TreeSet and compareTo Java 5

[Marcus Didius Falco]

Hallo I have the following class

  
package duplicate.finder;
 
import java.io.File;
 
public class JClass implements Comparable<JClass>{
private String name;
private File jarFile;

public JClass(String name, File jarFile) {
super();
this.name = name;
this.jarFile = jarFile;
}
 
public String getName() {
return name;
}
 
public File getJarFile() {
return jarFile;
}

@Override
public boolean equals(Object obj) {
boolean result = false;
if (obj instanceof JClass) {
JClass cl = (JClass) obj;
if (this.name.equals(cl.getName()) && this.jarFile.equals(cl.getJarFile())) {
result = true;
}
}
return result;
}

@Override
public int hashCode() {
return this.name.hashCode() ^ this.jarFile.hashCode();
}
 
public int compareTo(JClass otherClass) {
return this.name.compareTo(otherClass.getName());
}


}
 

I want to add instances of this class to a TreeSet.
As far I have understood Sets you should be able to add an element which is not already contained in this set. To test this the equals() method is used.
When I try to put two different instances of the above class, which differ only in the jarFile field but have the same name value (compareTo returns 0) only the first instance is put into the TreeSet.
When I switch to a HashSet both instances are put into the Set.

 
JClass cl1 = new JClass("bla", new File("C:\\temp"));
JClass cl2 = new JClass("bla", new File("C:\\temp2"));
System.out.println("cl1 equals cl2 " + cl1.equals(cl2));
TreeSet<JClass> t = new TreeSet<JClass>();
boolean a1 = t.add(cl1);
System.out.println(a1);
boolean a2 = t.add(cl2);
System.out.println(a2);
HashSet<JClass> h = new HashSet<JClass>();
a1 = h.add(cl1);
System.out.println(a1);
a2 = h.add(cl2);
System.out.println(a2);

Output:

  
cl1 equals cl2 false
true
false
true
true

This is totally unexpectated behavior for me, since I thought that only equals() is used to determined if an instance can be put into a Set, and compareTo() is only used for ordering.

Can anybody explain this behavior???

Many thanks,

Hans

[Rodrigo Lopes]

Extracted from http://java.sun.com/j2se/1.5.0/docs/api/java/util/TreeSet.html
:

"This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the set, equal."

[Joanne Neal]

A HashSet will use the hashCode method first to compare two objects. If two objects return different hashCodes they are assumed to be different and the HashSet will not call either the equals or comapreTo method. As JarFile does not override hashCode it will use Object's implementation.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects.

So as your hashCode method uses the JarFile hashCode method, unless the two JarFiles are the same object, it is very likely two different JClass objects will return different values from hashCode.