The Java Mixin Pattern, or Faking Multiple Inheritance

Unlike C++, Java supports single inheritance only -- it does not support multiple inheritance. Same with Ruby -- but Ruby has the concept of "Mixins", which are "partial classes" that you can bolt onto your class, giving you something very like multiple inheritance.

You can fake mixins in Java. They're great when you want to refactor common functionality from two classes having different superclasses. Just make a new class called FooMixin to hold the common functionality -- the magic of inner classes handles the rest. Here's a real-world example from the OpenJUMP open-source GIS project:

public class LoadDatasetFromFilePlugIn extends AbstractLoadDatasetPlugIn {
 protected void setSelectedFormat(String format) {
     loadSaveDatasetFileMixin.setSelectedFormat(format);
 }
 protected String getSelectedFormat() {
     return loadSaveDatasetFileMixin.getSelectedFormat();
 }
 protected Collection showDialog(WorkbenchContext context) {
     final JFileChooser fileChooser = GUIUtil
             .createJFileChooserWithExistenceChecking();
     fileChooser.setDialogType(JFileChooser.OPEN_DIALOG);
     fileChooser.setMultiSelectionEnabled(true);
     return loadSaveDatasetFileMixin.showDialog(fileChooser,
             LoadFileDataSourceQueryChooser.class, context);
 }
 private LoadSaveDatasetFileMixin loadSaveDatasetFileMixin = new LoadSaveDatasetFileMixin() {
     protected String getName() {
         return LoadDatasetFromFilePlugIn.this.getName();
     }
     protected String getLastDirectoryKey() {
         return LoadDatasetFromFilePlugIn.this.getLastDirectoryKey();
     }
     public boolean isAddingExtensionIfRequested() { return false; }
     public File initiallySelectedFile(File currentDirectory) { return null; }
 };
} 

Note that the LoadSaveDatasetFileMixin has access to the main class (via template methods like #getName, #getLastDirectoryKey), and the main class of course has access to the mixin (via #setSelectedFormat, #getSelectedFormat). Beautiful.