Development Guidelines

How your code should look like

  • make sure the code is Java 8 compatible
  • Document your code using inline comments and javadoc. Many people will thank you :)
  • Try to avoid public fields
  • JOSM has a lot of helper methods in the Utils, GuiUtils, Geometry ... classes. Use them if you need
  • Check parameters. You can use Objects.requireNonNull.
  • Don't write for performance - write for readability. Use Streams, Functions and other Java 8 features if they make the code more readable.

Threading / Locking

  • JOSM uses various locking mechanisms, depending on the object.
  • The data sets are protected by a RW lock. Some methods do not automatically lock for performance reasons. Make sure to acquire the locks required for your changes.
  • GUI components should only be modified in the EDT thread
  • Prefer to use SwingUtils.invokeLater if you need to run anything on the UI thread
  • Many listeners already run in the EDT thread (layer changes) or have a central manager that allows you to register listeners that run in EDT (dataset changes, selection changes).

How your formatting should look like

  • make sure there is no trailing white space
  • don't use multiple consecutive empty lines
  • JOSM uses 4 characters indentation and no tab stops. If you use Notepad++ you can change the default indentation in the "Preferences" -> "Language" -> "Tab Settings" -> check "Replace by spaces" (this is permanent) or with the "change indentation settings" button in the toolbar (this is a temopary setting and requires the plugin "Customize Toolbar").
  • add curly brackets for each if, unless it is followed by return (or maybe throw)
  • You should use checkstyle before patch/commit: ant checkstyle and check checkstyle-josm.xml; if you find running checkstyle slow for all files, run for changed files only:
        svn diff --summarize | awk '{ print $2 }' | xargs java -jar tools/checkstyle/checkstyle-7.1-all.jar -c tools/checkstyle/josm_checks.xml
        # or
        git diff --name-only | xargs java -jar tools/checkstyle/checkstyle-7.1-all.jar -c tools/checkstyle/josm_checks.xml

How your javadoc should look like

  • The Oracle Javadoc style guide is used as the base guide
  • @since is used for public classes and methods (visible to plugin developers) with the JOSM revision that introduced the element. Example: @since 5408
    • @since is updated / added when a public method signature changes or if a class is renamed.
    • @since can be omitted for public methods and fields introduced at the same time as the class, provided they do not have changed and the class is correctly documented.
    • There can be multiple @since tags, e.g. for adding interfaces to a class. The reason for those tags should be added Example: @since 12345 @FunctionalIterface was added
    • If you submit a patch and don't know the revision, add @since xxx any way. It can then be replaced when merging your patch.
  • @throws is preferred to @exception
  • check your changes before patch/commit by generating javadoc: ant javadoc, browse output messages; if you find running javadoc slow for all files, run for changed files only:
    svn diff --summarize | awk '{ print $2 }' | xargs javadoc -d javadoc
    # or
    git diff --name-only | xargs javadoc -d javadoc

Configuring Eclipse


  • make sure you use tr(...) for all localized strings
    import import static;
    // use tr(...) for exception messages
    throw new Exception(tr("error message always in tr()"));
    // use tr(...) for labels, title, tooltip texts and the like
    new JLabel(tr("Label always in tr()"));
    // etc.
  • never assemble localized messages with +. Use format placeholders instead.

DONT new JLabel(tr("My Label " + labelId));

DO new JLabel(tr("My Label {0}",labelId));

Only exception: + can be used to break long lines of non-variable texts.

  • When using apostrophe, the following rules apply:

For all tr the apostrophe is special. (Like backslash in C)
It needs to be escaped by another apostrophe:

new JButton(tr("Don''t press me more than {0} times!", n))

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Last modified 3 weeks ago Last modified on 2017-05-11T10:48:19+02:00

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