ICU-23091 Add Java source formatting configs and automated checks

.github/workflows/icu4j.yml

@@ -64,6 +64,33 @@ jobs:
           cd icu4j;
           mvn ${SHARED_MVN_ARGS} dependency:go-offline -P '!old_jdk_taglet'
 
+  # Using the Java style formatter google-java-style provided by the Spotless
+  # plugin configured in the root pom.xml using 4-space indents (AOSP style).
+  # Spotless is configured to run only on files in this branch (PR) that differ
+  # from origin/main
+  formatter:
+    name: Formatter + Style checker
+    needs: icu4j-mvn-init-cache
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout and setup
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0  # fetch all branches so that Spotless can resolve `origin/main`
+      - name: Restore read-only cache of local Maven repository
+        uses: actions/cache/restore@v4
+        id: cache
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-
+          lookup-only: true
+      - name: Check Java style
+        run: |
+          cd icu4j;
+          mvn spotless:check || (echo "Style checker failed. Formatting changes can be applied by 'mvn spotless:apply'" && exit 1)
+
   # ICU4J build and unit test using Maven
   # https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-java-with-maven
   icu4j-mvn-build-and-test:

docs/userguide/dev/codingguidelines.md

@@ -1449,6 +1449,20 @@ ICU Java classes and methods.
 
 ### Code style
 
+ICU uses a source formatter to ensure a consistent code style automatically,
+and it uses a single common formatter to avoid spurious diff noise in code reviews.
+This is now enforced via a
+[formatter](https://github.com/google/google-java-format)
+that is configured in the Maven build
+via a [Maven plugin](https://github.com/diffplug/spotless/tree/main/plugin-maven)
+and checked by continuous integration on pull requests.
+
+When creating pull requests, you can check the formatting locally using the command `mvn spotless:check`. You can apply the formatter's changes using the command `mvn spotless:apply`. Continuous integration errors for formatting can be fixed by committing the changes resulting from applying the formatter locally and pushing the new commit.
+
+The following are further guidelines for code style,
+but they will be superceded by the automated formatter in CI that is mentioned above
+if the guidelines differ in style.
+
 The standard order for modifier keywords on APIs is:
 
 * `public static final synchronized strictfp`

icu4j/demos/src/main/java/com/ibm/icu/dev/demo/Launcher.java

@@ -8,6 +8,9 @@
  */
 package com.ibm.icu.dev.demo;
 
+import com.ibm.icu.dev.demo.impl.DemoApplet;
+import com.ibm.icu.dev.demo.impl.DemoUtility;
+import com.ibm.icu.util.VersionInfo;
 import java.awt.BorderLayout;
 import java.awt.Button;
 import java.awt.Color;
@@ -22,26 +25,17 @@
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 
-import com.ibm.icu.dev.demo.impl.DemoApplet;
-import com.ibm.icu.dev.demo.impl.DemoUtility;
-import com.ibm.icu.util.VersionInfo;
-
-
 /**
- * @author srl
- * Application to provide a panel of demos to launch
+ * @author srl Application to provide a panel of demos to launch
  */
 public class Launcher extends DemoApplet {
     private static final long serialVersionUID = -8054963875776183877L;
-
-    /**
-     * base package of all demos
-     */
+
+    /** base package of all demos */
     public static final String demoBase = "com.ibm.icu.dev.demo";
-    /**
-     * list of classes, relative to the demoBase. all must have a static void main(String[])
-     */
-    public static final String demoList[] = { 
+
+    /** list of classes, relative to the demoBase. all must have a static void main(String[]) */
+    public static final String demoList[] = {
         "calendar.CalendarApp",
         "charsetdet.DetectingViewer",
         "holiday.HolidayCalendarDemo",
@@ -51,16 +45,16 @@ public class Launcher extends DemoApplet {
 
     public class LauncherFrame extends Frame implements ActionListener {
         private static final long serialVersionUID = -8054963875776183878L;
-        
+
         public Button buttonList[] = new Button[demoList.length]; // one button for each demo
         public Label statusLabel;
         private DemoApplet applet;
-        
+
         LauncherFrame(DemoApplet applet) {
             init();
             this.applet = applet;
         }
-        
+
         public void init() {
             // close down when close is clicked.
             // TODO: this should be factored..
@@ -74,114 +68,119 @@ public void windowClosing(WindowEvent e) {
                                 applet.demoClosed();
                             } else System.exit(0);
                         }
-                    } );
+                    });
 
             setBackground(DemoUtility.bgColor);
             setLayout(new BorderLayout());
 
             Panel topPanel = new Panel();
-            topPanel.setLayout(new GridLayout(5,3));
+            topPanel.setLayout(new GridLayout(5, 3));
 
