Added tag default value

src/main/java/net/minestom/server/tag/Tag.java

@@ -1,11 +1,13 @@
 package net.minestom.server.tag;
 
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jglrxavpok.hephaistos.nbt.NBT;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 import java.util.function.BiConsumer;
 import java.util.function.Function;
+import java.util.function.Supplier;
 
 public class Tag<T> {
 
@@ -13,6 +15,8 @@ public class Tag<T> {
     private final Function<NBTCompound, T> readFunction;
     private final BiConsumer<NBTCompound, T> writeConsumer;
 
+    protected volatile Supplier<T> defaultValue;
+
     private Tag(@NotNull String key,
                 @NotNull Function<NBTCompound, T> readFunction,
                 @NotNull BiConsumer<NBTCompound, T> writeConsumer) {
@@ -25,6 +29,20 @@ public class Tag<T> {
         return key;
     }
 
+    public Tag<T> defaultValue(@NotNull Supplier<T> defaultValue) {
+        this.defaultValue = defaultValue;
+        return this;
+    }
+
+    public Tag<T> defaultValue(@NotNull T defaultValue) {
+        defaultValue(() -> defaultValue);
+        return this;
+    }
+
+    public @Nullable Supplier<@Nullable T> getDefaultValue() {
+        return defaultValue;
+    }
+
     protected T read(@NotNull NBTCompound nbtCompound) {
         return readFunction.apply(nbtCompound);
     }

src/main/java/net/minestom/server/tag/TagGetter.java

@@ -5,4 +5,6 @@ import org.jetbrains.annotations.Nullable;
 
 public interface TagGetter {
     <T> @Nullable T getTag(@NotNull Tag<T> tag);
+
+    boolean hasTag(@NotNull Tag<?> tag);
 }