WIP collection deep cloning. Starting with PotionMeta

src/main/java/net/minestom/server/item/metadata/PotionMeta.java

@@ -4,7 +4,9 @@ import net.minestom.server.chat.ChatColor;
 import net.minestom.server.potion.CustomPotionEffect;
 import net.minestom.server.potion.PotionType;
 import net.minestom.server.registry.Registries;
+import net.minestom.server.utils.clone.CloneUtils;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 import org.jglrxavpok.hephaistos.nbt.NBTList;
 import org.jglrxavpok.hephaistos.nbt.NBTTypes;
@@ -22,7 +24,9 @@ import java.util.concurrent.CopyOnWriteArrayList;
 public class PotionMeta extends ItemMeta {
 
     private PotionType potionType;
-    private final List<CustomPotionEffect> customPotionEffects = new CopyOnWriteArrayList<>();
+
+    // Not final because of #clone()
+    private List<CustomPotionEffect> customPotionEffects = new CopyOnWriteArrayList<>();
 
     private boolean hasColor;
     private byte red, green, blue;
@@ -32,6 +36,7 @@ public class PotionMeta extends ItemMeta {
      *
      * @return the potion type
      */
+    @Nullable
     public PotionType getPotionType() {
         return potionType;
     }
@@ -41,15 +46,16 @@ public class PotionMeta extends ItemMeta {
      *
      * @param potionType the new potion type
      */
-    public void setPotionType(PotionType potionType) {
+    public void setPotionType(@Nullable PotionType potionType) {
         this.potionType = potionType;
     }
 
     /**
      * Get a list of {@link CustomPotionEffect}.
      *
-     * @return the custom potion effects
+     * @return the custom potion effect list
      */
+    @NotNull
     public List<CustomPotionEffect> getCustomPotionEffects() {
         return customPotionEffects;
     }
@@ -75,7 +81,8 @@ public class PotionMeta extends ItemMeta {
 
     @Override
     public boolean hasNbt() {
-        return potionType != null;
+        return potionType != null ||
+                !customPotionEffects.isEmpty();
     }
 
     @Override
@@ -155,7 +162,7 @@ public class PotionMeta extends ItemMeta {
     public ItemMeta clone() {
         PotionMeta potionMeta = (PotionMeta) super.clone();
         potionMeta.potionType = potionType;
-        potionMeta.customPotionEffects.addAll(customPotionEffects);
+        potionMeta.customPotionEffects = CloneUtils.cloneCopyOnWriteArrayList(customPotionEffects);
 
         potionMeta.hasColor = hasColor;
         potionMeta.red = red;

src/main/java/net/minestom/server/potion/CustomPotionEffect.java

@@ -1,9 +1,16 @@
 package net.minestom.server.potion;
 
+import net.minestom.server.utils.clone.PublicCloneable;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Objects;
+
 /**
  * Represents a custom effect in {@link net.minestom.server.item.metadata.PotionMeta}.
+ * <p>
+ * This is an immutable class.
  */
-public class CustomPotionEffect {
+public class CustomPotionEffect implements PublicCloneable<CustomPotionEffect> {
 
     private final byte id;
     private final byte amplifier;
@@ -45,4 +52,28 @@ public class CustomPotionEffect {
     public boolean showIcon() {
         return showIcon;
     }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        CustomPotionEffect that = (CustomPotionEffect) o;
+        return id == that.id && amplifier == that.amplifier && duration == that.duration && ambient == that.ambient && showParticles == that.showParticles && showIcon == that.showIcon;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id, amplifier, duration, ambient, showParticles, showIcon);
+    }
+
+    @NotNull
+    @Override
+    public CustomPotionEffect clone() {
+        try {
+            return (CustomPotionEffect) super.clone();
+        } catch (CloneNotSupportedException e) {
+            e.printStackTrace();
+            throw new IllegalStateException("Weird thing happened");
+        }
+    }
 }

src/main/java/net/minestom/server/utils/clone/CloneUtils.java

@@ -0,0 +1,19 @@
+package net.minestom.server.utils.clone;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+public final class CloneUtils {
+
+    @NotNull
+    public static <T extends PublicCloneable> CopyOnWriteArrayList cloneCopyOnWriteArrayList(@NotNull List<T> list) {
+        CopyOnWriteArrayList<T> result = new CopyOnWriteArrayList<>();
+        for (T element : list) {
+            result.add((T) element.clone());
+        }
+        return result;
+    }
+
+}