Initial work on cleaning up plugin message handling in Velocity

proxy/src/test/java/com/velocitypowered/proxy/util/collect/CappedCollectionTest.java

@@ -0,0 +1,35 @@
+package com.velocitypowered.proxy.util.collect;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import java.util.Collection;
+import java.util.Set;
+import org.junit.jupiter.api.Test;
+
+class CappedCollectionTest {
+
+  @Test
+  void basicVerification() {
+    Collection<String> coll = CappedCollection.newCappedSet(1);
+    assertTrue(coll.add("coffee"), "did not add single item");
+    assertThrows(IllegalStateException.class, () -> coll.add("tea"),
+        "item was added to collection although it is too full");
+    assertEquals(1, coll.size(), "collection grew in size unexpectedly");
+  }
+
+  @Test
+  void testAddAll() {
+    Set<String> doesFill1 = ImmutableSet.of("coffee", "tea");
+    Set<String> doesFill2 = ImmutableSet.of("chocolate");
+    Set<String> overfill = ImmutableSet.of("Coke", "Pepsi");
+
+    Collection<String> coll = CappedCollection.newCappedSet(3);
+    assertTrue(coll.addAll(doesFill1), "did not add items");
+    assertTrue(coll.addAll(doesFill2), "did not add items");
+    assertThrows(IllegalStateException.class, () -> coll.addAll(overfill),
+        "items added to collection although it is too full");
+    assertEquals(3, coll.size(), "collection grew in size unexpectedly");
+  }
+}