Missing Entity API

Co-authored-by: Nassim Jahnke <nassim@njahnke.dev>
Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>
Co-authored-by: William Blake Galbreath <blake.galbreath@gmail.com>
Co-authored-by: SoSeDiK <mrsosedik@gmail.com>
Co-authored-by: booky10 <boooky10@gmail.com>
Co-authored-by: Amin <amin.haddou@frg.wwschool.de>
Co-authored-by: TrollyLoki <trollyloki@gmail.com>
Co-authored-by: FireInstall <kettnerl@hu-berlin.de>
Co-authored-by: maxcom1 <46265094+maxcom1@users.noreply.github.com>
Co-authored-by: TotalledZebra <Holappa57@gmail.com>

paper-api/src/main/java/io/papermc/paper/entity/SchoolableFish.java

@@ -0,0 +1,47 @@
+package io.papermc.paper.entity;
+
+import org.bukkit.entity.Fish;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * Represents a fish that can school with other fish.
+ */
+@NullMarked
+public interface SchoolableFish extends Fish {
+
+    /**
+     * Forces this fish to follow the given fish.
+     *
+     * @param leader fish to follow
+     */
+    void startFollowing(SchoolableFish leader);
+
+    /**
+     * Causes the fish to stop following their current
+     * leader.
+     */
+    void stopFollowing();
+
+    /**
+     * Gets the amount of fish currently following this fish.
+     *
+     * @return school size
+     */
+    int getSchoolSize();
+
+    /**
+     * Gets the maximum number of fish that will naturally follow this fish.
+     *
+     * @return max school size
+     */
+    int getMaxSchoolSize();
+
+    /**
+     * Gets the fish that this entity is currently following.
+     *
+     * @return following fish
+     */
+    @Nullable SchoolableFish getSchoolLeader();
+
+}