contains(.) method in Collection framework

[Samik R]

Hello,

I have a questions about how the contains() method works for a class in Collection framework with a user defined object. Do I have to do something special?

I tried the following example. But the output is not what I expected.

--------------- CODE --------------------
import java.util.*;

class Lane
{ int SNode, DNode;
public Lane(int SN, int DN)
{ SNode=SN; DNode=DN; }

public boolean equals(Lane o)
{ if(o.SNode==SNode && o.DNode==DNode) return true;
else return false;
}
};

class Bundle
{ HashSet<Lane> Lanes;

public Bundle(){ Lanes=new HashSet<Lane>(); }

public boolean add(Lane l){ return Lanes.add(l); }

public boolean contains(Lane l){ return Lanes.contains(l); }

public void printAll()
{ System.out.print("[ ");
for(Lane l : Lanes){ System.out.print("("+l.SNode+","+l.DNode+") "); }
System.out.println("]");
}
};

public class Test
{
public static void main(String[] args)
{ Lane L1=new Lane(1,2), L2=new Lane(1,3);
Bundle B=new Bundle();
B.add(L1);
B.add(L2);
B.printAll();
if(B.contains(L2)) System.out.println("Bundle has L2");
if(B.contains(new Lane(1,3))) System.out.println("Bundle has (1,3)");
}
}


-------------- OUTPUT ------------
[ (1,3) (1,2) ]
Bundle has L2
---------------------------------

I was expecting to see "Bundle has (1,3)" in the output. My impression was that contains method internally calls equals method of the object concerned, so I even defined my own equals(). Still it did not work. What am I missing?

Also, one secondary question. I am sure this question has been discussed in many texts and newsgroup messages, but with google I couldn't find. What will be the exact set of keywords for my case to search? I tried [java "contains method" "user defined object"], [java Collection "contains method"] etc.

Thanks.

 

[Thomas Hawtin]

I have a questions about how the contains() method works for a class in
Collection framework with a user defined object. Do I have to do
something special?

public boolean equals(Lane o)
{ if(o.SNode==SNode && o.DNode==DNode) return true;
else return false;
}

This does not override Object.equals(Object). You need something like:

@Override // 1.5+ only
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
// If class was final, could use if (!(obj instanceof Lane))
if (obj == null || !(this.getClass() != obj.getClass)) {
return false;
}
Lane other = (Lane)obj;
return
this.sNode == other.sNode &&
this.dNode == other.dNode;
}

Whenever you override equals, you should also override hashCode (and
probably toString). See the java.lang.Object JavaDocs for details.

@Override
public int hashCode() {
return sNode*37 + dNode;
}

It's probably best to make the class and member variables final.

Tom Hawtin

 

[Samik R]

This does not override Object.equals(Object). You need something like:

@Override // 1.5+ only
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
// If class was final, could use if (!(obj instanceof Lane))
if (obj == null || !(this.getClass() != obj.getClass)) {
return false;
}
Lane other = (Lane)obj;
return
this.sNode == other.sNode &&
this.dNode == other.dNode;
}

Whenever you override equals, you should also override hashCode (and
probably toString). See the java.lang.Object JavaDocs for details.

@Override
public int hashCode() {
return sNode*37 + dNode;
}

It's probably best to make the class and member variables final.

Tom Hawtin

Thanks Tom. Really appreciate your time.

 

[Roedy Green]

public int hashCode() {
return sNode*37 + dNode;
}

for more notes on composing hashCode methods, see
http://mindprod.com/jgloss/hashcode.html