This is example demonstrates that we create a list of person objects and we compare each person's age by using compateTo() method of Comparable Interface. In short, we are sorting persons by age in ascending order.

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Sort User-Defined Objects in Ascending Order in Java

public class Person implements Comparable < Person > {

    private int id;

    private String name;

    private int age;

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

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return this.name;
    }

    @Override
    public int compareTo(Person otherPerson) {
        return (this.getAge() - otherPerson.getAge());
    }
}

Note that Person class implements Comparable interface and provides compareTo() method implementation.

Here is code to test and sort person objects:

import java.util.ArrayList;

import java.util.Collections;
import java.util.List;

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

        sortUserDefinedObjectsInAsc();
    }

    private static void sortUserDefinedObjectsInAsc() {
        List < Person > persons = new ArrayList < Person > ();
        Person person1 = new Person(59, "John", 40);
        Person person12 = new Person(67, "Roger", 25);
        Person person13 = new Person(45, "Steven", 30);
        persons.add(person1);
        persons.add(person12);
        persons.add(person13);

        System.out.println("Before Sorting : " + persons);
        Collections.sort(persons);
        System.out.println("After Sorting : " + persons);
    }
}

Output

Before Sorting : [John, Roger, Steven]
After Sorting : [Roger, Steven, John]

Collections Examples

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• Java LinkedList Example
  
• Java ArrayList Example
• Java Comparator Interface Example
• Java Comparable Interface Example
• Java IdentityHashMap Example
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• Java EnumSet Class Example
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• Iterate over TreeSet in Java Example
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• Iterate over LinkedList using an Iterator in Java
• Search an Element in an ArrayList in Java
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• Remove Element from HashSet in Java
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• How To Remove Duplicate Elements From ArrayList In Java?
  
• Different Ways to Iterate over List, Set, and Map in Java

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