Computer Science 15-111 (Sections A & B), Spring 2007

Class (Recitation) Notes: 01-Mar-2007

Here are some code samples we developed today in recitation:

1. Implementing Comparable:

import java.util.*;
import static java.lang.System.out;
class MyString implements Comparable {
public MyString(String s) { this.s = s; }

public int compareTo(Object obj) {
MyString that = (MyString) obj;
return this.s.compareTo(that.s);
}

public String toString() {
return "<" + this.s + ">";
}

private String s;

public static void main(String[] args) {
MyString[] y = { new MyString("yak"),
new MyString("bar"),
new MyString("foo")
};
Arrays.sort(y);
out.println(Arrays.toString(y));
}
}

2. Implementing Comparator:

import java.util.*;
import static java.lang.System.out;
class MyStringComparator implements Comparator {

public MyStringComparator(boolean forwards, boolean ignoreCase) {
this.forwards = forwards;
this.ignoreCase = ignoreCase;
}

private boolean forwards, ignoreCase;

public int compare(Object obj1, Object obj2) {
String s1 = (String) obj1;
String s2 = (String) obj2;
if (ignoreCase) {
s1 = s1.toUpperCase();
s2 = s2.toUpperCase();
}
if (forwards)
return s1.compareTo(s2);
else
return -s1.compareTo(s2);
}

public static void main(String[] args) {
String[] z = { "yak", "bar", "CAR", "foo" };

// MyStringComparator(boolean forwards, boolean ignoreCase)
Arrays.sort(z, new MyStringComparator(true,false));
out.println(Arrays.toString(z));
Arrays.sort(z, new MyStringComparator(true,true));
out.println(Arrays.toString(z));
Arrays.sort(z, new MyStringComparator(false,false));
out.println(Arrays.toString(z));
Arrays.sort(z, new MyStringComparator(false,true));
out.println(Arrays.toString(z));
}
}

3. A thought on Comparator as a Class rather than an Interface

 abstract class AbstractComparator implements Comparator {
     abstract public int compare(Object obj1, Object obj2);
  }

This allows you to replace "implements Comparator" with "extends AbstractComparator" in most cases.

Think about it!