Sorting array of objects by field

Question

I have objects

Person{
    String name;  
    int age;
    float gradeAverage;
    }

Is there an easy way to sort

Person[] ArrayOfPersons

by age?

Do I have to use Comparable or Comparator? I don't fully understand them.

Answer 1

Just for completeness, when using Java 8, you can use Comparator.comparing to create a simple comparator for some attribute, eg Comparator.comparing(Person::getAge) , or using lambda, like Comparator.comparing(p -> p.age) , if there is not getter method for the age.

This makes it particularly easy to chain comparators for different attributes, using thenComparing , eg for sorting primarily by age, and then by name in case of ties:

Comparator.comparing(Person::getAge).thenComparing(Person::getName)

Combine that with Arrays.sort , and you are done.

Arrays.sort(arrayOfPersons, Comparator.comparing(Person::getAge));

Answer 2

You can implement the Comparable interface to make your class comparable. Make sure to override the compareTo method then.

public class Person implements Comparable<Person> {
    String name;
    int age;
    float gradeAverage;

    @Override
    public int compareTo(Person p) {
        if(this.age < p.getAge()) return -1;
        if(this.age == p.getAge()) return 0;
        //if(this.age > p.getAge()) return 1;
        else return 1;
    }

    //also add a getter here
}

Answer 3

You can check for age using a getter in your loop

for (int i = 0 ; i < persons.length - 1; i++) {
    Person p = persons[i];
    Person next =  persons[i+1];
    if(p.getAge() > next.getAge()) {
        // Swap
    }
}

However implementing Comparable is the convenient way

class Person implements Comparable<Person> {
    String name;  
    int age;
    float gradeAverage;

    public int compareTo(Person other) {
        if(this.getAge() > other.getAge())
            return 1;
        else if (this.getAge() == other.getAge())
            return 0 ;
        return -1 ;
    }

    public int getAge() {
        return this.age ;
    }
}

You can check Comparable documentation also

Answer 4

Yes just implement the Comparable interface.

Here's an example :

class Person implements Comparable<Person> {
    public int age;
    public String name;

    public int compareTo(Person other){
        return this.age == other.age ? 0 : this.age > other.age ? 1 : -1;
    }
}

Answer 5

import java.util.Arrays;

public class PersonCompare {

public static void main(String[] args) {
    Person p1 = new Person("Test1",10);
    Person p2 = new Person("Test2",12);
    Person p3 = new Person("Test3",4);
    Person p4 = new Person("Test4",7);

    Person[] ArrayOfPersons = {p1,p2,p3,p4};
    Arrays.sort(ArrayOfPersons);

    for(Person p: ArrayOfPersons) {
        System.out.println(p.getName()+"--"+p.getAge());
    }
}
}


class Person implements Comparable<Person> {
String name;
int age;

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

}

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 int compareTo(Person other) {
    if(this.getAge() > other.getAge())
        return 1;
    else if (this.getAge() == other.getAge())
        return 0 ;
    return -1 ;
}
}