Sorting an ArrayList of objects using a custom sorting order

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说谎 2020-11-22 00:14

I am looking to implement a sort feature for my address book application.

I want to sort an ArrayList contactArray. Contact

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长发绾君心 (楼主)

2020-11-22 00:50

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 {

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

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

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

so that you can just do

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

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

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

}

which can be used as follows:

List contacts = new ArrayList();
// 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