Fix top ten ordering: replace LinkedHashMap with ConcurrentHashMap for thread safety

src/main/java/world/bentobox/level/LevelsManager.java

@@ -82,6 +82,8 @@ private boolean addToTopTen(Island island, long lv) {
                 && hasTopTenPerm(island.getWorld(), island.getOwner())) {
             topTenLists.computeIfAbsent(island.getWorld(), k -> new TopTenData(island.getWorld())).getTopTen()
                     .put(island.getUniqueId(), lv);
+            // Invalidate the cache for this world because of the update
+            cache.remove(island.getWorld());
             return true;
         }
         return false;

src/main/java/world/bentobox/level/objects/TopTenData.java

@@ -1,8 +1,8 @@
 package world.bentobox.level.objects;
 
-import java.util.LinkedHashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.bukkit.World;
 
@@ -20,7 +20,7 @@ public class TopTenData {
     @Expose
     private String uniqueId = "";
     @Expose
-    private Map<String, Long> topTen = new LinkedHashMap<>();
+    private Map<String, Long> topTen = new ConcurrentHashMap<>();
 
     public TopTenData(World k) {
 	uniqueId = k.getName().toLowerCase(Locale.ENGLISH);

src/test/java/world/bentobox/level/LevelsManagerTest.java

@@ -381,6 +381,43 @@ public void testSetIslandLevel() {
 
     }
 
+    /**
+     * Test method for
+     * {@link world.bentobox.level.LevelsManager#getTopTen(org.bukkit.World, int)}.
+     * Verifies that the top ten is sorted in descending order by level.
+     */
+    @Test
+    public void testGetTopTenSortOrder() {
+        lm.createAndCleanRankings(world);
+        Map<World, TopTenData> ttl = lm.getTopTenLists();
+        Map<String, Long> tt = ttl.get(world).getTopTen();
+        // Add islands in non-sorted order, mimicking the reported issue
+        String island65 = UUID.randomUUID().toString();
+        String island1065 = UUID.randomUUID().toString();
+        String island500 = UUID.randomUUID().toString();
+        String island200 = UUID.randomUUID().toString();
+        // Insert in arbitrary order
+        tt.put(island65, 65L);
+        tt.put(island1065, 1065L);
+        tt.put(island500, 500L);
+        tt.put(island200, 200L);
+        when(im.getIslandById(island65)).thenReturn(Optional.of(island));
+        when(im.getIslandById(island1065)).thenReturn(Optional.of(island));
+        when(im.getIslandById(island500)).thenReturn(Optional.of(island));
+        when(im.getIslandById(island200)).thenReturn(Optional.of(island));
+
+        Map<String, Long> topTen = lm.getTopTen(world, Level.TEN);
+        // Verify descending order
+        long previousLevel = Long.MAX_VALUE;
+        for (Long level : topTen.values()) {
+            assertTrue("Top ten not in descending order: " + level + " should be <= " + previousLevel,
+                    level <= previousLevel);
+            previousLevel = level;
+        }
+        // Verify highest is first
+        assertEquals(1065L, topTen.values().iterator().next().longValue());
+    }
+
     /**
      * Test method for
      * {@link world.bentobox.level.LevelsManager#getRank(World, UUID)}