SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/boss/BossBar.java

@@ -1,6 +1,9 @@
 package org.bukkit.boss;
 
 import org.bukkit.entity.Player;
+import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.List;
 
@@ -11,6 +14,7 @@ public interface BossBar {
      *
      * @return the title of the bar
      */
+    @NotNull
     String getTitle();
 
     /**
@@ -18,13 +22,14 @@ public interface BossBar {
      *
      * @param title the title of the bar
      */
-    void setTitle(String title);
+    void setTitle(@Nullable String title);
 
     /**
      * Returns the color of this boss bar
      *
      * @return the color of the bar
      */
+    @NotNull
     BarColor getColor();
 
     /**
@@ -32,13 +37,14 @@ public interface BossBar {
      *
      * @param color the color of the bar
      */
-    void setColor(BarColor color);
+    void setColor(@NotNull BarColor color);
 
     /**
      * Returns the style of this boss bar
      *
      * @return the style of the bar
      */
+    @NotNull
     BarStyle getStyle();
 
     /**
@@ -46,21 +52,21 @@ public interface BossBar {
      *
      * @param style the style of the bar
      */
-    void setStyle(BarStyle style);
+    void setStyle(@NotNull BarStyle style);
 
     /**
      * Remove an existing flag on this boss bar
      *
      * @param flag the existing flag to remove
      */
-    void removeFlag(BarFlag flag);
+    void removeFlag(@NotNull BarFlag flag);
 
     /**
      * Add an optional flag to this boss bar
      *
      * @param flag an optional flag to set on the boss bar
      */
-    void addFlag(BarFlag flag);
+    void addFlag(@NotNull BarFlag flag);
 
     /**
      * Returns whether this boss bar as the passed flag set
@@ -68,7 +74,7 @@ public interface BossBar {
      * @param flag the flag to check
      * @return whether it has the flag
      */
-    boolean hasFlag(BarFlag flag);
+    boolean hasFlag(@NotNull BarFlag flag);
 
     /**
      * Sets the progress of the bar. Values should be between 0.0 (empty) and
@@ -90,7 +96,7 @@ public interface BossBar {
      *
      * @param player the player to add
      */
-    void addPlayer(Player player);
+    void addPlayer(@NotNull Player player);
 
     /**
      * Removes the player from this boss bar causing it to be removed from their
@@ -98,7 +104,7 @@ public interface BossBar {
      *
      * @param player the player to remove
      */
-    void removePlayer(Player player);
+    void removePlayer(@NotNull Player player);
 
     /**
      * Removes all players from this boss bar
@@ -112,6 +118,7 @@ public interface BossBar {
      *
      * @return a immutable list of players
      */
+    @NotNull
     List<Player> getPlayers();
 
     /**