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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: hash map / collection choice
Date: Sun, 27 Apr 2014 17:05:39 -0400
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On 4/27/2014 3:58 PM, Philipp Kraus wrote:
> Hello,
>
> I need help to create a data structure. I have got a class
>
> class myEdge {

     Aside: It is customary to start class and interface names with
a capital letter.

>     private int m_edgeid = 0;
> ….
>
>        @Override

     Aside: I think an `implements Comparable<myEdge>' is missing
from the class declaration -- otherwise, what are you overriding?

>     public int compareTo(myEdgep_edgelink) {

     I think there's a space character missing here ...

>         if (m_edgeid > p_edgelink.m_edgeid)
>             return 1;
>         if (m_edgeid < p_edgelink.m_edgeid)
>             return -1;
>         return 0;

     Aside: `return Integer.compare(m_edgeid, p_edgelink.m_edgeid);'
does the whole job in one line of code.

>     }
>
>
>     @Override
>     public boolean equals(Object p_object) {
>         if ( (p_object == null) || (!(p_object instanceof myEdge)) )
>             return false;

     Aside: Since the class isn't final it might someday be extended,
and then equals() could mistake a MyDecoratedEdge instance for a
plain myEdge.  Try something like

	if (p_object == null || p_object.getClass() != getClass())
	    return false;

>         return this.m_edgeid == ((myEdge)p_object).m_edgeid;
>     }
>
>
>     @Override
>     public int hashCode() {
>         return m_edgeid;
>     }
> }
>
> The edge ID is always unique. So I would like to create a collection in
> which I can do something like
>
> myEdge x = edgecollection.get( searching edge id )
>
> So the collection stores the edge objects and I can get an object with
> the internal id. The collection
> should use the the edgeid value to identify the object.
>
> A set should be the correct structure, but I can not get the object
> itself of the map without iteration over all
> items. A HashMap needs a key, value pair, so I need also a key class
> which stores the edge id (redundant data).
>
> I'm a little bit uncertain, which collection is the correct choice.

     You want some kind of Map<Integer, myEdge>, most probably a
HashMap if iteration order is unimportant, or a TreeMap if you'd
like to traverse by edge ID.  (There are other Map candidates, too,
but these are "the usual suspects.")  Whatever kind of Map you
use, you'd insert (or update) with

	myEdge edge = ...;
	theMap.put(edge.getID(), edge);

... and retrieve with

	myEdge edge = theMap.get(someRandomIdValue);
	if (edge == null) {
	    // no such thing in the Map
	}

For traversals, you can iterate over the Map's values:

	for (myEdge edge : theMap.values()) {
	   ...
	}

... or you can use those newfangled Java 8 stream thingummies.

-- 
Eric Sosman
esosman@comcast-dot-net.invalid