Java: Sorting an ArrayList

How do you sort an ArrayList?

Given the following ArrayList

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List<String> colors = new ArrayList<String>();
colors.add("red");
colors.add("green");
colors.add("blue");

You can simply use the built-in Collections.sort method to sort your ArrayList.

Default ArrayList Sort

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List<String> colors = new ArrayList<String>();
colors.add("red");
colors.add("green");
colors.add("blue");

The ArrayList now contains the following elements in order

ArrayList Contents

blue
green
red

Okay, that’s great, but what if you want to sort a custom object for example a person object with an age attribute.

Let’s first define a custom Person Object

Person.java

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public class Person {
    public String name;
    public int age;

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

Next let’s populate the List

Populate ArrayList with People

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List<Person> people = new ArrayList<Person>();
people.add(new Person("Joe", 28));
people.add(new Person("Jane", 26));
people.add(new Person("Jeff", 53));

At this point we can again use the Collections.sort method but this time we need to provide a custom Comparator because the sort doesn’t know how to determine the order of your objects. We can create a Comparator that compares by age.

AgeComparator.java

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public class AgeComparator implements Comparator<Person> {
    @Override
    public int compare(Person a, Person b) {
        return a.age - b.age;
    }
}

Now all we need to do is use Collections.sort with a new AgeComparator

Sort with Comparator

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Collections.sort(people, new AgeComparator());

The ArrayList now contains the following elements in order

ArrayList Contents

Jane=26
Joe=28
Jeff=53