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Sorting Lists

To sort a List, pass it to the sort method of the java.util.Collections class.

Collections.sort(planets);
* Collections.sort modifies the actual object you pass, without creating and returning a new List.

  • This version of sort requires a list of Comparables and uses their natural sort order.

For a list of non-Comparable objects, or to customize the sort, use a Comparator with Collections.sort.

public class PlanetDiameterComparator implements Comparator<Planet> {
  public int compare(Planet a, Planet b) {
    if (a.getDiameter() < b.getDiameter())
      return -1;
    else if (a.getDiameter() > b.getDiameter())
      return 1;
    else
      return a.getName().compareTo(b.getName());
  }
}

PlanetDiameterComparator comp = new PlanetDiameterComparator();
Collections.sort(planets, comp);

Java 8 introduced the ability for a List to sort itself, using a Comparator.

planets.sort(new PlanetReverseDiameterComparator());

Practice Exercise

The List interface was first published in Java 1.2. The sort method was added in version 1.8. Adding a new abstract method to List would have broken the contract used by all the many existing implementing classes. However there were compelling reasons for sort and other new methods.

This need to add methods to an interface without breaking existing implementors is the reason Java 8 introduced default methods. Because the Java 8 List provides a default implementation of sort, no existing implementors had to be modified.

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