I would also recommend GeoTools: look at the CRS class:
http://docs.geotools.org/latest/userguide/library/referencing/crs.html
Using Maven:
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-api</artifactId>
<version>8-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-referencing</artifactId>
<version>8-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-epsg-hsql</artifactId>
<version>8-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-epsg-extension</artifactId>
<version>${version.geotools}</version>
</dependency>
The gt-epsg-hsql basically included a HSQL database with a load of CRS definitions already loaded. However, you can also manually load definitions using WKT:
CRS.parseWKT( txt )
This means you can pop on over to http://spatialreference.org/, grab an OGC WKT definition and plumb it in.
Also, Proj4 does have JNI bindings if you fancy going that route.