why the function GetMethodID in JNI always return 0?

[C++ Programming]

below is C++ code

jmethodID mid;j
class cls = env->FindClass( "test/MyTest");
mid = env->GetMethodID(cls, "append","(Ljava/lang/String;I)Ljava/lang/String;");
mid=env->GetMethodID(cls,"HelloWord","()Ljava/lang/String;");

below is display by javap
public test.MyTest(java.lang.String);
Signature: (Ljava/lang/String;)V
public java.lang.String HelloWord();
Signature: ()Ljava/lang/String;
public java.lang.String append(java.lang.String, int);
Signature: (Ljava/lang/String;I)Ljava/lang/String;

below is java code:
public String HelloWord()
{ System.out.println("JAVA-CLASS:MyTest method:HelloWord:" +helloword);
return helloword;}
public String append(String str, int i) { return str + i; }


notice: the class has been found successfully, and the construct function 's id is not return 0

question:why the other methods’s id always return 0??