java: NilType::create() returns NilType's one and only instance

java/src/main/java/org/msgpack/MessagePackObject.java

@@ -23,7 +23,7 @@ import java.util.Map;
 import java.math.BigInteger;
 
 public abstract class MessagePackObject implements Cloneable, MessagePackable {
-	public boolean isNull() {
+	public boolean isNil() {
 		return false;
 	}
 

java/src/main/java/org/msgpack/object/NilType.java

@@ -21,14 +21,16 @@ import java.io.IOException;
 import org.msgpack.*;
 
 public class NilType extends MessagePackObject {
-	private static NilType instance = new NilType();
+	private final static NilType INSTANCE = new NilType();
 
 	public static NilType create() {
-		return instance;
+		return INSTANCE;
 	}
 
+	private NilType() { }
+
 	@Override
-	public boolean isNull() {
+	public boolean isNil() {
 		return true;
 	}
 
@@ -52,7 +54,7 @@ public class NilType extends MessagePackObject {
 
 	@Override
 	public Object clone() {
-		return new NilType();
+		return INSTANCE;
 	}
 }
 

java/src/test/java/org/msgpack/TestPackUnpack.java

@@ -130,7 +130,7 @@ public class TestPackUnpack {
 		ByteArrayOutputStream out = new ByteArrayOutputStream();
 		new Packer(out).packNil();
 		MessagePackObject obj = unpackOne(out);
-		assertTrue(obj.isNull());
+		assertTrue(obj.isNil());
 	}
 
 	@Test