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Converter

The Converter API is a way to convert a value of one type into a value of another type. The difference is that we supply a plugins so you can teach the system how to perform additional conversions over time.

  • The most common case of this is to parse a string into another type of value.

The Converter interface looks like this:

public interface Converter {
    Object convert( Object source, Class target ) throws Exception;
}

When you implement this you can either:

  • Handle it correctly (returning an object of the target class
  • Return null (if you could not handle it correctly)
  • Throw an exception (if you could not handle it correctly)

Converter for Enum

Lets look at a real world example coming to us from the land of Java 5. Java 5 features a new type of construct called Enum. This is not something GeoTools can usually deal with as a Java 1.4 project - but by supplying a converter you can teach GeoTools to get along with your Java 5 application.

Example enum:

public enum Choice {
   THIS,
   THAT;
}

Example enum converter:

 class Choice2TextConverter {
    Object convert( Object source, Class target ){
         if( target != Choice.class ) return null;
         return Choice.valueOf( (String) source ) );
    }
}

In general the freedom to return null or throw an exception lets you program converters very quickly.

A common case that we see in many places in GeoTools is parsing a string into the type defined by an AttributeType.

GeoTools 2.3 code (before converters were around):

FeatureType featureType = .... ;
AttributeType intType = featureType.getAttributeType( "intProperty" );
String string = "1234";

Integer integer = (Integer) intType.parse( string );

Using the Converters utility class this becomes:

FeatureType featureType = .... ;
AttributeType intType = featureType.getAttributeType( "intProperty" );
String string = "1234";

Integer integer = Converters.convert( string, intType.getType() );

The Converters are the technology behind our great support for the Filter API Expression:

Expression expr = ff.literal("#FF0000")
Color color = expr.evaualte( null, Color.class );