public class Student implements Comparable {
    private String studentname;
    public int studentage;

    public Student(String studentname, int studentage) {
        this.studentname = studentname;
        this.studentage = studentage;
    }

    @Override
    public int compareTo(Object comparestu) {
        int compareage=((Student)comparestu).studentage;
        /* For Ascending order*/
        return this.studentage-compareage;

    }

    @Override
    public String toString() {
        return "[ name=" + studentname + ", age=" + studentage + "]";
    }

}

import java.util.ArrayList;
import java.util.Collections;

public class ArrayListSorting {
    public static void main(String args[]){
        ArrayList<Student> arraylist = new ArrayList<Student>();
        arraylist.add(new Student( "Chaitanya", 26));
        arraylist.add(new Student( "Rahul", 24));
        arraylist.add(new Student( "Ajeet", 32));
        arraylist.add(new Student( "aa", 10));

        Collections.sort(arraylist);

        for(Student str: arraylist){
            System.out.println(str);
        }
    }
}

ref:
https://beginnersbook.com/2013/12/java-arraylist-of-object-sort-example-comparable-and-comparator/