Java: added true solar time demo

demo/java/correct/solar_time.txt

@@ -0,0 +1 @@
+True solar time = 11.6286 hours (11:37:42.979)

demo/java/demotest

@@ -19,6 +19,7 @@ if ! ./gradlew jar test; then
     exit 1
 fi
 
+TestDemo solar_time +38.88 -77.03 2023-02-12T17:00:00Z
 TestDemo constellation 2021-06-01T00:00:00Z
 TestDemo jupiter_moons 2021-04-16T00:26:18Z
 TestDemo lunar_eclipse 1988-01-01T00:00:00Z

demo/java/demotest.bat

@@ -11,6 +11,7 @@ call gradlew.bat jar test || exit /b 1
 
 REM ----------------------------------------------------------------------------------
 
+call :TestDemo solar_time +38.88 -77.03 2023-02-12T17:00:00Z || exit /b 1
 call :TestDemo constellation 2021-06-01T00:00:00Z || exit /b 1
 call :TestDemo jupiter_moons 2021-04-16T00:26:18Z || exit /b 1
 call :TestDemo lunar_eclipse 1988-01-01T00:00:00Z || exit /b 1

demo/java/src/main/java/io/github/cosinekitty/astronomy/demo/Main.java

@@ -60,6 +60,10 @@ public class Main {
         "    seasons year",
         "        Given an integer year number, displays the solstices and equinoxes for that year.",
         "        The year must be in the range 0000..9999.",
+        "",
+        "    solar_time latitude longitude [yyyy-mm-ddThh:mm:ssZ]",
+        "        Displays the true solar time for the observer at the",
+        "        given geographic coordinates.",
         ""
     );
 
@@ -198,6 +202,12 @@ public Demo(String name, int minArgs, int maxArgs, DemoRunner runner) {
             Seasons.run(
                 parseYear(args[1])
             )
+        ),
+        new Demo("solar_time", 3, 4, args ->
+            SolarTime.run(
+                parseObserver(args, 1),
+                parseTime(args, 3)
+            )
         )
     );
 }

demo/java/src/main/java/io/github/cosinekitty/astronomy/demo/SolarTime.java

@@ -0,0 +1,29 @@
+package io.github.cosinekitty.astronomy.demo;
+import io.github.cosinekitty.astronomy.*;
+
+public class SolarTime {
+    /**
+     * Display true solar time for a given geographic location at a given time.
+     *
+     * @param observer
+     * The geographic location for which to calculate true solar time.
+     *
+     * @param time
+     * The date and time for which to calculate true solar time.
+     */
+    public static int run(Observer observer, Time time) {
+        double ha = Astronomy.hourAngle(Body.Sun, time, observer);
+        double solarTimeHours = (ha + 12.0) % 24.0;
+        int milli = (int) Math.round(solarTimeHours * 3.6e+6);
+        int second = milli / 1000;
+        milli %= 1000;
+        int minute = second / 60;
+        second %= 60;
+        int hour = minute / 60;
+        minute %= 60;
+        hour %= 24;
+
+        System.out.printf("True solar time = %7.4f hours (%02d:%02d:%02d.%03d)%n", solarTimeHours, hour, minute, second, milli);
+        return 0;
+    }
+}