Java: instance variable vs. local parameters

Question

I am puzzled by this:

private RenderingHints hints;

public void addRenderingHints(Map hints) {
    hints.putAll(hints);
}

(from http://dev.geogebra.org/trac/browser/trunk/geogebra/desktop/org/freehep/graphicsio/AbstractVectorGraphicsIO.java?rev=39574#L1238)

Both Map and RenderingHints have a putAll member functions: https://docs.oracle.com/javase/8/docs/api/java/awt/RenderingHints.html#putAll-java.util.Map- https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#putAll-java.util.Map-

I have learned that local parameters can shadow instance variables. So what does this part of code do (useful)?

That's a mistake. That code does not do what the author intended. — SLaks
– SLaks, Commented Jul 2, 2015 at 17:49
You can copy and paste that code into any Java IDE and check the behaviour — Cristian Meneses
– Cristian Meneses, Commented Jul 2, 2015 at 17:49
@bers: It's even worse than that. It mutates the parameter. — SLaks
– SLaks, Commented Jul 2, 2015 at 17:49

Luiggi Mendoza · Accepted Answer · 2015-07-02 17:49:51Z

4

The code is wrong. Currently, it adds the elements of the parameter Map hint in itself, that will end doing nothing.

The code should be:

public void addRenderingHints(Map hints) {
    this.hints.putAll(hints);
}

The difference is when using this.hints. Using this refers to the fields in the class.

answered Jul 2, 2015 at 17:49

Luiggi Mendoza

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Comments

T.J. Crowder · Accepted Answer · 2015-07-02 17:50:04Z

1

It adds contents of the Map referenced by the parameter to itself. That is, it calls putAll on the parameter, passing the parameter in. The instance member is not involved at all.

This is not likely what was intended. They probably meant:

this.hints.putAll(hints);

That calls putAll on the instance field, passing in the parameter.

This is one reason why some Java lint tools and IDEs have an option to require that you prefix instance fields and methods with this., even though the language specification makes it optional (er, when they're not shadowed by local parameters). In any case, hopefully any decent IDE would warn you that the parameter was shadowing the instance field.

answered Jul 2, 2015 at 17:50

T.J. Crowder

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