How to sort an arraylist of objects java?

Question

So I want an arraylist of objects in java.

I have object1.number and object2.number, object3.number, etc... but those objects have other properties besides number, such as name, distance, etc...

So if it was sorting a string in a array it would just be, put a string in a temporal and let the other string take its place... but in an araryList of objects, how can I do it?

Can I just move objects to that position of the array?

Thanks.

Answer 1

Implement your own comparer:

Arrays.sort(yourArray, new Comparator<YourClass>() {
        @Override
        public int compare(YourClass o1, YourClass o2) {
            //compare object properties
        }
});

Answer 2

You need to implement the comparable interface

implements Comparable

the method that does the work is

public int compareTo(Object obj)
{
}

Please note that object is often replaced by a full on type because of generic syntax which can be used in the implements statement (shown below).

A full example is here in the tutorial docs hope this helps

A full example (take from the above link is as follows), I have added this just in case the link goes dead at some point

import java.util.*;

public class Name implements Comparable<Name> {
    private final String firstName, lastName;

    public Name(String firstName, String lastName) {
        if (firstName == null || lastName == null)
            throw new NullPointerException();
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String firstName() { return firstName; }
    public String lastName()  { return lastName;  }

    public boolean equals(Object o) {
        if (o == null || !(o instanceof Name))
            return false;
        Name n = (Name) o;
        return n.firstName.equals(firstName) && n.lastName.equals(lastName);
    }

    public int hashCode() {
        return 31*firstName.hashCode() + lastName.hashCode();
    }

    public String toString() {
    return firstName + " " + lastName;
    }

    public int compareTo(Name n) {
        int lastCmp = lastName.compareTo(n.lastName);
        return (lastCmp != 0 ? lastCmp : firstName.compareTo(n.firstName));
    }
}

The client code from the article is:

import java.util.*;

public class NameSort {
    public static void main(String[] args) {
        Name nameArray[] = {
            new Name("John", "Smith"),
            new Name("Karl", "Ng"),
            new Name("Jeff", "Smith"),
            new Name("Tom", "Rich")
        };

        List<Name> names = Arrays.asList(nameArray);
        Collections.sort(names);
        System.out.println(names);
    }
}