August 2009


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Stuart Rossiter <[log in to unmask]>
Reply To:
MASON Multiagent Simulation Toolkit <[log in to unmask]>
Fri, 28 Aug 2009 11:57:00 +0100
text/plain (94 lines)

   Nothing major, but just a flaw I came across when writing some custom
portrayals. When they're instantiated, all the MASON non-simple
portrayal classes instantiate default simple portrayal instance fields,
rather than doing a lazy instantiation on request

e.g. NetworkPortrayal2D has:

SimpleEdgePortrayal2D defaultPortrayal = new SimpleEdgePortrayal2D();
public Portrayal getDefaultPortrayal() { return defaultPortrayal; }

instead of, say:

SimpleEdgePortrayal2D defaultPortrayal = null;
public Portrayal getDefaultPortrayal() {
    if (defaultPortrayal == null) {
        defaultPortrayal = new SimpleEdgePortrayal2D();
    return defaultPortrayal;

Not a biggie, but means that all non-simple portayals carry this
unnecessary baggage if you're not using the default.

Note that there are some backwards compatibility issues with fixing it,
since subclasses might use direct defaultPortrayal access to change the
default portrayal (and all MASON's own subclasses do, such as
HexaSparseGridPortrayal2D for SparseGridPortrayal2D). So
getDefaultPortrayal as above wouldn't work properly for subclasses
unless they overrode this accessor method as well.

A better (and more OO purist) alternative to force the change would be
something like the below:

// Force subclasses to change if they were accessing this directly
// before

private SimpleEdgePortrayal2D defaultPortrayal = null;

// Force this not to be overrideable (final)

public final Portrayal getDefaultPortrayal() {
    if (defaultPortrayal == null) {
        defaultPortrayal = createDefaultPortrayal();
    return defaultPortrayal;

// Subclasses should override this if using a new default portayal
// class

protected SimpleEdgePortrayal2D createDefaultPortrayal() {
    return new SimpleEdgePortrayal2D();

Note that, with Java 1.5, you can override to return a subclass of
SimpleEdgePortrayal2D explicitly, instead of only a
SimpleEdgePortrayal2D (this is apparently called a covariant return).

e.g. this, in a subclass, wouldn't give a compile error on Java 1.5:

// Override with explicitly returned subtype: Java 1.5 only
// (where SpecialSimpleEdgePortrayal2D extends SimpleEdgePortrayal2D)

protected SpecialSimpleEdgePortrayal2D createDefaultPortrayal() {
    return new SpecialSimpleEdgePortrayal2D();


// Version for Java < 1.5

protected SimpleEdgePortrayal2D createDefaultPortrayal() {
    return new SpecialSimpleEdgePortrayal2D();

This avoids callers of getDefaultPortrayal on the subclass having to do
explicit casts if they need the functionality of the subclassed
SimpleEdgePortayal2D (and makes subclass documentation much clearer).
Sorry, a bit of a tangent (esp. for MASON itself as it's Java 1.3
compatible), but an interesting feature of 1.5 that I hadn't come across
until now, and was burning to communicate ;-)

Appreciate that the benefit may be (far!) outweighed by the
compatibility issues, but don't know MASON's stance on compatibility vs.
performance/correctness (for example, I assume that MASON's support down
to Java 1.3 is a conscious compatibility design decision).


(Stuart Rossiter - University of Strathclyde)