Implement ItemStack#equals & use a soft reference to store meta cached NBT

src/main/java/net/minestom/server/item/ItemMeta.java

@@ -8,6 +8,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
+import java.lang.ref.SoftReference;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -29,7 +30,7 @@ public class ItemMeta implements Cloneable {
 
     private final int customModelData;
 
-    private NBTCompound cache = null;
+    private SoftReference<NBTCompound> cache;
 
     protected ItemMeta(@NotNull ItemMetaBuilder metaBuilder) {
         this.builder = metaBuilder.clone();
@@ -85,10 +86,12 @@ public class ItemMeta implements Cloneable {
     }
 
     public @NotNull NBTCompound toNBT() {
-        if (cache == null) {
-            this.cache = NBTUtils.metaToNBT(this);
+        var nbt = cache != null ? cache.get() : null;
+        if (nbt == null) {
+            nbt = NBTUtils.metaToNBT(this);
+            cache = new SoftReference<>(nbt);
         }
-        return cache;
+        return nbt;
     }
 
     @Override

src/main/java/net/minestom/server/item/ItemStack.java

@@ -111,21 +111,48 @@ public class ItemStack {
         return withLore(loreUnaryOperator.apply(getLore()));
     }
 
+    @Contract(pure = true)
     public @NotNull StackingRule getStackingRule() {
         return stackingRule;
     }
 
+    @Contract(pure = true)
     public @NotNull ItemMeta getMeta() {
         return meta;
     }
 
+    @Contract(pure = true)
+    public boolean isAir() {
+        return equals(AIR);
+    }
+
+    @Contract(pure = true)
     public boolean isSimilar(@NotNull ItemStack itemStack) {
         return material.equals(itemStack.material) &&
                 meta.equals(itemStack.meta);
     }
 
-    public boolean isAir() {
-        return equals(AIR);
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        ItemStack itemStack = (ItemStack) o;
+        if (uuid.equals(itemStack.uuid)) return true;
+
+        if (amount != itemStack.amount) return false;
+        if (!stackingRule.equals(itemStack.stackingRule)) return false;
+        if (material != itemStack.material) return false;
+        return meta.equals(itemStack.meta);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = stackingRule.hashCode();
+        result = 31 * result + material.hashCode();
+        result = 31 * result + amount;
+        result = 31 * result + meta.hashCode();
+        return result;
     }
 
     @Contract(value = "-> new", pure = true)