Recommended code patterns

This page lists some recommended code pattern for developing or using Apache SIS.


This section lists recommended code pattern when using the sis-referencing module.

Never explicitly swap coordinates for axis order

The axis order issue causes lot of confusion, and developers are sometimes tempted to swap their coordinate values in order to comply with some expected axis ordering. It should never be necessary, since the Apache SIS referencing engine manages axis order transparently — provided that the Coordinate Reference System (CRS) definition is accurate. If a code needs to swap coordinates, this is probably an indication that the CRS has not been properly defined. Instead of patching the coordinate values, try to make sure that the Source CRS (associated to the original data) and the Target CRS (the coordinate space where to perform the work) are properly defined, and let the referencing engine performs the conversion from the source to the target CRS.

Rasters and coverages

This section lists recommended code pattern when using the sis-coverage module.

Georeference images with affine transforms, not bounding boxes

Many users define the geographic extent of an image by its corner locations. This approach may be ambiguous as it does not specify whether the (x,y) axes are interchanged (see the axis order issue) or if the y axis is oriented downward. All images in SIS shall be georeferenced by at least an affine transform (more complex transforms are also possible), never by a rectangle or bounding box. In the two-dimensional case, the standard java.awt.geom.Affine­Transform class can be used.

Do not cast Raster to WritableRaster

Some images are writable. But modifying pixel values should be done by invoking the getWritableTile(…) and releaseWritableTile(…) methods of WritableRenderedImage interface. Do not cast directly a Raster to WritableRaster even when the cast is safe, because some raster data may be shared by many tiles having identical content. Furthermore changes in pixel values may be lost if {@code releaseWritableTile(…)} is not invoked.


This section lists recommended code pattern for internationalization.

Specify timezone

Geospatial data often cover a wide geographic area, spanning many time zones. Timezone are sometimes specified as metadata in the header of data files to be read, or is sometimes fixed to UTC by applications managing world-wide data. Some Apache SIS objects have Locale and TimeZone information. Such locale and timezone are given to java.text.DateFormat or java.util.Calendar constructors among others.

When reading dates or timestamps from a JDBC database, always use the ResultSet method accepting a Calendar argument, when such method is available. For example prefer the getTimestamp(int, Calendar) method instead of getTimestamp(int). The Calendar object should has been created with the appropriate timezone.

Replace underscores by spaces before sorting

Before to sort programmatic names for human reading, consider replacing all occurrences of the underscore character ('_') by the space character (' '). The ASCII value of the underscore character is greater than 'Z' but lower than 'a', which sometimes produce unexpected sort results. For example "Foo_bar" is sorted between "FooBar" and "Foobar". The space character produces more consistent sort results because its ASCII value is less than any printable character, so "Foo bar" is sorted before both "FooBar" and "Foobar".

Loop over character sequences using code points

Since Java 1.5, characters are no longer restricted to 16 bits. Some “characters” are actually represented using two consecutive char elements. Those “characters” are called code points. Consequently, when iterating over characters in a string, the following pattern should be used:

for (int i=0; i<string.length();) {
    final int c = string.codePointAt(i);
    // ... do some stuff ...
    i += Character.charCount(c);


Apache SIS uses the java.util.logging framework. It does not necessarily means that all SIS users are forced to use this framework, as it is possible to use java.util.logging as an API and have logging redirected to another system. For example the logging can be redirect to SLF4J by adding the jul-to-slf4j dependency to a project.

The logger names are usually the package name of the class emitting log messages, but not necessarily. In particular, we do not follow this convention if the class is located in an internal package (org.apache.sis.internal.*) since those packages are considered privates. In such cases, the logger name should be the package name of the public class invoking the internal methods. The reason for that rule is that logger names are considered part of the public API, since developers use them for configuring their logging (verbosity, destination, etc.).

All logging at Level.INFO or above shall be targeted to users or administrators, not to developers. In particular Level.SEVERE shall be reserved for critical errors that compromise the application stability — it shall not be used for exceptions thrown while parsing user data (file or database).