compator and treemap key/data

[Bill Goldsworthy]

I have a data relationship like this:

name -> [start,length]

I need to reference the values by their name but I need to sort by the 'start' value. Can a comparator sort on data or just a key? If so, how might I do this? I'm getting stack overflow exceptions when calling get() in the compare function.

 
public class FormatFileDataMap {
 
private Map dataMap = new TreeMap( new FormatFileComparator() );

public void add( String name, int start, int length ) {
dataMap.put( name, new Node( start, length ) );
}

public int getStart( String name ) { 
return ((Node) dataMap.get( name )).getStart();
}

public int getLength( String name ) {
return ((Node) dataMap.get( name )).getLength();
}

public Set keys() {
return dataMap.keySet();
}

public void dump() {
 
Iterator it = keys().iterator();

while( it.hasNext() ) {
String key = (String) it.next();
System.err.println( key + " " + getStart( key ) + " " + getLength( key ) );
}

}

// data node
protected class Node extends Object {

private int start;
private int length;

Node( int start, int length ) {
this.start = start;
this.length = length;
}

protected int getStart() { return this.start; }

protected int getLength() { return this.length; }
}

// comparator
protected class FormatFileComparator implements Comparator {

public int compare( Object element1, Object element2 ) {
// ??
}

}
}
 

[Keith Lynn]

I would do something like this.

protected class FormatFileComparator implements Comparator {
   public int compare( Object element1, Object element2 ) {
      Node node1 = (Node)element1;
      Node node2 = (Node)element2;
 
      if (node1.getStart() < node2.getStart())
         return(-1);
      else if (node1.getStart() > node2.getStart())
         return(1);
      else
         return(0);
   }
 
}

[Bill Goldsworthy]

Thanks for responding Keith- that's very similar to what I have but I have to get the Node out of the Map like such (which gives me a stack overflow exception).

I edited your example to illustrate:

protected class FormatFileComparator implements Comparator {
   public int compare( Object element1, Object element2 ) {
            
      Node node1 = (Node) dataMap.get( element1 );
      Node node2 = (Node) dataMap.get( element2 );
 
      if (node1.getStart() < node2.getStart())
         return(-1);
      else if (node1.getStart() > node2.getStart())
         return(1);
      else
         return(0);
   }
 
}
}
 

[Keith Lynn]

The stack overflow is probably occurring because that method is kinda going in circles. The purpose of the Comparator is to compare two Node objects. This will be used by the TreeMap in order to sort the list of objects. After the elements are all in the map, then you can iterate through the map.

[Bill Goldsworthy]

OK, I get it now. I converted my class to use a list instead of a map and it works as it should. I decided this was ok because I don't think I'll have enough nodes to really cause a problem. Thanks again for your help.

    
public class FormatFileDataList implements Iterable {
 
private List dataList = new LinkedList();
private Comparator fileFormatComparator = new FormatFileComparator();

public void add( String name, int start, int length ) {
// add
dataList.add( new Node( name, start, length ) );

// sort
Collections.sort( dataList, fileFormatComparator );
}

public int getStart( String name ) { 
return find( name ).getStart();
}

public int getLength( String name ) {
return find( name ).getLength();
}

public Iterator iterator() {
return dataList.iterator();
}

private Node find( String name ) {
Iterator it = iterator();
Node node;
while( it.hasNext() ) {
node = (Node) it.next();
if( node.getName().equals( name ) ) {
return node;
}
}
return null;
}

public void dump() {
 
Iterator it = iterator();

while( it.hasNext() ) {
System.err.println( ((Node) it.next()) );
}

}

// data node
protected class Node extends Object {

private String name;
private int start;
private int length;

Node( String name, int start, int length) {
this.name = name;
this.start = start;
this.length = length;
}

protected String getName() { return this.name; }

protected int getStart() { return this.start; }

protected int getLength() { return this.length; }

public String toString() { return name + " " + start + " " + length; }

}

// comparator
protected class FormatFileComparator implements Comparator {
public int compare( Object element1, Object element2 ) {

Node node1 = (Node) element1;
Node node2 = (Node) element2;

if (node1.getStart() < node2.getStart())
return(-1);
else if (node1.getStart() > node2.getStart())
return(1);
else
return(0);
}
}

}