-            for(int i=0;i<buttonList.length;i++) {
+            for (int i = 0; i < buttonList.length; i++) {
                 String demo = demoList[i];
                 Button b = new Button(demo);
                 b.addActionListener(this);
-                buttonList[i]=b;
+                buttonList[i] = b;
                 topPanel.add(b);
             }
-            add(BorderLayout.CENTER,topPanel);
+            add(BorderLayout.CENTER, topPanel);
             statusLabel = new Label("");
             statusLabel.setAlignment(Label.LEFT);
             String javaVersion = "";
-            try { 
-                javaVersion = "* Java: "+System.getProperty("java.version");
+            try {
+                javaVersion = "* Java: " + System.getProperty("java.version");
             } catch (Throwable t) {
                 javaVersion = "";
             }
-            add(BorderLayout.NORTH, new Label(
-                   "ICU Demos * ICU version "+VersionInfo.ICU_VERSION +
-                   " * https://icu.unicode.org/ "+javaVersion));
-            add(BorderLayout.SOUTH,statusLabel);
+            add(
+                    BorderLayout.NORTH,
+                    new Label(
+                            "ICU Demos * ICU version "
+                                    + VersionInfo.ICU_VERSION
+                                    + " * https://icu.unicode.org/ "
+                                    + javaVersion));
+            add(BorderLayout.SOUTH, statusLabel);
             // set up an initial status.
-            showStatus(buttonList.length+" demos ready. ");
+            showStatus(buttonList.length + " demos ready. ");
         }
-        
+
         /**
          * Change the 'status' field, and set it to black
+         *
          * @param status
          */
         void showStatus(String status) {
             statusLabel.setText(status);
             statusLabel.setForeground(Color.BLACK);
             statusLabel.setBackground(Color.WHITE);
-//            statusLabel.setFont(Font.PLAIN);
+            //            statusLabel.setFont(Font.PLAIN);
             doLayout();
         }
+
         void showStatus(String demo, String status) {
-            showStatus(demo+": "+status);
+            showStatus(demo + ": " + status);
         }
+
         void showFailure(String status) {
             statusLabel.setText(status);
             statusLabel.setBackground(Color.GRAY);
             statusLabel.setForeground(Color.RED);
-//            statusLabel.setFont(Font.BOLD);
+            //            statusLabel.setFont(Font.BOLD);
             doLayout();
         }
+
         void showFailure(String demo, String status) {
-            showFailure(demo+": "+status);
+            showFailure(demo + ": " + status);
         }
 
-
         public void actionPerformed(ActionEvent e) {
             // find button
-            for(int i=0;i<buttonList.length;i++) {
-                if(e.getSource() == buttonList[i]) {
+            for (int i = 0; i < buttonList.length; i++) {
+                if (e.getSource() == buttonList[i]) {
                     String demoShort = demoList[i];
-                    String demo = demoBase+'.'+demoShort;
+                    String demo = demoBase + '.' + demoShort;
                     showStatus(demoShort, "launching");
                     try {
                         Class c = Class.forName(demo);
                         String args[] = new String[0];
                         Class params[] = new Class[1];
                         params[0] = args.getClass();
-                        Method m = c.getMethod("main", params );
-                        Object[] argList = { args };
+                        Method m = c.getMethod("main", params);
+                        Object[] argList = {args};
                         m.invoke(null, argList);
                         showStatus(demoShort, "launched.");
                     } catch (ClassNotFoundException e1) {
-                        showFailure(demoShort,e1.toString());
+                        showFailure(demoShort, e1.toString());
                         e1.printStackTrace();
                     } catch (SecurityException se) {
-                        showFailure(demoShort,se.toString());
+                        showFailure(demoShort, se.toString());
                         se.printStackTrace();
                     } catch (NoSuchMethodException nsme) {
-                        showFailure(demoShort,nsme.toString());
+                        showFailure(demoShort, nsme.toString());
                         nsme.printStackTrace();
                     } catch (IllegalArgumentException iae) {
-                        showFailure(demoShort,iae.toString());
+                        showFailure(demoShort, iae.toString());
                         iae.printStackTrace();
                     } catch (IllegalAccessException iae) {
-                        showFailure(demoShort,iae.toString());
+                        showFailure(demoShort, iae.toString());
                         iae.printStackTrace();
                     } catch (InvocationTargetException ite) {
-                        showFailure(demoShort,ite.toString());
+                        showFailure(demoShort, ite.toString());
                         ite.printStackTrace();
                     }
                     repaint();
                 }
             }
         }
-
     }
-    
+
     /* This creates a Frame for the demo applet. */
     protected Frame createDemoFrame(DemoApplet applet) {
         return new LauncherFrame(applet);
     }
 
     /**
-     * The main function which defines the behavior of the Demo
-     * applet when an applet is started.
+     * The main function which defines the behavior of the Demo applet when an applet is started.
      */
     public static void main(String[] args) {
         new Launcher().showDemo();

icu4j/demos/src/main/java/com/ibm/icu/dev/demo/calendar/CalendarApp.java

@@ -9,23 +9,17 @@
 
 package com.ibm.icu.dev.demo.calendar;
 
-import java.awt.Frame;
-
 import com.ibm.icu.dev.demo.impl.DemoApplet;
+import java.awt.Frame;
 
-/**
- * CalendarApp demonstrates how Calendar works.
- */
-public class CalendarApp extends DemoApplet
-{
-    /**
-     * For serialization
-     */
+/** CalendarApp demonstrates how Calendar works. */
+public class CalendarApp extends DemoApplet {
+    /** For serialization */
     private static final long serialVersionUID = -4270137898405840825L;
 
     /**
-     * The main function which defines the behavior of the CalendarDemo
-     * applet when an applet is started.
+     * The main function which defines the behavior of the CalendarDemo applet when an applet is
+     * started.
      */
     public static void main(String argv[]) {