More robust check

proxy/src/main/java/com/velocitypowered/proxy/util/collect/CappedCollection.java

@@ -36,16 +36,16 @@ public class CappedCollection<T> extends ForwardingCollection<T> {
 
   @Override
   public boolean add(T element) {
-    Preconditions.checkState(this.delegate.size() + 1 <= upperSize, "collection is too large (%s)",
-        this.delegate.size());
+    if (this.delegate.size() >= upperSize) {
+      Preconditions.checkState(this.delegate.contains(element), "collection is too large (%s >= %s)",
+          this.delegate.size(), this.upperSize);
+      return false;
+    }
     return this.delegate.add(element);
   }
 
   @Override
   public boolean addAll(Collection<? extends T> collection) {
-    Preconditions.checkState(this.delegate.size() + collection.size() <= upperSize,
-        "collection would grow too large (%s + %s > %s)",
-        this.delegate.size(), collection.size(), upperSize);
     return this.standardAddAll(collection);
   }
 }

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

@@ -32,4 +32,18 @@ class CappedCollectionTest {
         "items added to collection although it is too full");
     assertEquals(3, coll.size(), "collection grew in size unexpectedly");
   }
+
+  @Test
+  void handlesSetBehaviorCorrectly() {
+    Set<String> doesFill1 = ImmutableSet.of("coffee", "tea");
+    Set<String> doesFill2 = ImmutableSet.of("coffee", "chocolate");
+    Set<String> overfill = ImmutableSet.of("coffee", "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");
+  }
 }