Comparable and Comparator

Comparable

• Comparable Interface
• Imposes a total ordering over a collection of objects
• Allows objects to be sorted by natural order
• [Collections.sort](http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List\)) and [Arrays.sort](http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#sort(java.lang.Object[]\))
• Best used when always sorting by the same attribute

Comparable Example

public class Person implements Comparable<Person> {
  private String name;

  ...

  public String getName() {
    return this.name;
  }
  
  ...

  @Override
  public int compareTo(Person o) {
    return this.name.compareTo(o.getName());
  }
}

List<Person> persons = new ArrayList<Person>();
persons.add(new Person("Bob"));
persons.add(new Person("Alice"));
persons.add(new Person("Susie"));
persons.add(new Person("Dave"));

Collections.sort(persons);
System.out.println(persons);
// ["Alice", "Bob", "Dave", "Susie"]

Comparator

• Comparator Interface
• Imposes a total ordering over a collection of objects
• Allows objects to be sorted in multiple different ways
• [Collections.sort](http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List, java.util.Comparator)) and [Arrays.sort](http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#sort(T[], java.util.Comparator/))

Sorting Strings by Length

• Natural ordering by alphabetical order
• Need a comparator to sort differently

List<String> strings = new ArrayList<String>();
strings.add("aaaa");
strings.add("a");
strings.add("aaa");
strings.add("aa");

Collections.sort(strings, new LengthComparator());

System.out.println(strings);
// ["a", "aa", "aaa", "aaaa"]

public class LengthComparator implements Comparator<String> {
  @Override
  public int compare(String o1, String o2) {
    if(o1.length < 02.length) {
      return -1;
    }
    if(o1.length == o2.length) {
      return 0;
    }
    return 1;
  }
}

• Can we use built in compareTo for Strings? Yes we can.

public class LengthComparator implements Comparator<String> {
  @Override
  public int compare(String o1, String o2) {
    return ((Integer) o1.length()).compareTo((Integer) o2.length());
  }
}

• We don't need to create another file, we can use anonymous classes as well

List<String> strings = new ArrayList<String>();
strings.add("aaaa");
strings.add("a");
strings.add("aaa");
strings.add("aa");

Collections.sort(strings, new Comparator<Meep>() {
  @Override
  public int compare(Meep o1, Meep o2) {
    return ((Integer) o1.length()).compareTo((Integer) o2.length());
  }
});

In-Lab

• Download the two class files at https://github.com/anthonyjchriste/ics211f13/tree/master/src/comparisons
• Complete the compareTo method in Meep.java
• Complete the compare method in AgeComparator.java
• Complete the anonymous compare method in Meep.java