We build for JDK 1.5.

src/main/java/net/minecraft/server/EntityPlayer.java

@@ -67,7 +67,7 @@ public class EntityPlayer extends EntityHuman implements ICrafting {
         if (world == null) {
             dead = false;
             ChunkCoordinates position = null;
-            if (!spawnWorld.isEmpty()) {
+            if (spawnWorld != null && !spawnWorld.equals("")) {
                 CraftWorld cw = (CraftWorld) Bukkit.getServer().getWorld(spawnWorld);
                 if (cw != null && M() != null) {
                     world = cw.getHandle();

src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -598,7 +598,7 @@ public final class CraftServer implements Server {
             if (node != null) {
                 String name = node.getString("generator");
 
-                if ((name != null) && (!name.isEmpty())) {
+                if ((name != null) && (!name.equals(""))) {
                     String[] split = name.split(":", 2);
                     String id = (split.length > 1) ? split[1] : null;
                     Plugin plugin = pluginManager.getPlugin(split[0]);