• Study the compareTo method of the String class.
• See how to define a new class with its own compareTo method.
• Study references to objects, including the null reference. (Text, Section 5.1.)
• Study local data, global (class level) data, and scope. (A formal parameter behaves just like a local variable.)
• Study the connection between actual and formal parameters.
• Begin the study of arrays, including the creation of simple arrays, and the use of an array of counters to keep track of the results of throwing a single die, and of the resulting sum of spots when thowing two dice.

// StringCompare.java: compare Strings
public class StringCompare {
   public static void main (String[] args) {
      String s1 = new String("Aardvark");
      String s2 = new String("Aaron");
      String s3 = new String("Aragon");
      System.out.println("s1.compareTo(s1): " + s1.compareTo(s1));
      System.out.println("s1.compareTo(s2): " + s1.compareTo(s2));
      System.out.println("s1.compareTo(s3): " + s1.compareTo(s3));
      System.out.println("s2.compareTo(s1): " + s2.compareTo(s1));
      System.out.println("s2.compareTo(s2): " + s2.compareTo(s2));
      System.out.println("s2.compareTo(s3): " + s2.compareTo(s3));
      System.out.println("s3.compareTo(s1): " + s3.compareTo(s1));
      System.out.println("s3.compareTo(s2): " + s3.compareTo(s2));
      System.out.println("s3.compareTo(s3): " + s3.compareTo(s3));
   }
} 
    
/* Output is:

s1.compareTo(s1): 0
s1.compareTo(s2): -11
s1.compareTo(s3): -17
s2.compareTo(s1): 11
s2.compareTo(s2): 0
s2.compareTo(s3): -17
s3.compareTo(s1): 17
s3.compareTo(s2): 17
s3.compareTo(s3): 0

*/
 

• s1 comes before s1 in alphabetic order means that s1.compareTo(s2) is less than 0.
• s1 equals s1 (as strings) means that s1.compareTo(s1) is equal to 0.
• s2 comes after s1 in alphabetic order means that s2.compareTo(s1) is greater than 0.

First is the class StringCompare2 that tests this out:

// StringCompare2.java: compare Strings in reverse order
public class StringCompare2 {
   public static void main (String[] args) {
      ReverseString s1 = new ReverseString("Aardvark");
      ReverseString s2 = new ReverseString("Aaron");
      ReverseString s3 = new ReverseString("Aragon");
      System.out.println("s1.compareTo(s1): " + s1.compareTo(s1));
      System.out.println("s1.compareTo(s2): " + s1.compareTo(s2));
      System.out.println("s1.compareTo(s3): " + s1.compareTo(s3));
      System.out.println("s2.compareTo(s1): " + s2.compareTo(s1));
      System.out.println("s2.compareTo(s2): " + s2.compareTo(s2));
      System.out.println("s2.compareTo(s3): " + s2.compareTo(s3));
      System.out.println("s3.compareTo(s1): " + s3.compareTo(s1));
      System.out.println("s3.compareTo(s2): " + s3.compareTo(s2));
      System.out.println("s3.compareTo(s3): " + s3.compareTo(s3));
   }

} 
    
/* Output is: (the reverse of the normal compareTo for strings)

s1.compareTo(s1): 0
s1.compareTo(s2): 1
s1.compareTo(s3): 1
s2.compareTo(s1): -1
s2.compareTo(s2): 0
s2.compareTo(s3): 1
s3.compareTo(s1): -1
s3.compareTo(s2): -1
s3.compareTo(s3): 0

*/

// ReverseString.java: compare Strings in reverse order
public class ReverseString implements Comparable {
   String internal;
   public ReverseString(String s) {
      internal = s;
   }
   public int compareTo(Object t) {
      if (internal.compareTo(((ReverseString)t).internal) < 0) return 1;
      else if (internal.compareTo(((ReverseString)t).internal) > 0) return -1;
      else return 0;
   }
} 

public interface Comparable
{
   int compareTo(Object obj);
}