , S3 (str:s3, marks:2)

I want to ensure that for Student objects that this condition holds true if sub1 > sub2 > sub3 then marks1 > marks2 > marks3

Answer 1

Have that class implementing comparable:

class Student implements Comprable<Student> { 
....
    public int compareTo( Student other ) { 
        return this.marks - other.marks;
    }
 ...
}

Here’s a working demo. In this demo, the natural order of your class is “descending based on the marks attribute”. That is, greater mark will be placed before a lower mark.

import java.util.*;
class Student implements Comparable<Student> {
   public String str;   
   public int  marks;

   public Student( String s, int m ) { 
     this.str = s;
     this.marks = m;
   }
   public int compareTo( Student other ) { 
    return other.marks - this.marks;
   }
   public String toString() { 
     return String.format("Student(str=%s,marks=%s)", str, marks);
   }

}
class Main { 
  public static void main( String ... args ) { 
    List<Student> l;
    Collections.sort(( l =  new ArrayList<Student>(Arrays.asList(
      new Student("sub3", 2),
      new Student("sub1", 10),
      new Student("sub2", 5)
    ))));
    System.out.println( l );
  }
}


C:\>java Main
[Student(str=sub1,marks=10), Student(str=sub2,marks=5), Student(str=sub3,marks=2)]