Sorting an ArrayList of objects using a custom sorting order

Here's a tutorial about ordering objects:

  • The Java Tutorials - Collections - Object Ordering

Although I will give some examples, I would recommend to read it anyway.

There are various way to sort an ArrayList. If you want to define a natural (default) ordering, then you need to let Contact implement Comparable. Assuming that you want to sort by default on name, then do (nullchecks omitted for simplicity):

public class Contact implements Comparable<Contact> {

    private String name;
    private String phone;
    private Address address;

    @Override
    public int compareTo(Contact other) {
        return name.compareTo(other.name);
    }

    // Add/generate getters/setters and other boilerplate.
}

so that you can just do

List<Contact> contacts = new ArrayList<Contact>();
// Fill it.

Collections.sort(contacts);

If you want to define an external controllable ordering (which overrides the natural ordering), then you need to create a Comparator:

List<Contact> contacts = new ArrayList<Contact>();
// Fill it.

// Now sort by address instead of name (default).
Collections.sort(contacts, new Comparator<Contact>() {
    public int compare(Contact one, Contact other) {
        return one.getAddress().compareTo(other.getAddress());
    }
});

You can even define the Comparators in the Contact itself so that you can reuse them instead of recreating them everytime:

public class Contact {

    private String name;
    private String phone;
    private Address address;

    // ...

    public static Comparator<Contact> COMPARE_BY_PHONE = new Comparator<Contact>() {
        public int compare(Contact one, Contact other) {
            return one.phone.compareTo(other.phone);
        }
    };

    public static Comparator<Contact> COMPARE_BY_ADDRESS = new Comparator<Contact>() {
        public int compare(Contact one, Contact other) {
            return one.address.compareTo(other.address);
        }
    };

}

which can be used as follows:

List<Contact> contacts = new ArrayList<Contact>();
// Fill it.

// Sort by address.
Collections.sort(contacts, Contact.COMPARE_BY_ADDRESS);

// Sort later by phone.
Collections.sort(contacts, Contact.COMPARE_BY_PHONE);

And to cream the top off, you could consider to use a generic javabean comparator:

public class BeanComparator implements Comparator<Object> {

    private String getter;

    public BeanComparator(String field) {
        this.getter = "get" + field.substring(0, 1).toUpperCase() + field.substring(1);
    }

    public int compare(Object o1, Object o2) {
        try {
            if (o1 != null && o2 != null) {
                o1 = o1.getClass().getMethod(getter, new Class[0]).invoke(o1, new Object[0]);
                o2 = o2.getClass().getMethod(getter, new Class[0]).invoke(o2, new Object[0]);
            }
        } catch (Exception e) {
            // If this exception occurs, then it is usually a fault of the developer.
            throw new RuntimeException("Cannot compare " + o1 + " with " + o2 + " on " + getter, e);
        }

        return (o1 == null) ? -1 : ((o2 == null) ? 1 : ((Comparable<Object>) o1).compareTo(o2));
    }

}

which you can use as follows:

// Sort on "phone" field of the Contact bean.
Collections.sort(contacts, new BeanComparator("phone"));

(as you see in the code, possibly null fields are already covered to avoid NPE's during sort)


In addition to what was already posted by BalusC it may be worth pointing that since Java 8 we can shorten our code and write it like:

Collection.sort(yourList, Comparator.comparing(YourClass::getSomeComparableField));

or since List now have sort method also like

yourList.sort(Comparator.comparing(YourClass::getSomeComparableField));

Explanation:

Since Java 8, functional interfaces (interfaces with only one abstract method - they can have more default or static methods) can be easily implemented using:

  • lambdas arguments -> body
  • or method references source::method.

Since Comparator<T> has only one abstract method int compare(T o1, T o2) it is functional interface.

So instead of (example from @BalusC answer)

Collections.sort(contacts, new Comparator<Contact>() {
    public int compare(Contact one, Contact other) {
        return one.getAddress().compareTo(other.getAddress());
    }
});

we can reduce this code to:

Collections.sort(contacts, (Contact one, Contact other) -> {
     return one.getAddress().compareTo(other.getAddress());
});

We can simplify this (or any) lambda by skipping

  • argument types (Java will infer them based on method signature)
  • or {return ... }

So instead of

(Contact one, Contact other) -> {
     return one.getAddress().compareTo(other.getAddress();
}

we can write

(one, other) -> one.getAddress().compareTo(other.getAddress())

Also now Comparator has static methods like comparing(FunctionToComparableValue) or comparing(FunctionToValue, ValueComparator) which we could use to easily create Comparators which should compare some specific values from objects.

In other words we can rewrite above code as

Collections.sort(contacts, Comparator.comparing(Contact::getAddress)); 
//assuming that Address implements Comparable (provides default order).

Tags:

Java

Sorting

Collections

Arraylist

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