Question on Collections & Generics for SCJP 5.0

[Rupak Khurana]

import java.util.*;
 
class fruit implements Comparable {
protected int sweetness=0;
protected String name = "FRUIT";

public boolean equals(Object o)
{
if (o instanceof fruit) 
{
fruit ofruit = (fruit)o;
if(this.sweetness == ofruit.sweetness)
return true;
else
return false;
}else {
throw new ClassCastException("object is not instanceof fruit");
}

}
 
public int compareTo (Object o)
{
//return 0;

if (o instanceof fruit) 
{
fruit ofruit = (fruit)o;
if(this.sweetness < ofruit.sweetness)
return -1;
else if(this.sweetness > ofruit.sweetness)
return 1;
else
return 0;
}else {
throw new ClassCastException("object is not instanceof fruit");
}

}
 
public String toString()
{
   return ("Name = "+name+", Sweetness = "+sweetness);
}
}
 
class apple extends fruit {
public apple()
{
  sweetness = 1;
  name = "APPLE";
}
}
 
class galaApple extends apple {
public galaApple()
{
  sweetness = 3;
  name = "GALAAPPLE";
}
}
 
class macApple extends apple {
public macApple()
{
  sweetness = 5;
  name = "MACAPPLE";
}
 
}
 
public class genericsLab
{
public static void main (String args[])
{
TreeSet<fruit> ta = new TreeSet<fruit>();
ta.add(new macApple());
ta.add(new apple());
ta.add(new fruit());
ta.add(new galaApple());
ta.add(new apple());
 
for(fruit a : ta)
System.out.println(a);
}
}
 

With the code above, I have the following questions :-

The above code prints (as expected):-

Name = FRUIT, Sweetness = 0
Name = APPLE, Sweetness = 1
Name = GALAAPPLE, Sweetness = 3
Name = MACAPPLE, Sweetness = 5

1. Now when I comment out the entire equals(), I still get the same output as above, i.e.,

Name = FRUIT, Sweetness = 0
Name = APPLE, Sweetness = 1
Name = GALAAPPLE, Sweetness = 3
Name = MACAPPLE, Sweetness = 5

If you note above, I am adding two new apple objects to TreeSet. Since I dont have my own equals(), the equals() of the parent Object class will be used which only checks for == operator, but I added two new apple objects, so why doesnot my collection have 5 objects as I added?

2. Now If I comment out the contents of the compareTo() and let it return 0 all the time, the output now becomes :-

Name = MACAPPLE, Sweetness = 5

I understand this since when you add the second element, the compareTo() will return 0 which means the object is already present and so it wont be added.

However if I let it return -1, then the output becomes :-

Name = APPLE, Sweetness = 1
Name = GALAAPPLE, Sweetness = 3
Name = FRUIT, Sweetness = 0
Name = APPLE, Sweetness = 1
Name = MACAPPLE, Sweetness = 5

And when it returns +1, when the output becomes :-

Name = MACAPPLE, Sweetness = 5
Name = APPLE, Sweetness = 1
Name = FRUIT, Sweetness = 0
Name = GALAAPPLE, Sweetness = 3
Name = APPLE, Sweetness = 1

How do we explain this behavior ? What order is being used here ?

Also notice that my collection now show 5 elements. If I revert the compareTo() to its original good logic, then the size drops to 4 elements..

pls enlighten me on whats going on here....
[ July 13, 2005: Message edited by: Barry Gaunt ]

[Barry Gaunt]

Please, as a courtesy to your readers, use

tags to suround formatted code.

And please use class names with capital first letters (that is, follow the normal Java conventions). For example Apple, MacApple, GalaApple.
[ July 13, 2005: Message edited by: Barry Gaunt ]

[lauren bai]

 import java.util.*; 
class fruit implements Comparable { 

Rupak:
I saw you implemented java.lang.Comparable which only has one method compareTo(T o) and doesn't have equal method.
I thought you may want to implement java.util.Comparator which has methods compare(T o1, T o2) and equals(Object obj).

I don't know if this the reason you can't get expected result since I don't have JAVA 5.0 to run.

[Rupak Khurana]

TreeSets & TreeMap rely on their elements to implement java.lang.Comparable interface.