Re: hash map / collection choice
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