why the function GetMethodID in JNI always return 0?

jmethodID mid;
jclass 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;");

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;

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